Grease trap sizing calculator
There are two ways to size grease equipment, and which one applies depends on what you are sizing. A small indoor trap is sized by flow, from the sinks that drain into it. A large outdoor interceptor is sized by volume, from how many meals you serve. This calculator does both and shows its working.
Enter each sink compartment that drains to the trap. Measure inside the bowl, in inches.
An estimate for planning, using the methods shown below. Your city's sewer or FOG program, and your plumber or engineer, set the size you actually install; many cities also set a minimum.
Method 1: indoor traps, sized by the sinks
Indoor (hydromechanical) traps are rated by flow in gallons per minute. The fixture method, as set out in the City of Independence, Missouri's sizing requirements and Josam's sizing guide:
- Measure each sink compartment: length × width × depth, in inches, gives cubic inches.
- Divide by 231 to get gallons (231 cubic inches is one gallon).
- Multiply by 0.75, because a sink is normally filled to about 75% of capacity.
- Divide by the drainage period: one minute for interior interceptors; two minutes is used for exterior interceptors, or where conditions allow.
- Choose the next standard size up when the result falls between sizes.
Worked example (Josam's): a 48 × 24 × 12 inch sink is 13,824 cubic inches, or 59.8 gallons. At 75% that's 44.9 gallons. Draining in one minute gives 44.9 gpm, so a 50 gpm trap; over two minutes, 22.5 gpm and a 25 gpm trap. The calculator's indoor tab reproduces this if you enter one sink of 48 × 24 × 12.
Josam's table lists PDI sizes of 10, 15, 25, 35 and 50 gpm. The calculator rounds up to a common rating (4, 7, 10, 15, 20, 25, 35, 50, 75 or 100 gpm); check the ratings for the model you are actually buying.
Method 2: outdoor interceptors, sized by seats
Outdoor (gravity) interceptors are sized by liquid volume in gallons. The Uniform Plumbing Code formula, as laid out on the Texas Commission on Environmental Quality's sizing worksheet, multiplies four factors:
| Factor | Values |
|---|---|
| Meals per peak hour = seats × meal factor | Fast food 1.33 · Restaurant 1.00 · Leisure dining 0.67 · Dinner club 0.50 |
| Waste flow rate, gallons per meal | With dishwasher 6 · Without 5 · Single-service 2 · Food waste disposer only 1 |
| Retention time, hours | Commercial kitchen with dishwasher 2.5 · Single-service 1.5 |
| Storage factor, by hours open | 8 hours 1.0 · 12 hours 1.5 · 16 hours 2.0 · 24 hours 3.0 · Single-service 1.5 |
Worked example: a 100-seat restaurant with a dishwasher, open 12 hours: 100 × 1.00 × 6 × 2.5 × 1.5 = 2,250 gallons. Interceptors are made in standard sizes, so you would choose the next size up.
The worksheet applies the 2.5-hour retention time to commercial kitchens generally, so the calculator uses it for kitchens with or without a dishwasher.
Bigger isn't automatically better
An oversized interceptor costs more to install and more to pump — pumping is priced by tank size. On the City of Tempe's 2025 cooperative price list, a 1,000-gallon interceptor costs $270 a visit and a 2,000-gallon one $500. The calculator shows the matching figure for your result. Josam's guide also warns that the older fixture-unit method can grossly oversize interceptors, and that most jurisdictions are moving away from it.
Undersized is worse: a trap that is too small for the flow can't separate grease before the water leaves, and it reaches the 25% limit faster, so it needs pumping more often. See how often should a grease trap be pumped and what grease trap pumping costs.
Before you buy anything
- Ask your city first. Grease equipment is approved by the local sewer utility or FOG program, and many set minimum sizes or require a specific method.
- Have a plumber or engineer confirm it against the plumbing code your state has adopted.
- Count everything that drains to it — prep sinks, mop sinks and floor drains in the kitchen, not just the pot sink.
- Check the manufacturer's sizing for the specific model: the TCEQ worksheet's last step is to select an interceptor "as recommended by the manufacturer".