Rebar Calculator — The Fifty Percent Step Nobody Chooses

Overview

What to Look at First

How to Use This Calculator

1. Pick the bar size and the grade. The diameter and the unit weight come from the published ASTM A615 figures, not from the bar number divided by eight, because that rule stops holding above a number eight: a number eleven is 1.410 inches rather than 1.375.

2. Enter the specified concrete strength from the drawing. Its square root is in the denominator, and section 25.4.1.4 caps that square root at 100 psi, so above 10,000 psi concrete the length stops shortening.

3. Answer the spacing and cover condition from the section rather than from preference. This is the fifty percent step. If you have the actual confinement term from the detailing, choose the last option and enter it instead, and the page will run the general equation on it.

4. Answer the casting position, the coating and the concrete weight. A top bar takes 1.3, a tightly covered or closely spaced epoxy bar takes 1.5, and lightweight concrete lengthens rather than shortens the result.

5. Pick the splice class, or say it is not known. Class A needs both of its conditions and partial compliance is not allowed, so Not known falls back to Class B and the result says that it did.

6. Read the development length with the other condition beside it, then the splice. Both classes are reported whichever one you chose, because the drawing is where the class is settled.

7. Add a run length only if you want the quantity. The page then gives the sticks, the splices, the lap actually used, the total length of one line with the laps in it, and, with a line count, the weight and the tonnage.

8. Add a price per ton if you want a cost. It is arithmetic on the number you entered, in your own currency, and bar is normally quoted by weight rather than by length.

Inputs & Outputs

Rebar Development Length Formula

ld = (3 / 40) x (fy / (lambda x sqrt(fc))) x ((psi_t x psi_e x psi_s x psi_g) / ((cb + Ktr) / db)) x db

Class A lap = 1.0 x ld
Class B lap = 1.3 x ld

sticks  = roundup(run / stick length)
splices = sticks - 1
total length of one line = run + splices x lap
weight  = total length x lines x unit weight

Key Facts

• ACI 318-19 equation 25.4.2.4a gives the tension development length of a straight deformed bar. Its leading coefficient is three fortieths.
• Table 25.4.2.3 sets its coefficient from the clear spacing and the clear cover. The ratio between its two readings is exactly 1.5 on every bar size, so the step is fifty percent.
• Commentary R25.4.2.1 states that the table is based on two preselected values of the confinement term and the general equation on the actual one, which makes them one expression rather than two methods.
• Three fortieths divided by 1.5 is one twentieth, and divided by 1.0 is three fortieths. Multiplied by the 0.8 size factor those become one twenty-fifth and three fiftieths, which are the four constants Table 25.4.2.3 prints.
• Twenty divided by 0.8 is 25, so the simplified table constant already contains the bar size factor. Applying it again returns a length twenty percent short, in the unsafe direction.
• Section 25.4.2.4 caps the confinement term at 2.5.
• Section 25.4.1.4 caps the square root of the specified compressive strength at 100 psi, which corresponds to 10,000 psi concrete.
• The code limits the product of the casting position factor and the coating factor to 1.7. The maximum that product reaches unlimited is 1.3 times 1.5, which is 1.95, so the limit takes 12.8 percent off the length.
• Table 25.4.2.5 gives the casting position factor as 1.3 for a top bar with more than twelve inches of fresh concrete cast below it and 1.0 otherwise.
• The coating factor is 1.5 for epoxy coated bar with cover under three bar diameters or clear spacing under six, 1.2 for other epoxy coated bar, and 1.0 for uncoated or zinc coated bar.
• The bar size factor is 0.8 for a number six and smaller and 1.0 for a number seven and larger, with no intermediate value.
• The lightweight factor is 1.0 for normal weight, 0.85 for sand-lightweight and 0.75 for all-lightweight concrete. It divides into the denominator, so lightweight concrete lengthens the development.
• The grade factor is 1.0 for Grade 60, 1.15 for Grade 80 and 1.3 for Grade 100. Grade 80 is therefore 53 percent longer than Grade 60 on the same bar, not 33 percent.
• The published minimum development length is 12 inches, given as 305 mm in the metric edition.
• Section 25.5 gives a Class A tension lap splice as 1.0 times the development length and a Class B splice as 1.3 times it.
• Published guidance names Class B as the safe default, states that Class A requires both of its conditions, no more than half the bars spliced within the required lap length and at least twice the reinforcement required, and that partial compliance is not allowed.
• The ASTM A615 nominal diameter of a number eleven bar is 1.410 inches. The bar number divided by eight gives 1.375, which is 2.5 percent low and feeds straight into the length, so the eighths rule holds only to a number eight.
• At 4,000 psi, Grade 60, bottom cast, uncoated, normal weight and the favourable condition, a number six Class B splice is 37.00 inches and a number seven is 53.96, which is 45.8 percent for one step in bar size.
• The same conditions give a Class B splice of 24.67 inches on a number four, which is 10.3 percent of a 20 foot stick, and 53.96 inches on a number seven, which is 22.5 percent.
• A number eight bar at Grade 60 in 4,000 psi concrete, bottom cast, uncoated and normal weight develops in 47.43 inches under the favourable condition and 71.15 under the other cases.
• Bar numbers are US designations. A 25 mm bar is not a number eight, so the bar series does not convert between unit systems.
• One US ton of 2,000 pounds is 0.90718474 tonnes, and one pound per foot is 1.48816 kilograms per metre.

Applications

• A detailer checking a lap length on a shop drawing against the code rather than against the last job.
• An engineer seeing what the spacing and cover condition is worth before deciding whether to add ties through a development length.
• A reviewer reconciling two lap schedules that differ by fifty percent and looking for which condition each assumed.
• A contractor converting a run length into sticks, splices and tonnage for a bar order.
• Someone comparing a number six mat against a number seven mat and wanting the lap cost of the step rather than only the bar count.
• An estimator putting a price per ton against a bar order that already carries its laps.
• A detailer working in metric off a drawing written in US bar numbers.
• Anyone who has been handed a flat ten percent lap allowance and wants to know whether it fits the bar in front of them.

Standards & References

• ACI 318-19 Building Code Requirements for Structural Concrete and Commentary The source of the development length equation 25.4.2.4a, Table 25.4.2.3 and its two condition sets, the 2.5 cap on the confinement term in 25.4.2.4, the modification factors of Table 25.4.2.5, the limit of 1.7 on the product of the casting position and coating factors, the 100 psi cap on the square root of the concrete strength in 25.4.1.4, the 12 inch minimum, the Class A and Class B tension lap splices of section 25.5 and the two-tier statement in commentary R25.4.2.1.
• ASTM A615/A615M Standard Specification for Deformed and Plain Carbon-Steel Bars for Concrete Reinforcement The source of the US bar designations, the nominal diameters and the nominal unit weights used here, including the number eleven diameter of 1.410 inches that the eighths rule does not give.
• ASTM A706/A706M Standard Specification for Deformed and Plain Low-Alloy Steel Bars for Concrete Reinforcement The specification covering the low-alloy bar commonly supplied in the higher grades, where the grade factor of 1.15 or 1.3 applies on top of the yield strength.
• American Concrete Institute The body publishing ACI 318 and its commentary, which are the governing documents for everything computed on this page.
• Concrete Reinforcing Steel Institute The trade institute publishing detailing guidance on lap splice classes, development length tables and bar placement practice in the United States.

The equation, the table, the condition sets, the factors, the three limits, the minimum and the splice classes are quoted from ACI 318-19 and its commentary. The bar diameters and unit weights are the ASTM designations. Every length, splice, step, percentage, weight and tonnage on this page is computed from those provisions at full precision and rounded only for display, rather than copied from a published table of results. Codes are adopted by jurisdiction and amended locally, so confirm the edition in force and the lap schedule on the drawing before a result here is used for construction.

Units

Common Mistakes to Avoid

• Treating the spacing and cover condition as a preference. It is read off the section, and the step between the two readings is fifty percent on every bar size.
• Applying the 0.8 bar size factor to a length already read off Table 25.4.2.3. The constant contains it: twenty divided by 0.8 is 25, and applying it twice gives a length twenty percent short.
• Assuming the limit of 1.7 on the casting position and coating product makes the answer longer. It caps a product that reaches 1.95, so it takes 12.8 percent off.
• Using the bar number divided by eight as the diameter above a number eight. A number eleven is 1.410 inches, not 1.375, and the 2.5 percent difference feeds straight into the length.
• Reading Grade 80 as 33 percent longer than Grade 60. The yield strength is 33 percent higher and a grade factor of 1.15 applies on top of it, so the length is 53 percent longer.
• Expecting lightweight concrete to shorten the development. The lightweight factor divides into the denominator, so 0.85 and 0.75 lengthen it.
• Claiming Class A because one of its two conditions is met. Published guidance states that both are required and that partial compliance is not allowed.
• Stepping a mat from a number six to a number seven for the bar count alone. The 0.8 size factor disappears at that step and the lap grows 45.8 percent.
• Taking the flat ten percent lap allowance from a slab quantity onto a number seven run. The lap there is 22.5 percent of a 20 foot stick.
• Counting splices as sticks. A run of exactly one stick has no splice, so the stick count rounds up and the splice count is one fewer.
• Entering a cylinder break instead of the specified compressive strength. The equation runs on the specified figure from the drawing.
• Entering 4,000 in the megapascal field. The page stops on it as a unit entry error rather than returning a length nobody would question.
• Expecting a stronger mix to keep shortening the length indefinitely. The square root of the concrete strength is capped at 100 psi, so above 10,000 psi nothing further is credited.
• Assuming a metric selection changes the bar series. It changes the units the same US bar is reported in.

Frequently Asked Questions