Concrete Block Calculator — Do Not Add the Joint Twice

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This selector governs every field, label, result and export here, and it overrides the site header switch. Block designations stay US nominal sizes: a 190 by 190 by 390 mm block is a different product, not a conversion of an 8 by 8 by 16.

The Wall

One wall takes a length and a height. Several walls take a pair for each, and the course check then runs on each wall height separately, because a wall that is a whole number of courses and a wall that is not are two different findings.

In feet, measured along the face of the wall. Where several walls were chosen this is the first of them.

In feet, from the top of the footing to the top of the wall. The course check runs on this figure, and a height that is not a multiple of the course module is reported rather than rounded.

The Block

All three published sizes share the 8 by 16 face, so the block count is the same in any of them. The size is asked because the width changes the grout, which is the number it moves.

In inches. Leave it blank and the published 3/8 inch is used and named. That is the joint the nominal block size already contains, and it is the only one that returns the published 1.125 blocks per square foot.

The Openings

Each opening comes off the gross area before the blocks per square foot factor is applied, because a window is not made of blocks. A solid wall is an answer here and it is offered as one.

The Allowance

Published guidance ties the allowance to the openings, because masonry cutoffs at door jambs, window sills and wall corners are rarely reusable. Cut coursing raises it again, and published guidance states that it lifts the labour rate as well.

The Grout

Whether a wall is grouted, and at what spacing, comes from a structural design and a seismic category this page cannot read. Not known is an answer here and no volume is issued against it.

The Mortar

In standard blocks laid per bag, read off the bag you are buying. Published yields run from 8 to 16 for an 80 pound bag, so a bag count from an assumed yield is a guess with a unit on it. No bag count is issued without this figure.

Bond Beams, Lintels and Bars

How many courses are bond beam courses. A bond beam unit is a different unit from a standard block and it substitutes for one in that course rather than adding to the count.

In feet, the total length of lintel over the openings. Lintel units are counted on the same module as the wall and substitute for standard units in the course above the opening.

In inches, on centre, from the structural drawing. The bars are counted here and not sized: the development and lap splice lengths come from the Rebar Calculator.

In inches, on centre. A horizontal bar sits in a bond beam course, so a spacing that is not a multiple of the course module is reported as one that cannot be built as drawn.

Prices

Optional. Whatever your supplier quotes for one standard unit. Bond beam and lintel units are priced differently and are not included in this figure.

Optional. The bag count it multiplies comes from the yield you entered, so no cost appears without one.

Optional, per cubic yard delivered. The grout volume is reported as a band, so the cost is reported as one too.

Overview

An eight by eight by sixteen block measures 7-5/8 by 7-5/8 by 15-5/8.

The name is not the block. It is the block plus its mortar joint, and published guidance across every source consulted says so: the nominal dimensions already include the three eighths inch joint.

That matters because the module on every unit page is the same thing, the unit plus one shared joint, and a block is the only one sold under the name of its module. A tile sold as twelve by twelve is twelve by twelve of tile. A block sold as eight by sixteen is 7-5/8 by 15-5/8 of block with the joint already added.

So two errors run in opposite directions from one naming quirk. Using the actual face gives 1.209 blocks per square foot against 1.125, which is 7.4 percent too many. Adding a joint to the nominal size gives 1.0500, which counts the joint twice.

The published 1.125 is the answer at a three eighths joint and not a property of the block. At a quarter inch joint the module is 7.875 by 15.875 and the count is 1.1519. At a half inch it is 1.0991.

And the grout figure is worth checking before you order. Published figures per block run from 0.24 to 0.27 cubic feet with both cells filled, and one published calculator gives 0.014 per cell, which is 0.028 for a block. That is nearly ten times, and the low figure comes from a site whose own estimating guide publishes 0.24.

What to Look at First

Read the two blocks per square foot figures side by side before anything else. The module figure is the one the count runs on and the actual face figure is reported beside it, so a reader who has measured a block with a tape and found 15-5/8 can see what that would have given and why the page did not use it.

Then read the course remainder. A wall height that is not a multiple of the course module means either the top course is being cut or the footing elevation is wrong, and published guidance states that cut coursing raises the waste factor and the labour rate together, so the cost lands twice.

How to Use This Calculator

  1. Enter the wall and the openings. Each opening comes off the gross area before the blocks per square foot factor is applied, because a window is not made of blocks.

  2. Leave the joint at three eighths unless you are laying something else. The published 1.125 is what the module equation returns at that joint, and another joint moves it: a quarter inch gives 1.1519 and a half inch gives 1.0991.

  3. Read the module figure and the actual figure side by side. If you have measured a block and found 15-5/8, the page shows you what that would have given and why it did not use it.

  4. Check the course remainder. A wall height that is not a multiple of eight inches means either the top course is being cut or the footing elevation is wrong, and both cost money.

  5. Say whether the wall is grouted and how. Not known is an answer, and the page reports no volume rather than guessing, because whether a wall is grouted is a structural question.

  6. Give the mortar bag yield in blocks per bag. Published yields swing from 8 to 16 for an 80 pound bag, so a bag count from an assumed yield is a guess with a unit on it.

  7. Add the bar spacings if the wall is reinforced. The page counts the bars and sends the length question to the Rebar Calculator, which is where the development and lap splice lengths come from.

Inputs & Outputs

Inputs

•Unit System : Options: US / Imperial (ft, in, sq ft, cu ft, cu yd), SI / Metric (m, mm, sq m, cu m)
•Wall Shape : Options: Not selected, One wall, from a length and a height, Several walls, entered one at a time
•Wall Length
•Wall Height
•Wall 2 Length
•Wall 2 Height
•Wall 3 Length
•Wall 3 Height
•Wall 4 Length
•Wall 4 Height
•Block Size : Options: Not selected, 6 by 8 by 16, partition walls, 8 by 8 by 16, standard walls, 12 by 8 by 16, heavy walls
•Mortar Joint
•Openings : Options: Not selected, None, a solid wall, One opening, Two openings, Three openings, Four openings, Five openings, Six openings
•Opening 1 Width
•Opening 1 Height
•Opening 2 Width
•Opening 2 Height
•Opening 3 Width
•Opening 3 Height
•Opening 4 Width
•Opening 4 Height
•Opening 5 Width
•Opening 5 Height
•Opening 6 Width
•Opening 6 Height
•Waste Allowance : Options: Not selected, 5 percent, a rectangular wall with no openings, 10 percent, two to four openings, 15 percent, many openings or a non standard layout, A percentage of my own, None
•Waste Percentage
•Grout Pattern : Options: Not selected, No grout, Every cell, Cells at a stated spacing, Not known
•Cell Spacing
•Mortar Bag Yield
•Bond Beam Courses
•Lintel Length
•Vertical Bar Spacing
•Horizontal Bar Spacing
•Price Per Block
•Price Per Mortar Bag
•Price Per Cubic Yard Of Grout

Outputs

•The wall area gross and net, with each opening deducted before the blocks per square foot factor
•The module and the actual face side by side, with the count each one returns and the count that adds the joint twice
•The block count before and after the waste allowance
•The courses, the remainder on the module and the blocks in one course
•The mortar bags at your yield, the published 8 to 16 range and the mortar volume
•The grout by pattern, with the low published figure beside it and all three cross-checks named
•The bond beam units, the lintel units and the bar counts
•The mortar types named and not selected
•What this page is not: no grout mix, no bar length, no footing and no pass or fail

Concrete Block Formula

Every quantity here comes from one wall area, one block size and one joint width, and the module is the block plus the joint rather than a hard coded nominal face.

The Wall

gross_sf = wall_len_ft * wall_hgt_ft
open_sf = open_1_sf + open_2_sf + open_3_sf
net_sf = gross_sf - open_sf

The three terms in the openings equation are representative. The page sums as many openings as were entered, and where several walls were entered the gross area equation runs once per wall and the results are summed.

The Module and the Two Faces

mod_hgt_in = act_hgt_in + joint_in
mod_len_in = act_len_in + joint_in
face_mod_sf = mod_hgt_in * mod_len_in / 144
face_act_sf = act_hgt_in * act_len_in / 144
per_sf_mod = 1 / face_mod_sf
per_sf_act = 1 / face_act_sf

The module is the actual block plus one joint in each direction, which is the same rule the tile and paver pages use. At the published three eighths joint the actual 7.625 by 15.625 becomes exactly 8 by 16, which is 128 square inches and is what the block is sold as. The nominal face is not a separate rule, it is this equation's answer at the standard joint.

That is why a non standard joint moves the count. At a quarter inch the module is 7.875 by 15.875 and the count is 1.1519 per square foot. At a half inch it is 8.125 by 16.125 and 1.0991. Only three eighths returns the published 1.125.

The actual face and its count are reported beside the module figures rather than used. Using the actual face returns 1.209 against 1.125, which is 7.4 percent too many, and adding a joint to the nominal size gives 8.375 by 16.375 and 1.0500, which counts the joint twice.

The Count and the Allowance

blocks_raw = net_sf * per_sf_mod
blocks = blocks_raw * (1 + waste_pct / 100)

The count before the allowance is carried unrounded into it. Block, bag and bar counts round up once, at the end.

The Courses

courses = wall_hgt_in / mod_hgt_in
course_rem_in = wall_hgt_in - courses_whole * mod_hgt_in
course_blocks = wall_len_in / mod_len_in

The course equation divides by the module height rather than the actual block height. A ten course wall is eighty inches, which is six feet eight, and not 77.5 inches. Counting on the actual height produces a wall two and a half inches short of the drawing.

The remainder is reported rather than absorbed, because a fractional course means either a cut top course or a wrong footing elevation and neither is something a calculator resolves. The course count is not rounded, because the remainder is the finding.

The Mortar

mortar_bags = blocks / blocks_per_bag
mortar_cf = blocks / 100 * cf_per_100

No bag count is issued without a yield in blocks per bag, because published yields for an eighty pound bag run from 8 to 16 and the spread is the product rather than the arithmetic.

The Grout

grout_cf = blocks_raw * cf_per_block
cells_across = wall_len_in / cell_sp_in
cell_count = cells_across * courses
grout_sp_cf = cell_count * cf_per_cell
grout_cy = grout_cf / 27

Where the wall is grouted at a spacing, the cells across are counted as one at each end of the wall and one every spacing between them, and each cell runs the full height of the wall, which is why the count multiplies by the courses. That is also the rule the vertical bar count uses, and it has to be the same one: a vertical bar sits in a grouted cell.

The cubic feet per block is the published 0.24 to 0.27 band for an 8 inch unit, and the cubic feet per cell is half of it. On a 6 or a 12 inch unit the published per block figures do not apply, and the page runs the published 45 to 50 percent void against the actual unit volume instead and names that as its own arithmetic.

The Bars

vert_bars = wall_len_in / vert_sp_in
horiz_bars = wall_hgt_in / horiz_sp_in

Both counts add one bar for the far end of the run, so a twenty foot wall at sixteen inches on centre is sixteen bars rather than fifteen. What neither count gives is a bar length, and that is the Rebar Calculator's.

Unit Conversions

sq m = sq ft * 0.09290304
mm = in * 25.4
cu m = cu ft * 0.0283168

A Block Is Sold Under the Name of Its Own Module

The Tile Calculator on this site says a tile does not cover its own dimensions, because the module is the tile plus one shared joint. The Paver Calculator says the same about a paver. It would be easy to read a block as the exception to that, and it is not one.

The module is the unit plus one joint on all three pages. What differs is what the unit is sold as.

A tile sold as twelve by twelve is twelve by twelve of tile. A paver sold as four by eight is four by eight of paver. A block sold as eight by eight by sixteen measures 7-5/8 by 7-5/8 by 15-5/8, because published guidance across every source consulted states that the nominal dimensions already include the three eighths inch mortar joint.

So the block carries the name of its module rather than the name of its face. At the standard joint the actual 7.625 by 15.625 plus three eighths in each direction is exactly 8 by 16, which is 128 square inches, 0.889 square feet and 1.125 blocks per square foot. Published sources give it as about 113 blocks per 100 square feet.

Two errors follow from the naming and they run opposite ways.

Using the actual face gives 119.14 square inches and 1.209 blocks per square foot, which is 7.4 percent too many. On a two hundred square foot wall that is seventeen blocks nobody needs.

Adding a joint to the nominal size counts the joint twice, which is the correction the other two pages teach and the one a reader arriving from them will reach for. It gives 8.375 by 16.375, which is 1.0500 per square foot and fourteen blocks short on the same wall.

And because the module is the actual plus the joint rather than a fixed nominal, a non standard joint moves the count. A quarter inch joint gives 1.1519 per square foot and a half inch gives 1.0991. Only three eighths returns the published figure, and the page says so rather than leaving 1.125 in place.

The Grout Figure Is Published Across a Factor of Nine

Grout per block with both cells filled is published three ways in the same span and a fourth well outside it. Three sources give 0.24, 0.25 and 0.27 cubic feet. One published calculator gives 0.014 per grouted cell, which is 0.028 for a block.

Nearly ten times is not a range and it is not a matter of opinion. One of them is wrong.

Three checks point the same way.

The first is published on the same site as the low figure, in its own estimating guide: an 8 inch cell is about 0.12 cubic feet, two cells to a block, and about one cubic yard per 110 to 120 solid grouted blocks. Twenty seven cubic feet over 110 to 120 blocks is 0.225 to 0.245 per block, which lands on the guide and nowhere near the calculator.

The second is a third source giving 100 blocks per cubic yard, which is 0.27 per block.

The third is the block itself. A 7.625 by 7.625 by 15.625 unit is 908 cubic inches. Published void for a hollow concrete masonry unit is forty five to fifty percent, which gives 0.237 to 0.263 cubic feet. The three per block figures sit inside that and the low one is nowhere near it.

On five hundred blocks the two ends are about 4.4 cubic yards and 0.5. That is not an estimating difference, it is a different wall.

So this page uses the 0.24 to 0.27 band, reports the low figure beside it as published, and names all three checks.

One more published figure is not used. One source quotes the association technical note as giving about 55 cubic feet per 100 square feet for a solid grouted 8 inch wall, which converts to 0.489 per block. A hundred square feet of an 8 inch wall is 63.5 cubic feet of wall in total, so 55 would be 87 percent void, and no hollow unit reaches that.

The Wall Height Is a Module, Not a Dimension

Published guidance states it plainly: the wall height in inches divided by eight should come out even. A six foot eight inch wall is exactly ten courses. A seven foot wall is ten and a half.

And it states what a half course means: either every block in the top course is being cut, or the footing elevation is wrong.

Neither of those is a calculator's problem to solve and both are worth knowing before the order goes in. Published guidance adds that cut coursing affects both the waste factor and the labour rate, so the cost lands twice.

The related trap is counting courses on the actual block height rather than the module. Published guidance gives the example: a ten course wall is eighty inches, which is six feet eight, and not 77.5 inches. Counting on 7-5/8 produces a wall two and a half inches short of the drawing.

So this page divides by the module height, reports the remainder rather than rounding it away, and names both causes of a fractional course.

The Mortar Spread Is a Bag Yield Problem

Published mortar figures per one hundred blocks look like the grout disagreement and are not the same thing.

Four sources give three bags per hundred. One gives one bag per thirty five blocks and one gives one per thirty seven, so those three agree closely at about 2.7 to 2.9 bags.

Against that, manufacturer figures for an eighty pound bag run from 8 to 16 standard blocks. One branded mason mix lays up to 13 and a preblended bag runs closer to 16. That is 6.25 to 12.5 bags per hundred, and one published calculator lets the reader pick the yield rather than hard coding one.

The figure that sits outside the manufacturer range is the rule of three bags per hundred, which implies about 33 blocks per bag.

So this page takes the yield from the reader rather than assuming one, reports the published range with the bag size it belongs to, and states the published site variables: rough textured or lightweight block, full bedding and hot weather all increase consumption, and most masons carry a bag or two of cushion per hundred.

The volume figures carry their own ambiguity. Published mortar volume is about 8.5 cubic feet per 100 blocks for face shell bedding and about 11 for full bedding, and a bag of masonry cement yields about 4.5 cubic feet of mortar, which is a different product from a preblended mason mix bag. One source publishes the 8.5 figure per 100 square feet of wall rather than per 100 blocks, and 100 square feet is 112.5 blocks, so the two differ by that factor and neither says it is the other.

This Page Counts Bars and Does Not Size Them

A reinforced block wall has vertical bars in grouted cells and horizontal bars in bond beams, and the spacings come from a structural drawing.

Counting them is arithmetic: a wall length divided by a spacing, a wall height divided by another, with one bar added for the far end of the run.

What that count does not tell you is how long each bar has to be, and that is where the money and the failure modes are. The Rebar Calculator on this site takes a bar size and a set of conditions and returns a development length and a lap splice length, and a Class B splice on a number five bar runs from 30.8 inches to 46.3 depending on the spacing and cover.

So this page counts the bars at the spacing entered, reports the count, and sends the length question to the page that answers it. No development length, no splice length and no bar weight appears here.

The same split applies to the grout. This page produces a volume in cubic feet and cubic yards. The Concrete Calculator turns a volume into a mix and a bag count, and repeating that here would give two pages that disagree the first time either yield figure moved.

What Is a Concrete Block Calculation

A block wall is a grid, and the grid spacing is the block plus its joint.

That grid is what a block count comes from, and it is why the count is not a division by the size of the block you can hold. It is a division by the space each block occupies on the wall, which is larger by one joint in each direction.

Above the grid sits a course count, which is the same arithmetic vertically and has a harder consequence: block courses do not stretch, so a wall height that is not a multiple of the course module is a cut course or a wrong footing.

Beside the grid sit three materials that are counted differently from each other. Mortar goes in the joints and is bought by the bag, at a yield that varies by a factor of two across products. Grout goes in the cells and is bought by the cubic yard, at a figure published across a factor of nearly ten. Reinforcement goes in the grouted cells and is counted here and sized somewhere else.

Openings are the one subtraction that has to happen in the right order. Each opening comes off the gross wall area before the blocks per square foot factor is applied, and published guidance gives a two hundred square foot wall with three standard windows as a difference of roughly thirty five to forty blocks, which at 1.125 per square foot is three windows of about eleven to twelve square feet each.

What the arithmetic cannot decide is whether the wall is grouted at all, where the bars go and which mortar type applies. Those come from a structural drawing.

Where the Published Figures Come From

The actual 7-5/8 by 7-5/8 by 15-5/8 size and the statement that the nominal dimensions include the three eighths inch joint are consistent across every source consulted and are attributed to ASTM C90.

The blocks per square foot figure, the mortar per 100 square feet and the grout tables are attributed by published sources to NCMA and CMHA TEK 04-02A, the technical note series of the association formed by the unification of the National Concrete Masonry Association and the Interlocking Concrete Pavement Institute. This page reports them as attributed rather than as read from the note, because the sources that cite the figure do not quote the note.

The grout per block figures of 0.24, 0.25 and 0.27 come from three separate sources and are used because two published cross-checks and a physical void check land on them. The low figure of 0.014 per cell is reported beside them as published.

The mortar bag yields of 8 to 16 blocks per eighty pound bag are manufacturer figures for specific products, and the mortar type strengths are ASTM C270 designations.

The bar count rule of one bar at each end and one every spacing between them is this page's arithmetic rather than a published figure, and so is the grout figure for a 6 or a 12 inch unit, which runs the published void fraction against the actual unit volume because the published per block figures are 8 inch figures. Both are named as such on the row that carries them.

Key Facts

  • A nominal 8 by 8 by 16 block actually measures 7-5/8 by 7-5/8 by 15-5/8, because published guidance states that the nominal dimensions already include the three eighths inch mortar joint.
  • The module on this page is the actual block plus one joint in each direction, which is the same rule the tile and paver pages use.
  • At a three eighths joint that module is exactly 8 by 16, which is 128 square inches, 0.889 square feet and 1.125 blocks per square foot, published as about 113 blocks per 100 square feet.
  • Published sources attribute the blocks per square foot figure to NCMA and CMHA TEK 04-02A, and one gives 119 per 100 square feet with waste.
  • Using the actual 7.625 by 15.625 face returns 1.209 blocks per square foot, which is 7.4 percent too many.
  • Adding a joint to the nominal size gives 8.375 by 16.375 and 1.0500 per square foot, which counts the joint twice.
  • The published 1.125 is the module equation's answer at a three eighths joint: a quarter inch joint gives 1.1519 and a half inch gives 1.0991.
  • The 6, 8 and 12 inch blocks all share the 8 by 16 face, so all three are 1.125 per square foot and the width changes the grout rather than the count.
  • Published guidance states that the wall height in inches divided by eight should come out even, that a six foot eight inch wall is exactly ten courses and that a seven foot wall means a cut course.
  • A ten course wall is eighty inches nominal and not 77.5 inches actual, so counting courses on the actual block height produces a wall height error.
  • Published guidance states that a fractional course means either every block in the top course is being cut or the footing elevation is wrong, and that cut coursing affects both the waste factor and the labour rate.
  • Grout per block with both cells filled is published at 0.24, 0.25 and 0.27 cubic feet across three sources, and at 0.014 per cell, which is 0.028 per block, by one calculator.
  • That low figure comes from a site whose own estimating guide publishes an 8 inch cell at about 0.12 cubic feet, so the same publisher carries both ends of a spread of nearly ten times.
  • Two cross-checks land on the higher figures: one cubic yard per 110 to 120 solid grouted blocks gives 0.225 to 0.245, and 100 blocks per cubic yard gives 0.27.
  • A physical check agrees: a 7.625 by 7.625 by 15.625 block is 908 cubic inches, and at the published 45 to 50 percent void that is 0.237 to 0.263 cubic feet.
  • The published grout figures are 8 inch figures, so on a 6 or a 12 inch unit this page runs the published void fraction against the actual unit volume and names that as its own arithmetic.
  • The published fraction for a wall grouted at 32 inches on centre is given as roughly a quarter in one source and roughly half in another, so this page counts the cells from the spacing instead of applying a fraction.
  • Published mortar yields swing from 8 to 16 standard blocks per 80 pound bag, with one branded mason mix laying up to 13 and a preblended bag closer to 16.
  • Published mortar volume is about 8.5 cubic feet per 100 blocks for face shell bedding and about 11 for full bedding, and a bag of masonry cement yields about 4.5 cubic feet of mortar.
  • Published waste allowances are 5 percent for a rectangular wall with no openings, 10 for two to four openings and 15 for many openings or a non standard layout.
  • Published guidance states that masonry cutoffs at door jambs, window sills and wall corners are rarely reusable.
  • Published mortar types: Type S at 1,800 psi for structural CMU walls, Type N at 750 psi for above grade non load bearing walls and veneer, and Type M at 2,500 psi for below grade and foundation walls, with Type N named as not appropriate below grade.
  • Openings are deducted from the gross wall area before the blocks per square foot factor is applied.

Applications

  • Someone who has measured a block and found 15-5/8 sees why the page did not use it.
  • A reader arriving from the tile or paver calculator finds out why adding a joint here counts it twice.
  • Someone laying a half inch joint sees the count move off the published 1.125.
  • A reader whose wall is seven feet tall finds the half course before the blocks arrive.
  • Someone quoted two grout figures nearly ten times apart sees which one survives a cross-check.
  • A reader with a mason mix bag in front of them enters its yield rather than accepting an assumed one.
  • Someone specifying below grade finds that Type N is named as not appropriate for it.
  • A reader planning reinforcement gets a bar count here and a bar length from the page that computes lengths.

Example Calculations

Example 1. The name is not the block

Given: a 200 square foot wall in nominal 8 by 8 by 16 block at a three eighths inch joint, no openings and no waste allowance.

The actual block is 7.625 by 15.625 inches. Add three eighths in each direction and the module is 8 by 16, which is 128 square inches and 0.889 square feet.

144 divided by 128 is 1.125 blocks per square foot, so the wall takes 225 blocks.

Result: 225 blocks, and two wrong answers to avoid.

Using the actual face gives 144 over 119.14, which is 1.209 per square foot and 242 blocks. That is 17 blocks too many.

Adding a joint to the nominal size gives 8.375 by 16.375, which is 1.0500 per square foot and 211 blocks. That counts the joint twice and is 14 blocks short.

Example 2. The joint is the module, so a different joint is a different count

Given: the same 200 square foot wall at three joint widths.

At a quarter inch the module is 7.875 by 15.875, which is 125.02 square inches and 1.1519 per square foot: 231 blocks.

At three eighths it is 8 by 16, which is 128 and 1.1250: 225 blocks.

At a half inch it is 8.125 by 16.125, which is 131.02 and 1.0991: 220 blocks.

Result: 231, 225 and 220 on one wall. The published 1.125 is the middle answer and belongs to the middle joint, and a page leaving it in place on a half inch joint would report a figure the wall does not have.

Example 3. Seven feet is ten and a half courses

Given: a wall specified at 7 feet 0 inches.

The course module is the actual 7.625 plus the three eighths joint, which is 8 inches.

84 inches divided by 8 is 10.5 courses.

Result: ten full courses reach 80 inches and there are 4 inches left.

Published guidance names the two causes: either every block in the top course is being cut, or the footing elevation is wrong. It adds that cut coursing affects the waste factor and the labour rate, so the cost lands twice.

A six foot eight inch wall is 80 inches and exactly ten courses, which is why that height appears on so many drawings.

Example 4. The grout figure, and the publisher who disagrees with themselves

Given: the same 225 blocks, every cell grouted.

At the published 0.24 to 0.27 cubic feet per block the volume is 54.00 to 60.75 cubic feet, which is 2.00 to 2.25 cubic yards. The middle published figure of 0.25 gives 56.25 cubic feet, or 2.08 cubic yards.

At the low published figure of 0.028 per block it is 6.30 cubic feet, which is 0.23 cubic yards.

Result: about two cubic yards against a quarter of one, and one of them is wrong.

Three checks settle it. One published source gives a cubic yard per 110 to 120 solid grouted blocks, which is 0.225 to 0.245 per block. Another gives 100 blocks per cubic yard, which is 0.27. And a 908 cubic inch block at the published 45 to 50 percent void is 0.237 to 0.263.

The low figure comes from a calculator whose own estimating guide, on the same site, gives an 8 inch cell as about 0.12 cubic feet, which is 0.24 for a block.

Example 5. The bag yield is the question, not the bag count

Given: the same 225 blocks and an 80 pound bag of mason mix.

Published yields for an 80 pound bag run from 8 to 16 standard blocks. One branded mason mix lays up to 13 and a preblended bag runs closer to 16.

At 8 blocks per bag the wall takes 29 bags. At 13 it takes 18. At 16 it takes 15.

Result: 15 to 29 bags on one wall, from the product rather than the arithmetic.

That is why the page asks for the yield in blocks per bag rather than assuming one. A bag count from an assumed yield is a guess with a unit on it, and the published range is a factor of two.

Standards & References

The 7-5/8 by 7-5/8 by 15-5/8 actual size and the statement that the nominal dimensions include the three eighths inch mortar joint are consistent across every source consulted and are attributed to ASTM C90. The blocks per square foot figure, the mortar per 100 square feet and the grout tables are attributed by published sources to NCMA and CMHA TEK 04-02A, and this page reports them as attributed rather than as read from the note. The grout per block figures of 0.24, 0.25 and 0.27 come from three separate sources and are used because two published cross-checks and a physical void check land on them; the low figure of 0.014 per cell is reported beside them as published and is contradicted by its own publisher's estimating guide. One source quotes TEK 04-02A as giving about 55 cubic feet per 100 square feet for a solid grouted 8 inch wall, which converts to 0.489 per block and would be 87 percent of the wall volume, and that figure is not used. A mortar figure of 8.5 cubic feet appears per 100 square feet of wall in one source and per 100 blocks in another, and the two differ by the 1.125 factor with neither saying it is the other. The mortar bag yields of 8 to 16 blocks per 80 pound bag are manufacturer figures for specific products, and the mortar types and their strengths are ASTM C270 designations. This page reports quantities and issues no pass or fail: whether a wall must be grouted, where the reinforcement goes and which mortar type applies come from a structural design and a seismic category it cannot read, and it computes no grout mix, no reinforcement length and no footing.

Units

Wall areas are in square feet with square metres alongside at 0.09290304. A 200 square foot wall is 18.58 square metres.

Block dimensions, joints, courses and remainders are in inches with millimetres alongside at 25.4. A three eighths inch joint is 9.53 mm, the actual block height of 7.625 inches is 193.7 mm and the 8 inch module is 203.2.

Grout volumes are in cubic feet and cubic yards with cubic metres alongside at 0.0283168. 56.25 cubic feet is 2.08 cubic yards and 1.59 cubic metres.

Block designations are US nominal sizes. An 8 by 8 by 16 is a name for a module rather than a measurement of a unit, and metric markets sell a different block list rather than a conversion of these: a 190 by 190 by 390 mm block is not an 8 by 8 by 16.

Blocks per square foot is a count against an Imperial area. In metric the same figure is blocks per square metre, which is 10.764 times it, so 1.125 per square foot is 12.11 per square metre.

Mortar bag yields are in blocks per bag and are dimensionless, and the bag weight is a product figure in pounds.

The course count, the remainder, the waste percentage and all the counts are dimensionless or identical in both systems.

The internal unit selector governs the fields, labels, math, on-screen result and any exported result, and it takes priority over any site-wide unit switch. Switching converts the values you entered rather than reinterpreting them.

Limitations

  • This page reports quantities and issues no pass or fail. Whether a wall must be grouted, where the reinforcement goes and which mortar type applies come from a structural design and a seismic category.
  • It computes no grout mix and no bag count for grout. That is the Concrete Calculator's.
  • It computes no reinforcement length. The bar count comes from the spacing you enter and the development and splice lengths are the Rebar Calculator's.
  • It does not compute the footing under the wall.
  • It does not count half blocks, corner blocks, bullnose units or control joint details separately.
  • It does not place the cut course. It reports the remainder and names the two causes.
  • It reports no grout volume where the grout pattern is not known.
  • The association figures are attributed by published sources rather than read from the technical note, and one quoted TEK figure for solid grout does not survive a physical check and is not used.
  • The published grout per block figures are 8 inch figures. On a 6 or a 12 inch unit the volume is this page's arithmetic on the published void fraction and is named as such.
  • The mortar figures mix a bag yield with a bedding method, and the page takes the yield from you rather than assuming one.
  • Any cost is arithmetic on a price you entered.

Common Mistakes to Avoid

  • Adding a mortar joint to the nominal block size. The nominal already includes it, so adding one gives 8.375 by 16.375 and counts the joint twice.
  • Using the actual 7-5/8 by 15-5/8 face for the count. It returns 1.209 blocks per square foot against 1.125, which is 7.4 percent too many.
  • Keeping the published 1.125 on a non standard joint. It is the module equation's answer at three eighths, and a half inch joint gives 1.0991.
  • Counting courses on the actual block height. A ten course wall is 80 inches and not 77.5, so counting on 7-5/8 leaves the wall two and a half inches short.
  • Rounding a fractional course away. It means either a cut top course or a wrong footing elevation, and both cost money.
  • Assuming a mortar bag yield. Published yields run from 8 to 16 blocks per 80 pound bag, which is 29 bags against 15 on a 225 block wall.
  • Taking the low published grout figure. It is 0.014 per cell against a resolved 0.24 to 0.27 per block, and it is contradicted by its own publisher's estimating guide.
  • Treating grout as mortar. Mortar goes in the joints and grout goes in the cells, and they are different products with different specifications.
  • Using Type N below grade. Published guidance names it as not appropriate there and gives Type S or Type M instead.
  • Calculating blocks on the gross wall area. Openings come off first, and three standard windows on a 200 square foot wall are roughly 35 to 40 blocks.
  • Assuming a wider block needs more blocks. The 6, 8 and 12 inch units share the same 8 by 16 face, so the width changes the grout and not the count.
  • Ordering grout before the pattern is decided. Whether a wall is grouted is a structural question, and a volume for a wall that turns out to need none is worse than no volume.
  • Sizing rebar from a block calculator. The bar count is arithmetic and the lap splice length is not.

Frequently Asked Questions

How many blocks per square foot?
1.125 for a standard 8 by 8 by 16 at a three eighths inch joint, which is about 113 per 100 square feet. The figure comes from the module of 8 by 16, which is the actual 7-5/8 by 15-5/8 block plus its joint.
Why is a standard block only 15-5/8 inches long?
15-5/8 plus a three eighths joint is 16, and 16 is the name. The nominal size is the space the block occupies on the wall rather than the block itself.
Should I add the mortar joint to the block size?
No. It is already in the nominal size, and adding it again gives 8.375 by 16.375 and 1.0500 blocks per square foot, which is 14 blocks short on a 200 square foot wall.
How much grout does a block take?
Published figures are 0.24 to 0.27 cubic feet per block with both cells filled, which is about one cubic yard per 100 to 120 blocks. A figure of 0.014 per cell also circulates and is contradicted by its own publisher's guide.
How many blocks does a bag of mortar lay?
Published yields run from 8 to 16 standard blocks for an 80 pound bag. One branded mason mix lays up to 13 and a preblended bag runs closer to 16, so take the figure from the bag in front of you.
How many courses is a wall?
The height in inches divided by eight. A six foot eight inch wall is exactly ten courses and a seven foot wall is ten and a half, which means a cut top course or a wrong footing elevation.
Do 6 inch and 12 inch blocks need a different count?
No. All three share the 8 by 16 face, so all three are 1.125 per square foot. The width changes how much grout the cells take.
Does this calculator size the rebar?
No. It counts bars at the spacing you enter. The development and lap splice lengths come from the Rebar Calculator, where a Class B splice on a number five bar runs from 30.8 inches to 46.3 depending on conditions this page does not ask for.

Frequently Used Together

Engineers often use these calculators in combination for complete project workflows:

Concrete Calculator

Rebar Calculator

Concrete Slab Calculator

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