Grease Trap Size Calculator
Sink boxes to interceptor gallons and gpm rating.
What it does
Kitchen drains carry fat, oil, and grease that harden downstream and choke sewers. Interceptors slow the flow so grease floats and solids sink before the line leaves the building. Sizing one starts at the sink, not the catalog.
The tool above turns sink boxes into gallons, takes 75 % for the trap, and halves it for a gpm flow rating. Imperial mode works in inches and gallons; metric mode works in centimeters and liters. Shop a rated unit at or above both answers.
Plumbing codes in the IPC and UPC families size interceptors from connected fixture load with minimum volumes, and testing standards rate boxes by flow with grease capacity near twice the gpm. The 75 % and 2-minute factors on this page mirror that fixture-based planning method used across local guides.
Treat the result as a shopping estimate. Dishwashers and disposals change the picture, gravity tanks follow different tables, and the authority having jurisdiction signs the final size and the pump schedule.
How to use
- Pick imperial (inches, gallons) or metric (cm, liters).
- Count sink compartments draining to the trap.
- Measure one box inside: length, width, depth.
- Read fixture volume, trap size, and flow rating.
- Shop standard units at or above both figures.
- Confirm the final size with the local inspector.
Formula
- compartments
- Count of sink boxes draining to the trap.
- L, W, D
- Inside length, width, depth per box in inches or cm.
- 231
- Gallons per cubic inch divisor (231); metric uses 1000.
Metric divides cubic cm by 1000 for liters with the same 0.75 and 2-minute factors. The 75 % share reflects displacement by water in service; the 2-minute divisor is a planning retention figure, not a permit. Match a standard rated unit at or above both numbers.
Worked example
Example: 3-comp sink, 24 in boxes
Three-compartment sink, each box 24 by 24 by 14 inches.
- Three compartments, each 24 x 24 x 14 in.
- One box: 24 x 24 x 14 / 231 = 34.9 gal.
- Fixtures: 3 x 34.9 = 104.7 gal.
- Trap: 104.7 x 0.75 = 78.5 gal.
- Flow: 78.5 / 2 = 39.3 gpm, so shop near a 40 gpm unit.
Reference table
| Flow rating | Grease capacity | Fits |
|---|---|---|
| 20 gpm | About 40 lb | Small 2-comp sinks |
| 35 gpm | About 70 lb | Busy 3-comp sinks |
| 50 gpm | About 100 lb | Large prep lines |
| 75 gpm | About 150 lb | Heavy cafeterias |
| 100 gpm | About 200 lb | Top hydromechanical range |
Limits
- Planning estimate only. Permits need the local code and inspector.
- Hydromechanical boxes only; gravity tanks use other tables.
- Dishwashers and disposals need separate review.
- Pump schedule matters as much as size.
Common mistakes
- Sizing from meals instead of fixture volume.
- Counting only one box of a 3-comp sink.
- Draining the dishwasher through a small box.
- Buying below both the gallons and gpm figures.
- Skipping pump-outs until it backs up.
Try it yourself
Quick checks. Answers sit tucked under each question.
Trap for 2 boxes of 24 x 18 x 12 in?
One box is 5184 / 231 = 22.4 gal. Fixtures total 44.9 gal, trap 33.7 gal, flow 16.8 gpm.
Trap for 2 boxes of 50 x 45 x 30 cm?
One box is 67.5 L. Fixtures total 135.0 L, trap 101.3 L, flow 50.6 L/min.
Questions
Trap or interceptor: which do I need?
A hydromechanical box under or near the sink for small flows, or a big gravity tank outside for large ones. Volume and flow point to the class; the inspector confirms which one the address needs.
How is fixture volume figured?
Length times width times depth per box, divided by 231 for gallons (or by 1000 for liters from cm). Add the boxes. The tool then takes 75 % for the trap and halves it for gpm.
Why divide gallons by 2 for gpm?
About 2 minutes of retention at rated flow, a common planning figure. A 40 gpm unit thus holds about 80 gallons working volume. Codes and makers round to standard sizes from there.
How much grease does a rating hold?
Often near 2 lb of grease capacity per gpm (a 40 gpm box rates about 80 lb). Pump before grease and solids pass a quarter of the volume. Frequency beats size: a small trap pumped often outperforms a big one ignored.
Does the dishwasher drain into the same trap?
No. Dishwashers, garbage disposals, and some pre-rinse stations bypass small traps or need their own math. List every connected fixture; the inspector may exclude or upsize for them.
What fails inspections most?
Missing baffles, cracked lids, cold water flushing grease through, and no pump log. Size correctly, then keep the maintenance card current, because inspectors read it first.