It depends on the device. A hydromechanical grease interceptor is sized by flow rate, commonly 10 to 100 gpm, with connected fixtures not exceeding 2.5 times the certified rating. A gravity interceptor is sized by gallons, commonly 300 to 3,000, using flow times a 30-minute detention period. Always verify the size your local FOG ordinance requires.
The two interceptor types and the units that size them
A restaurant grease interceptor comes in one of two forms, and each is sized in its own unit.
A hydromechanical grease interceptor (HGI), sometimes called a grease trap, is sized by flow rate in gallons per minute (gpm). These are the smaller, often indoor units that sit under or near a sink. Common certified ratings run from 10 gpm up to 100 gpm, in steps such as 10, 15, 20, 25, 35, 50, 75, and 100. The unit is rated for how fast wastewater can pass through it while still separating grease.
A gravity grease interceptor (GGI) is sized by total volume in gallons. These are the large concrete or fiberglass tanks set in the ground outside the building. Common sizes range from 300 to 3,000 gallons. They hold wastewater long enough for grease to float and settle, so the size is about holding capacity, not flow speed.
The simplest way to keep them straight: a hydromechanical unit is rated by speed (gpm), and a gravity unit is rated by size (gallons). Once you know which device your kitchen and your local authority call for, the sizing math follows from there.
How a hydromechanical interceptor is sized by flow
A hydromechanical unit is matched to the flow rate of the fixtures draining into it. The drainage fixtures connected to the trap should not produce more flow than the unit can handle. A widely used code limit is that the connected fixture capacity should not exceed 2.5 times the certified gpm rating of the interceptor.
Here is what that means in practice. Say you install a unit certified at 20 gpm. The fixtures feeding it should not have a combined drainage capacity above 50 gpm (20 times 2.5). If the connected sinks can dump water faster than that, the trap cannot slow the flow enough to let grease separate, and grease rides straight through.
Sizing usually starts with the fixture itself. A three-compartment sink is the classic example. You find the drainage volume of the sink, divide it by the drain time, and match that flow to a certified gpm rating that covers it. Makers publish flow tables for their fixtures. The certified rating stamped on the unit is the number that counts for code.
The takeaway: a hydromechanical unit is only correct if its certified gpm rating comfortably covers the flow from every fixture tied to it. Undersize the rating, and the device is technically installed but functionally useless.
How a gravity interceptor is sized by gallons
A gravity interceptor is sized so wastewater stays inside long enough for grease to rise and solids to settle. The standard formula is flow rate multiplied by a 30-minute detention time. You estimate the peak flow into the tank, then provide enough volume to hold that flow for half an hour.
There is also a floor. If the formula produces a number below 750 gallons, the practical minimum for an exterior gravity unit is 750 gallons. Many jurisdictions will not approve a buried gravity interceptor smaller than that, because anything less pumps out too often and separates grease too poorly to be worth installing.
A utility sizing guide describes the goal plainly:
"Grease interceptors are sized to allow adequate retention time for wastewater to cool and for the grease to separate and float to the top."
That retention time is the whole point. A bigger tank gives grease more time to cool, congeal, and float before the water moves on. That is also why gravity units are favored for high-volume kitchens: full-service restaurants, cafeterias, and anything with heavy frying loads tend to need the holding capacity a buried tank provides.
What drives the size, and the role of local code
A few factors decide which number you land on:
- Fixtures. The sinks, dishwashers, woks, and floor drains tied to the interceptor set the baseline flow. More grease-producing fixtures mean more flow to handle.
- Seating and meals served. Many gravity-sizing methods factor in seats, meal counts, and hours of operation as a proxy for how much wastewater and grease the kitchen generates.
- Flow rate. For both device types, the peak flow is the engine of the calculation. Everything else feeds into estimating it.
Here is a side-by-side summary:
| Factor | Hydromechanical (HGI) | Gravity (GGI) |
|---|---|---|
| Sizing unit | Flow rate (gpm) | Volume (gallons) |
| Common range | 10 to 100 gpm | 300 to 3,000 gal |
| Key rule | Fixture capacity should not exceed 2.5x certified gpm | Flow x 30-minute detention; 750-gal exterior minimum |
| Typical location | Indoor, under or near sink | Buried, exterior |
| Best fit | Lower-volume kitchens, single fixtures | High-volume kitchens, whole-facility loads |
The single most important point: local code and your jurisdiction's FOG ordinance set the requirement. The Uniform Plumbing Code (UPC) and International Plumbing Code (IPC) provide the framework, but your city or county sewer authority can impose its own sizing method, minimum volumes, and approval process. In metro Phoenix, the interceptor size and the rules around it are set by the local jurisdiction, so the gpm or gallon figure that satisfies one city may not satisfy the next.
This matters because FOG is the leading cause of sewer blockages. Per the EPA's Report to Congress, grease causes roughly 47% of reported sewer blockages. In the arid Southwest, nearly three-quarters of sewer overflow events were caused by blockages. An interceptor sized to code is the barrier that keeps your kitchen's grease out of that number.
Why an undersized unit fails, and how to verify your size
An undersized unit does not fail slowly. It fails fast. When flow is more than a hydromechanical trap can slow down, or a gravity tank is too small to give grease time to separate, the hot grease passes through while still liquid. It then cools inside the building sewer, hardens on the pipe walls, narrows the line, and backs up into the kitchen.
The cost shows up in several ways. You pump the unit far more often, because it fills with grease in days instead of weeks. You risk sewer backups during your busiest hours. And you can face violations, because a unit that is too small does not meet the discharge limits your FOG ordinance enforces. A trap that is too small is, in effect, no trap at all. The fix is to size it right the first time, with margin to spare over your real peak load.
That means one more step before you buy. Sizing rules are not the same from city to city, so the gpm or gallon figure you calculate is a starting point, not a final answer. Call your local sewer or wastewater authority and ask three things: which interceptor type they require, which sizing method they accept, and whether they enforce a minimum size. Many jurisdictions publish a sizing worksheet and an approval form you must submit before you install. A licensed commercial plumber can run the math against your fixture list and meal volume, then match it to the code your inspector will hold you to. Confirming that number up front is far cheaper than replacing a unit after it fails.
Once your interceptor is sized and installed, two related questions come up fast: how often to service it, covered in our guide on how often to pump a grease trap, and what the system is fighting, covered in what is FOG (fats, oils, and grease). Both round out the picture so your interceptor keeps doing its job through every shift.