Grease Interceptor Sizing Chart
Sizing grease interceptors — the point-of-use hydromechanical units (HGIs, rated in GPM) and the large in-ground gravity tanks (GGIs, sized in gallons). Hydromechanical units are sized by flow with 2 pounds of grease capacity per GPM; gravity tanks are sized from fixture units or meals served. Sizing is heavily locally amended, so the AHJ has the final word.
Check local amendments first. Grease interceptor sizing is one of the most heavily locally-amended areas of plumbing code — the sewer authority or health department often overrides the model-code method with its own minimums. Confirm with the AHJ before finalizing.
Hydromechanical (HGI) — by flow rate
| Flow-through rating (GPM) | Grease retention (lb) |
|---|---|
| 10 | 20 |
| 15 | 30 |
| 20 | 40 |
| 25 | 50 |
| 35 | 70 |
| 50 | 100 |
| 75 | 150 |
| 100 | 200 |
Gravity (GGI) — by fixture units
| Drainage fixture units | Interceptor volume (gal) |
|---|---|
| 8 | 500 |
| 21 | 750 |
| 35 | 1,000 |
| 90 | 1,250 |
| 172 | 1,500 |
| 216 | 2,000 |
| 307 | 2,500 |
| 342 | 3,000 |
| 428 | 4,000 |
| 576 | 5,000 |
| 720 | 7,500 |
| 2112 | 10,000 |
| 2640 | 15,000 |
Two types, two sizing logics
The two interceptor types capture grease differently, so they are sized differently. A hydromechanical unit works fast, using flow restriction and baffles to separate grease as water passes through, so it is rated by the flow it can handle — GPM — with a grease-holding capacity of two pounds per GPM. A gravity interceptor works slowly, giving the water a long detention time in a big tank so grease can rise and collect, so it is sized by volume in gallons against the fixture load. Read the fixture drainage load from the DFU chart or the flow with the flow rate calculator.
Common questions
How do you size a grease interceptor?
It depends on the type. A hydromechanical grease interceptor (HGI, the point-of-use "grease trap") is sized by flow rate in GPM — from the drainage load of the connected fixtures, with a minimum of 20 GPM under the IPC — and each has a grease retention capacity of 2 pounds per GPM. A gravity grease interceptor (GGI, the large in-ground tank) is sized in gallons: the UPC sizes it from drainage fixture units at 30-minute retention, or by meals served.
What is the difference between a grease trap and a grease interceptor?
They are the same idea at two scales. A hydromechanical unit (grease trap) is small, sits at or near the fixtures, and is rated by flow in GPM. A gravity interceptor is a large buried tank sized in gallons that gives grease long enough to cool, separate, and float before the water moves on. Heavy-grease operations often need the gravity type; smaller kitchens use hydromechanical units.
Why does the chart say to check local amendments?
Because grease interceptor sizing is one of the most heavily locally-amended areas of plumbing code. Municipal sewer and pretreatment authorities and county health departments routinely override the model-code method with their own minimums — mandatory 1,000–1,500 gallon tanks, meals-served multipliers, or bans on hydromechanical units in food service. The model-code number is usually a floor, so confirm with the AHJ before finalizing.
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