Grease Trap Size Calculator — Flow & Interceptor Volume
Two devices answer to the name "grease trap", and they are sized by two unrelated methods. The under-sink hydromechanical interceptor is rated in gpm and sized from how fast the sink empties. The buried gravity interceptor is rated in gallons and sized from how many meals the kitchen serves. Pick a method below — and note that on most commercial jobs the local FOG programme, not the arithmetic, has the last word.
Size the interceptor
Measure the interior of one compartment, not the outside of the sink.
A one-minute drain period is the usual assumption. Some jurisdictions work it at two, which halves the required rating — check before you rely on it.
Seatings in the busiest hour. Fast food is higher.
Editable — jurisdictions differ.
Most sewer authorities impose one. Set it to 0 to see the raw calculation.
Interceptor rating
Smallest PDI rating covering 67.3 gpm, holding 150 lb of grease — 2× the flow rating, by definition.
Required flow
67.3 gpm
Grease capacity
150 lb
Probably an outdoor job
Past about 50 gpm most authorities want a gravity grease interceptor outside rather than a hydromechanical unit under the sink. Check the FOG programme before specifying either.
See the breakdown
Interceptor size
Smallest stocked vessel covering the 1,500 gallon requirement, well past the 1,000 gallon floor.
Volume required
1,500 gal
Peak-hour meals
100
Past a single vessel
Expect two interceptors in series or a custom-poured structure, and a design the authority will want to review rather than approve over the counter.
See the breakdown
Planning estimate. The local sewer authority's FOG programme governs, and it routinely imposes a minimum size, an outdoor location, a pump-out interval, and a sample port that no calculation predicts. Confirm before ordering.
Both formulas, explained in plain English
One asks how fast water arrives. The other asks how long it has to sit still.
Flow versus retention
A hydromechanical unit separates grease by slowing flow, so its rating is the speed it can cope with. A gravity interceptor separates by waiting, so its rating is how much it can hold while that happens. Sizing one with the other's method produces a number that looks plausible and means nothing.
Pounds and gpm are one number
PDI-G101 fixes grease capacity at 2 lb per gpm across the whole ladder, so a 10 gpm unit is a 20 lb unit and a 75 gpm unit is a 150 lb unit. There is nothing to look up.
The flow control fitting
A hydromechanical interceptor is rated at a design flow, not a peak one, so an orifice fitting upstream throttles the line to match it. Without one, a fast-emptying sink pushes flow straight through and the unit separates nothing — a correctly sized trap installed without flow control is still a failure.
The floor usually wins
On these defaults the meal-count formula only overtakes a 1,000 -gallon minimum at around 67 seats. Below that, every small kitchen in the jurisdiction gets the same tank regardless of what the arithmetic says.
Worked examples
The standard three-compartment sink, the small kitchen the floor governs, and the same restaurant open around the clock.
A 24 × 24 × 12 three-compartment sink
three of them = 89.77 gal
× 75% = 67.32 gal draining in 1 min
→ 67.32 gpm → 75 gpm PDI unit, 150 lb
Result: a standard commercial sink already needs the 75 gpm unit — near the top of the hydromechanical range. Drop to a 20 × 20 × 12 sink and it falls to 50 gpm. This is why full-service kitchens so often end up outside with a gravity interceptor instead.
A 50-seat café — where the floor takes over
formula = 50 × 6 × 2.5 × 1 = 750 gal
local floor = 1,000 gal
→ 1,000 gal, governed by the minimum
Result: the arithmetic asks for 750 gallons and the job gets 1,000. Worth knowing before you quote, because the excavation for a 1,000-gallon vessel is a different job from the one the customer imagined.
The same 100 seats, open 24 hours
Open about 16 hours a day (×2): 3,000 gal → 3000 gal vessel
Open 24 hours a day (×3): 4,500 gal → 5000 gal vessel
Single-service kitchen (×1.5): 2,250 gal → 2500 gal vessel
Result: hours of operation is a straight multiplier, so the same dining room needs 3× the vessel running around the clock. It is the input most often guessed at and the one with the largest single effect.
PDI interceptor ratings
The published ladder of hydromechanical units. The pound column is computed from the gpm column at 2 lb per gpm rather than transcribed alongside it, because PDI-G101 defines it that way — the two can never disagree here.
| Flow rating | Grease capacity | Typical application |
|---|---|---|
| 4 gpm | 8 lb | Single sink or a bar |
| 7 gpm | 14 lb | Single sink or a bar |
| 10 gpm | 20 lb | Single sink or a bar |
| 15 gpm | 30 lb | Small two-compartment sink |
| 20 gpm | 40 lb | Small two-compartment sink |
| 25 gpm | 50 lb | Small two-compartment sink |
| 35 gpm | 70 lb | Three-compartment sink |
| 50 gpm | 100 lb | Three-compartment sink |
| 75 gpm | 150 lb | Large sink — usually an outdoor job instead |
| 100 gpm | 200 lb | Large sink — usually an outdoor job instead |
Rating by sink size
Three compartments, 12 inches deep, drained in one minute at 75% fill.
| Compartment | Total volume | Required flow | PDI unit |
|---|---|---|---|
| 16 × 16 × 12 in | 39.9 gal | 29.92 gpm | 35 gpm / 70 lb |
| 18 × 18 × 12 in | 50.5 gal | 37.87 gpm | 50 gpm / 100 lb |
| 20 × 20 × 12 in | 62.3 gal | 46.75 gpm | 50 gpm / 100 lb |
| 24 × 24 × 12 in | 89.8 gal | 67.32 gpm | 75 gpm / 150 lb |
| 30 × 24 × 12 in | 112.2 gal | 84.16 gpm | 100 gpm / 200 lb |
Gravity interceptor by seat count
Full kitchen with dishwashing, one turnover per hour, 2.5 hour retention, eight-hour day, and a 1,000-gallon local floor.
| Seats | Formula | Required | Governed by | Vessel |
|---|---|---|---|---|
| 30 | 450 gal | 1,000 gal | The local floor | 1000 gal |
| 50 | 750 gal | 1,000 gal | The local floor | 1000 gal |
| 75 | 1,125 gal | 1,125 gal | The calculation | 1250 gal |
| 100 | 1,500 gal | 1,500 gal | The calculation | 1500 gal |
| 150 | 2,250 gal | 2,250 gal | The calculation | 2500 gal |
| 200 | 3,000 gal | 3,000 gal | The calculation | 3000 gal |
| 300 | 4,500 gal | 4,500 gal | The calculation | 5000 gal |
Sources & standards: IPC 2021 1003.3 (grease interception) and the PDI-G101 rating standard for hydromechanical units. The volume arithmetic is derived from 231 cubic inches per US gallon and the pound capacities are computed from the flow ratings, so neither is transcribed. The meal-count factors — gallons per meal, retention time and storage factor — are commonly published figures rather than values verified here against a primary source, which is why all three are editable. IPC 1003.3.4 permits the authority having jurisdiction to approve a sizing method, and in practice the local FOG programme sets the minimum size, the location, the pump-out interval and the sampling requirement. Confirm before ordering.
Frequently asked questions
Common questions about trap versus interceptor, PDI ratings, and FOG programme requirements.
What size grease trap do I need?
It depends which device you mean, and that is the whole problem. For a standard three-compartment sink at 24 × 24 × 12 inches, the hydraulic method gives 67.3 gpm, which takes a 75 gpm PDI interceptor holding 150 lb of grease. For a 100-seat restaurant, the meal-count method gives 1,500 gallons of gravity interceptor. Both answers can be right on the same job, because they size two different devices.
What is the difference between a grease trap and a grease interceptor?
In everyday use the words are swapped freely, but the devices are not. A hydromechanical interceptor — the small steel or plastic box under the sink — is rated in gpm and works by slowing flow enough for grease to float. A gravity grease interceptor — the buried concrete tank — is rated in gallons and works by holding the wastewater long enough for separation to happen on its own. Sizing one with the other's method is the commonest mistake on this subject.
Why is the grease capacity always double the flow rating?
Because PDI-G101 defines it that way. A 20 gpm unit is rated for 40 lb of grease, a 50 gpm unit for 100 lb, and so on at 2 lb per gpm right across the range. So "how many pounds" and "how many gpm" are not two specifications to look up — they are one number expressed twice.
Where does the 75% figure come from?
The fixture-volume method assumes a sink compartment is filled to about 75% of its geometric capacity before it drains, because nobody fills a sink to the rim. The full 6,912 cubic inches of one 24 × 24 × 12 compartment is 29.92 gallons; three of them at 75% is 67.3 gallons, and draining that in one minute is the 67.3 gpm the interceptor has to pass. The fill fraction and the drain period are both editable here because jurisdictions vary them.
Why did my calculation come out bigger than the units on the shelf?
Because a full-size three-compartment sink genuinely outruns the small end of the hydromechanical range. A 20 × 20 × 12 sink needs 50 gpm; a 24 × 24 × 12 needs 75. Past about 50 gpm most authorities want an outdoor gravity interceptor instead, and many will not permit an indoor unit for a commercial kitchen at all. A flow control fitting is the other half of the answer — it throttles the line to the interceptor's rated flow, which is why the rating is a design figure rather than a peak one.
Does the plumbing code tell me which size to fit?
Not on its own. IPC 1003.3.4 requires interception and permits the authority having jurisdiction to approve a sizing method, which means the real decision usually sits with your local sewer authority's FOG programme — fats, oils and grease. Those programmes routinely impose a flat minimum interceptor size, mandate outdoor installation, set pump-out intervals, and require a sample port. Run the numbers here to scope the job, then confirm with the authority before you order anything.
Why does a small restaurant still need a 1,000 gallon tank?
Because the floor governs, not the formula. At 50 meals in the peak hour the calculation gives only 750 gallons, but the common 1,000-gallon minimum takes over. That crossover happens at around 67 seats on these defaults — below it, every small kitchen in the jurisdiction gets the same tank.
What is out of scope here?
Flow control fittings and their vent, sample ports, the pump-out schedule, effluent limits, and any automatic grease removal unit. Those are set by the FOG programme rather than by arithmetic. This tool sizes the vessel; it does not design the installation, and the meal-count factors are commonly published figures rather than values verified against a primary source — which is why every one of them is an editable field.
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