Grease Trap Sizing Calculator — Flow Rate & Capacity
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Size mode returns the required certified flow rate and grease capacity from the fixture drainage load. Check mode compares a proposed interceptor against the requirement.
Number of sink compartments or connected fixtures draining to this interceptor. A 3-compartment sink is typical for commercial kitchens.
Choose how to enter the sink volume. Dimensions compute the volume as L × W × D × 0.004329. Direct volume entry is for when the fixture volume is already known.
Interior length of each sink compartment.
Interior width of each sink compartment.
Interior depth of each sink compartment.
Default 0.75 — a code and profile assumption that the sink is about 75% full in use (items being washed displace the remaining volume). Do not change without confirming the adopted code basis.
Default 2 min — from the adopted code. Some codes use 1 min, which doubles the drainage flow rate and requires a larger interceptor. Confirm with the AHJ.
Use when the authority having jurisdiction specifies a required flow rate that overrides the table lookup. Labelled as AHJ-override in the result.
Use when the AHJ specifies a required grease capacity that overrides the standard rating-table basis.
High-temperature dishwasher discharge can re-liquefy grease. This flag adds a soft check; it does not change the computed size.
Food-waste disposers raise solids loading and may be prohibited or require special handling under local code. This flag adds a soft check; it does not change the computed size.
Overview
This calculator sizes a hydromechanical grease interceptor — the unit most people call a grease trap — for a commercial kitchen sink, so that fats, oils, and grease are captured before the wastewater reaches the sewer. From the sink dimensions it works out the drainage flow rate, then returns the two ratings that a hydromechanical unit is certified on: a flow rate in gallons per minute and a grease-retention capacity in pounds. Size mode returns the required ratings; Check mode tests a proposed interceptor against them.
The sizing rests on a simple chain. A sink is filled to about three-quarters of its volume in use, and that water drains over a short, fixed period — one or two minutes depending on the adopted code. The volume divided by the drain time is the drainage flow rate, and the interceptor is matched to the next certified size at or above that flow. Multi-compartment sinks are normally taken to drain simultaneously, so their volumes add. The certified flow rate then carries a minimum grease capacity with it, set by the rating standard at twice the flow value in pounds.
Two points shape the whole tool. First, a hydromechanical trap has two ratings, not one: the flow rate and the grease capacity, and both have to be adequate. Sizing on gallons-per-minute alone is not enough, because a unit can pass the flow check yet hold too little grease for the cleaning interval. Second, the method itself is set by local plumbing code, and codes differ. This calculator uses the widely adopted PDI G101 / ASME A112.14.3 fixture-flow basis as a screening method, shows the factors it used, and lets the authority having jurisdiction's required size override it. The final unit is selected from a manufacturer's certified rating.
What to Look at First
Required Certified Flow Rate (Size mode). The first output is the required certified flow rate — the smallest standard interceptor size at or above the computed drainage flow rate. This is the floor, not a target.
Required Grease Capacity. The second required output is the minimum grease-retention capacity in pounds. Both the flow rate and the grease capacity must be met; a unit that passes the flow check alone is not sufficient.
Check mode verdict. In Check mode, the results show the ratio for each rating (required ÷ proposed). A ratio at or below 1.00 is adequate. The governing shortfall identifies which rating controls the verdict — flow rate, grease capacity, or both — so you know which spec to change on the replacement unit.
Soft checks. Read any soft checks about dishwashers, food-waste disposers, or drain-time assumptions. These do not change the computed size but flag conditions your AHJ or the manufacturer may require you to address.
How to Use This Calculator
Choose the mode. Size returns the required flow rate and grease capacity; Check verifies a proposed interceptor.
Enter the sink. Give the number of compartments and each compartment's length, width, and depth, or enter the fixture volume directly in gallons or litres.
Confirm the fill fraction and drain time, or accept the code defaults shown (0.75 fill and 2-minute drain).
In Check mode, choose the proposed unit basis — flow and capacity together, or flow only — and enter the interceptor's certified ratings.
Open the advanced fields if needed: an AHJ-specified size override, and flags for a dishwasher or food-waste disposer.
Read the result. Size mode leads with the required flow rate and grease capacity; Check mode leads with proposed against required, naming which rating governs.
This is a screening and preliminary-sizing aid using the PDI G101 / ASME A112.14.3 fixture-flow basis. Local code and the authority having jurisdiction govern the method and the required size. The final unit is selected from a manufacturer's certified rating.
Inputs & Outputs
Inputs
Mode & Units
- •Mode — Size — compute required flow rate and grease capacity; Check — evaluate a proposed interceptor against the requirement
- •Unit System — US (gpm, gal, lb, in) or Metric (L/min, L, kg, mm)
Fixture
- •Number of Compartments — Number of sink compartments or connected fixtures; 1–10; used with ALL_COMPARTMENTS_SIMULTANEOUS policy
- •Volume Entry Basis — Compartment dimensions (L × W × D) or direct volume per compartment
- •Compartment Length / Width / Depth (in or mm) — Interior dimensions of each compartment; volume = L × W × D × 0.004329 (in³ → gal)
- •Volume per Compartment (gal or L) — Known fixture volume per compartment; used when Volume Entry Basis = Direct volume
Profile Factors (code basis)
- •Fill Fraction — Default 0.75 (dimensionless) — code assumption that the sink is 75% full in use; shown as 'from profile' in the result
- •Drain Time (min) — Default 2 min — from the adopted code; 1 min doubles the drainage flow rate; shown as 'from profile' in the result
Check Mode Parameters
- •Proposed Unit Basis — Flow + capacity (full check) or flow only (partial check with capacity-not-verified flag)
- •Proposed Certified Flow Rating (gpm or L/min) — Certified flow rating of the proposed interceptor from the manufacturer's data sheet
- •Proposed Grease Capacity (lb or kg) — Certified grease-retention capacity from the manufacturer's data sheet; required for a full check
Advanced (Optional)
- •AHJ Flow Override (gpm or L/min) — Overrides the table lookup for the required flow rate; use when the AHJ specifies a flow requirement
- •AHJ Capacity Override (lb or kg) — Overrides the standard 2× capacity basis; use when the AHJ specifies a required grease capacity
- •Dishwasher Connected — Flags high-temperature dishwasher discharge for a soft check; does not change the computed size
- •Food-Waste Disposer Connected — Flags elevated solids loading for a soft check; does not change the computed size
Outputs
Both Modes
- •Compartment Volume (gal or L) — Volume of one compartment — from dimensions or direct entry
- •Drainage Volume per Compartment (gal or L) — Compartment volume × fill fraction
- •Total Drainage Volume (gal or L) — Sum over all compartments (ALL_COMPARTMENTS_SIMULTANEOUS)
- •Drain Time (min) — From the profile or entered value, labelled as 'from profile'
- •Computed Drainage Flow Rate (gpm or L/min) — Total drainage volume ÷ drain time — the flow rate the interceptor must handle
- •Required Certified Flow Rate (gpm or L/min) — Next certified trap size at or above the drainage flow rate (CONSERVATIVE_BRACKET lookup)
- •Required Grease Capacity (lb or kg) — 2 × certified flow rating in pounds (PDI G101 / ASME A112.14.3 rating basis); labelled 'from profile'
- •Sizing Basis — PDI G101 / ASME A112.14.3 fixture-flow basis — fill fraction, drain time, and simultaneity policy used, each labelled from profile
Check Mode
- •Proposed Certified Flow Rating (gpm or L/min) — As entered
- •Flow Ratio (required ÷ proposed) — ≤ 1.00 = adequate; 1.00–1.15 = at limit; 1.15–1.50 = undersized; > 1.50 = significantly undersized
- •Proposed Grease Capacity (lb or kg) — As entered (when Proposed Unit Basis = Flow + capacity)
- •Capacity Ratio (required ÷ proposed) — Same banding as flow ratio; shown only when capacity was provided
- •Governing Shortfall — FLOW_RATING | GREASE_CAPACITY | MULTIPLE | NONE — names which rating controls the verdict
Formula
Grease Trap Sizing Formulas
Compartment Volume (from dimensions)
compartment_volume (gal) = L(in) × W(in) × D(in) × 0.004329
Fixture Drainage Volume
drainage_volume = compartment_volume × fill_fraction
total_drainage = drainage_volume × number_of_compartments (simultaneous)
The fill fraction reflects that a sink is filled to roughly 75% in use, since the items being washed displace about a quarter of the volume. The drain time is one or two minutes depending on the adopted code.
Drainage Flow Rate
drainage_flow (gpm) = total_drainage (gal) / drain_time (min)
Required Certified Flow Rate
required_flow = smallest certified trap with rated flow ≥ drainage_flow
Lookup uses CONSERVATIVE_BRACKET: snap to the next certified size at or above the computed flow, never interpolate.
Required Grease Capacity
required_grease_capacity (lb) = 2 × rated_flow (gpm)
PDI G101 and ASME A112.14.3 set the minimum grease-retention capacity at twice the certified flow rating in pounds: a 35 gpm unit carries at least 70 lb, a 50 gpm unit at least 100 lb.
Check Mode Ratios
flow_ratio = required_flow / proposed_flow
capacity_ratio = required_capacity / proposed_capacity
Ratio ≤ 1.00 → Adequate; 1.00–1.15 → At limit; 1.15–1.50 → Undersized; > 1.50 → Significantly undersized.
Overall verdict = worst of the two ratios. Governing shortfall names FLOW_RATING | GREASE_CAPACITY | MULTIPLE | NONE.
PDI G101 Certified Trap Size Table
| Rated Flow (gpm) | Grease Capacity (lb) |
|---|---|
| 5 | 10 |
| 10 | 20 |
| 15 | 30 |
| 20 | 40 |
| 25 | 50 |
| 35 | 70 |
| 50 | 100 |
| 75 | 150 |
| 100 | 200 |
Variable Reference
| Variable | Meaning | Units |
|---|---|---|
| L, W, D | Compartment interior dimensions | in or mm |
| fill_fraction | Fill level in use (code basis) | dimensionless |
| drain_time | Code drainage time | min |
| drainage_flow | Computed fixture drainage flow rate | gpm or L/min |
| required_flow | Required certified flow rating | gpm or L/min |
| required_grease_capacity | Required certified grease capacity | lb or kg |
Profile Basis
All factors (fill fraction, drain time, capacity table) come from the active SizingProfile (PDI G101 / ASME A112.14.3 fixture-flow basis), never a hardcoded assumption. Profile factors are displayed in the result with their source labelled.
What is Grease Trap Sizing
A grease trap, or grease interceptor, is a chamber on a kitchen's drain line that slows the wastewater enough for fats, oils, and grease to separate and be held back while the water passes through. Grease congeals in cooler sewer pipes and is a leading cause of blockages and sanitary overflows, so codes require food-service kitchens to intercept it on site. Sizing the trap means giving it enough capacity to do that for the flow it sees.
There are two physical types, and they are sized differently. A hydromechanical grease interceptor is the smaller, certified unit, often installed under or near the sink, that uses internal baffles and a flow-control fitting to separate grease quickly. It is rated by a certified flow rate in gallons per minute and a grease-storage capacity in pounds, tested to standards like PDI G101 and ASME A112.14.3. A gravity grease interceptor is the large, below-grade vault that serves a whole kitchen, sized by volume from the meals served and a retention time. This calculator sizes the hydromechanical type, which covers most individual-fixture and small-kitchen applications.
For a hydromechanical unit, the sizing chain is short. A sink filled to about three-quarters of its volume drains over one or two minutes, and that volume per minute is the drainage flow rate. A three-compartment sink of moderate size drains at roughly 30 to 35 gallons per minute, which calls for a 35 gpm certified interceptor. That 35 gpm rating brings a 70-pound minimum grease capacity with it, because the rating standards set grease capacity at twice the flow value. The interceptor is selected at the next certified size at or above the computed flow.
The detail that catches people is that flow rate and grease capacity are two separate requirements. The flow rate decides whether the sink drains without backing up; the grease capacity decides how long the unit runs between cleanings before it loses efficiency. A unit can have ample flow rating and still hold too little grease for a busy kitchen's maintenance schedule, which is why the certified capacity matters alongside the flow. Both, and the method used to size them, are ultimately set by the local plumbing code and the authority having jurisdiction.
Hydromechanical vs Gravity Grease Interceptors
The two interceptor types serve different load scales and are selected by different methods. A hydromechanical unit is the right choice for an individual sink, a small prep area, or a light-duty application. It installs at or near the fixture, is serviced by removing the unit or opening an access port, and carries a certified rating for flow and grease retention. A gravity interceptor is used when the whole kitchen drains to a single large underground vault, which retains grease by allowing it to float while water exits at a lower outlet. It is sized from a meals-per-day or gallon-per-meal formula, not from a sink drainage flow rate.
Fill Fraction and Drain Time
Two factors convert the sink volume into a flow rate: the fill fraction and the drain time. The fill fraction reflects that a sink is not filled to its brim when in use — the items being washed displace roughly a quarter of the capacity, so the effective volume is about 75% of the geometric volume. This 0.75 value appears in the PDI G101 sizing method and in many plumbing codes, though the exact value is set by the adopted code, not by a universal physical constant.
The drain time is similarly a code assumption. Most codes use one or two minutes. A one-minute drain time produces twice the drainage flow rate for the same sink volume, which requires a larger interceptor. The fill fraction and drain time are made visible in this calculator and labelled as profile factors, because changing either changes the required size, and the adopted code — not a remembered value — governs both.
Grease Capacity and Cleaning Interval
The grease capacity of a hydromechanical interceptor is not just a compliance number; it determines how often the unit must be cleaned before it stops working. When grease accumulates to a point where it enters the outlet baffle, the unit passes FOG to the sewer. Many FOG programs and manufacturers recommend service when grease and solids reach about 25% of the certified capacity. A kitchen that generates a lot of grease — a burger grill, a fry station — will reach that threshold faster than a light-duty prep sink.
This calculator reports the required grease capacity from the PDI G101 rating basis: twice the certified flow rate in pounds. A 35 gpm unit carries at least 70 lb minimum capacity. Whether that capacity is adequate for the actual cleaning schedule depends on the kitchen's FOG load, which this tool does not calculate. The capacity check ensures the unit is not so small that it cannot function for a reasonable interval; it does not design the maintenance schedule.
The Certified Rating and the AHJ
The output of this calculator is a required certified flow rate and a required grease capacity. These are the minimum specifications a unit must carry. The actual unit must then be selected from a manufacturer's certified rating list — a unit tested and rated to PDI G101, ASME A112.14.3, or CSA B481 — at or above both required values. No calculator can substitute for the certified product listing.
The authority having jurisdiction may require a different sizing method, a larger minimum size, or a specific interceptor for the kitchen type. The AHJ override fields are provided for that: when the AHJ specifies a required flow or capacity, those values replace the table lookup, and the result labels them as AHJ-override so the basis is transparent.
Units
The calculator works in US units by default and converts to metric on selection. Sink dimensions are in inches or millimetres, volumes in gallons or litres, the flow rate in gallons per minute or litres per minute, and grease capacity in pounds or kilograms.
A sink volume in cubic inches converts to gallons by multiplying by 0.004329. Gallons convert to litres by multiplying by 3.78541, and gallons per minute to litres per minute by the same factor. Grease capacity in pounds converts to kilograms by multiplying by 0.4536.
Key Facts
- A hydromechanical grease interceptor has two certified ratings: a flow rate in gpm and a grease-retention capacity in lb. Both must be adequate; sizing on flow rate alone is not sufficient.
- The drainage flow rate is the sink volume times the fill fraction, divided by the drain time. A sink is taken as filled to about 75% in use, draining over one or two minutes per the adopted code.
- Multi-compartment sinks are normally taken to drain simultaneously, so their drainage volumes are added before dividing by the drain time.
- The interceptor is selected at the next certified size at or above the computed flow rate, not by rounding to the nearest convenient number.
- Certified grease capacity is set at twice the flow rate in pounds: a 20 gpm unit holds at least 40 lb, a 35 gpm unit at least 70 lb, a 50 gpm unit at least 100 lb.
- Grease capacity is only meaningful with a cleaning interval. Many FOG programs pump out when grease and solids reach about 25% of capacity.
- The sizing method is set by local plumbing code and varies by jurisdiction. PDI G101, ASME A112.14.3, and CSA B481 are the common rating standards.
- This calculator sizes the hydromechanical type. A whole-kitchen load usually needs a gravity grease interceptor, sized by volume from meals and retention time.
Applications
- Sizing a hydromechanical grease interceptor for a one-, two-, or three-compartment commercial sink.
- Confirming the required certified flow rate and grease capacity before selecting a manufacturer's model.
- Checking whether an existing or proposed interceptor is adequate on both flow rate and grease capacity.
- Screening a kitchen drain during design review or a permit submittal.
- Comparing how the required size changes with a one-minute versus a two-minute drain basis under different codes.
- Identifying when a fixture load is beyond the hydromechanical range and a gravity interceptor or engineered design is needed.
- Checking whether a unit chosen on flow rate alone also carries enough grease capacity for the kitchen's cleaning interval.
Example Calculation
Example 1 — Three-compartment sink, Size mode
A three-compartment sink has compartments each 24 inches long, 24 wide, and 12 deep. Code basis: 0.75 fill fraction, 2-minute drain, all compartments simultaneous.
Calculation:
Compartment volume: 24 × 24 × 12 × 0.004329 = 29.9 gal
Drainage volume per compartment: 29.9 × 0.75 = 22.4 gal
Total for three compartments: 3 × 22.4 = 67.3 gal
Drainage flow rate: 67.3 / 2 = 33.6 gpm
Required certified flow rate: next size at or above 33.6 = 35 gpm
Required grease capacity: 2 × 35 = 70 lb
Result: The sink needs a 35 gpm certified interceptor with at least 70 lb of grease capacity.
Example 2 — Check, grease capacity governs
The same sink needs 35 gpm and 70 lb. Proposed unit: 35 gpm certified, 50 lb grease capacity.
Flow ratio: 35 / 35 = 1.00 → Adequate
Capacity ratio: 70 / 50 = 1.40 → Undersized
Result: The flow rate passes but the grease capacity does not. Unit is Undersized — grease capacity governs. It would drain the sink fine but need cleaning too often to be practical.
Example 3 — Check, flow rate governs
Same requirement: 35 gpm / 70 lb. Proposed unit: 25 gpm, 75 lb.
Flow ratio: 35 / 25 = 1.40 → Undersized
Capacity ratio: 70 / 75 = 0.93 → Adequate
Result: The grease capacity is fine but the flow rate is short. Unit is Undersized — flow rating governs. The sink could back up at peak draining.
Standards & References
- PDI G101, Testing and Rating Procedure for Hydromechanical Grease Interceptors (Plumbing & Drainage Institute) — the certified flow-rate and grease-capacity rating method used as the sizing basis in this calculator
- ASME A112.14.3, Grease Interceptors and CSA B481 — the standards hydromechanical interceptors are tested and rated to; verify the current edition with the manufacturer
- International Plumbing Code (IPC), grease-interceptor sizing provisions and the local adopting authority — the governing method and any required size for the jurisdiction
- The authority having jurisdiction's adopted code edition and any municipal FOG-program requirements govern the method and the required size. The final unit is selected from a manufacturer's certified rating; this tool is a screening aid.
Units
The calculator works in US units by default and converts to metric on selection. Sink dimensions are in inches or millimetres, volumes in gallons or litres, the flow rate in gallons per minute or litres per minute, and grease capacity in pounds or kilograms.
A sink volume in cubic inches converts to gallons by multiplying by 0.004329; gallons convert to litres by 3.78541, and gallons per minute to litres per minute by the same factor. Grease capacity in pounds converts to kilograms by multiplying by 0.4536. The fill fraction and drain time are dimensionless and in minutes respectively, set by the adopted code rather than the unit system.
Limitations
- This is a screening and preliminary-sizing aid, not a permit document. The sizing method is set by the local plumbing code and the authority having jurisdiction, and the required size can differ from this screening result.
- It sizes the hydromechanical type only. A whole-kitchen or high-volume load usually needs a gravity grease interceptor, sized by volume from meals or seats and a retention time.
- It reports a certified flow rate and a grease capacity, but it does not select a specific manufacturer model or verify a product's certification or listing. The final unit must come from a certified rating.
- It does not decide whether a given fixture may connect to the interceptor. Dishwashers, food-waste disposers, mop sinks, floor drains, and pre-rinse stations are governed by local FOG rules.
- It does not calculate FOG mass loading from the menu, cooking process, or cleaning interval, and it does not predict pump-out frequency.
- It does not evaluate venting, the flow-control fitting, the installation location, or access and maintenance clearance.
- It does not address indoor/outdoor installation, traffic loading, corrosion resistance, sampling ports, or cleanouts.
- It does not evaluate local FOG discharge limits or wastewater pretreatment permits. Final design follows the adopted code and professional judgment.
Common Mistakes to Avoid
- Sizing on flow rate alone. A hydromechanical interceptor has two ratings; a unit can pass the gpm check and still hold too little grease for the cleaning interval. Check both the flow rate and the grease capacity.
- Confusing the two ratings when selecting a unit. The flow rate decides whether the sink drains without backing up; the grease capacity decides how long the unit runs between cleanings.
- Forgetting that compartments drain simultaneously. Multi-compartment sink volumes are normally added, since the compartments are taken to drain at once, which raises the flow rate above any single compartment.
- Including the sanitizing compartment. Many codes exclude the sanitizing compartment from the grease interceptor, so adding it can oversize the unit; follow the adopted code.
- Rounding to the nearest size instead of up. The interceptor is selected at the next certified size at or above the computed flow, not the closest one, so the rating is never below the requirement.
- Assuming the drain time and fill fraction are universal. The fill fraction and the one- or two-minute drain are code assumptions, not fixed physical facts, and the adopted code sets them.
- Using a hydromechanical trap for a whole-kitchen load. Past the hydromechanical range, a gravity grease interceptor sized by volume is required.
- Treating the result as the permitted size. The method and required size are set by local code and the AHJ.
Frequently Asked Questions
How do I size a grease trap?
Why does a grease trap have two ratings?
What is the difference between a grease trap and a grease interceptor?
How is grease capacity related to flow rate?
Do all the sink compartments count?
What size grease trap do I need for a 3-compartment sink?
Does this calculator size a large in-ground grease interceptor?
Can I use this result for a permit?
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Calculate
Size mode returns the required certified flow rate and grease capacity from the fixture drainage load. Check mode compares a proposed interceptor against the requirement.
Number of sink compartments or connected fixtures draining to this interceptor. A 3-compartment sink is typical for commercial kitchens.
Choose how to enter the sink volume. Dimensions compute the volume as L × W × D × 0.004329. Direct volume entry is for when the fixture volume is already known.
Interior length of each sink compartment.
Interior width of each sink compartment.
Interior depth of each sink compartment.
Default 0.75 — a code and profile assumption that the sink is about 75% full in use (items being washed displace the remaining volume). Do not change without confirming the adopted code basis.
Default 2 min — from the adopted code. Some codes use 1 min, which doubles the drainage flow rate and requires a larger interceptor. Confirm with the AHJ.
Use when the authority having jurisdiction specifies a required flow rate that overrides the table lookup. Labelled as AHJ-override in the result.
Use when the AHJ specifies a required grease capacity that overrides the standard rating-table basis.
High-temperature dishwasher discharge can re-liquefy grease. This flag adds a soft check; it does not change the computed size.
Food-waste disposers raise solids loading and may be prohibited or require special handling under local code. This flag adds a soft check; it does not change the computed size.