The volume is easy, the minute is the argument
Every flow-based interceptor sizing method starts the same way: add up the volume of everything that drains through the unit, decide how quickly it can empty, and divide. The volume part is a tape measure. A twenty-four by twenty-four by fourteen inch compartment filled to three quarters is about 26 gallons, so a three-compartment sink is about 78. Add a prep sink, a dish machine tank and a mop sink and you are near 115 gallons of connected fixture.
Then comes the divisor. At one minute that is 115 GPM. At two minutes it is 57. Nothing physical changed, and the size of the unit you are told to buy just halved. That is why the drainage period is an input here rather than a constant, and why flow control devices exist: a vented restrictor caps what can physically reach the interceptor, which turns the argument into a specification.
Why an oversized interceptor is not automatically better
The instinct is to buy the biggest unit that fits, on the reasoning that more capacity means less pumping. Two things push back. A unit sized far above its flow does not develop the velocity it was designed around, and separation performance is not simply proportional to volume. And service frequency is often set by rule at a fixed interval or at a fixed fraction of capacity, so you may end up paying to pump a large half-empty tank on the same schedule you would have pumped a small full one.
The pumping arithmetic on this page shows the other side of it. If the interval doubles and the service cost is the same per visit, the annual cost roughly halves — but only up to whatever fixed minimum interval your authority imposes, at which point the extra capacity buys nothing.
What actually goes down the drain
The accumulation rate is the softest number in the whole calculation. Grease per cover depends on the menu in a way that no table captures: a fried chicken operation and a sandwich shop serving the same number of people are not comparable. The only reliable source is your own pumper, who knows what came out of your unit last time and how long it took to get there.
What you can control is what goes in. Dry-wiping pans before they hit the sink, scraping into the bin rather than the disposer, and keeping fryer oil out of the drain entirely all show up directly in the interval. That is not a code point, it is just arithmetic on the accumulation rate.
Where this sits in the rest of the plumbing
The flow figure this page produces is the same kind of number the drain slope calculator and the pipe volume calculator work with, and the connected-fixture thinking is the same as in the household wastewater flow calculator, at a much smaller scale. On the hot side, the same sink that sets the interceptor size also sets the hot water peak — the commercial kitchen hot water demand calculator uses the same compartment dimensions for that. The annual pumping cost belongs in the fixed costs on the cafe and restaurant profit calculator.
Questions people ask
What size grease trap do I need for a restaurant?
It depends on which sizing method your jurisdiction uses, and there are several in circulation that give materially different answers for the same kitchen. Some size on fixture drainage flow, as this page does. Some size on the number of meals served. Some use drainage fixture units. Before you calculate anything, find out which method the plan reviewer will apply, because arriving with a correctly performed calculation from the wrong method is the same as arriving with nothing.
Why does the drainage period change the answer so much?
Because flow is volume divided by time, and the volume is fixed. The same 115 gallons of connected fixture gives 115 GPM at one minute and 38 at three. The unit is then sized on that flow, so the drainage period propagates straight through to the tank you buy. This is exactly what a vented flow control device is for: it makes the drainage period a property of the installed plumbing rather than an assumption, and some jurisdictions require one for that reason.
How often does a grease interceptor need pumping?
Two separate answers, and the stricter one wins. There is the physical answer, which is when accumulation reaches whatever fraction of capacity triggers service — this page estimates that interval from your cover count and accumulation rate. And there is the regulatory answer, which in many places is a fixed maximum interval regardless of how full the unit is, with a manifest to prove each visit happened. Ask your sewer authority for the rule before relying on the arithmetic.
Can I put a small unit under the sink instead of a tank outside?
Sometimes, and it is entirely a jurisdiction question. Whether a hydromechanical interceptor is accepted, at what size, for what kind of operation, and whether it can serve the whole kitchen or only some fixtures, varies between places and between editions of the code. The physical trade is that a small unit under the sink is easy to install and needs servicing far more often. Get the answer from the plan reviewer before the plumbing is roughed in, not after.
Does a garbage disposer affect interceptor sizing?
It affects everything downstream of it, and many jurisdictions restrict or prohibit them in commercial kitchens for exactly that reason. Ground food solids load the interceptor with material that is not grease, filling the solids compartment faster and shortening the interval, and they carry through into the lateral. If you have one, the accumulation rate you use on this page should reflect what your pumper actually removes rather than a grease-only estimate.