What size trap do you need?
A quick estimate. Your matched installer confirms the final size to local FOG code.
How grease traps get sized
Two paths exist. A smaller kitchen with a light grease load can sometimes use an indoor hydromechanical grease trap, a compact unit rated in gallons per minute that sits under or near the sink line. A busier kitchen needs a below-ground gravity interceptor, a large in-ground tank rated in gallons that gives fats, oils and grease time to separate before the water moves on.
The real calculation is not a guess. An installer counts your drainage fixture units (every sink, dishwasher, floor drain and prep station that carries grease), applies your city's FOG formula, and sizes the unit to pass plan-check and the sewer district's review. Meals per day, whether you run a dish machine, and your menu all move the number. That is why this tool gives a range, not a single answer, and why the final spec has to come from the pro who signs the permit.
One more thing worth knowing up front: many local FOG programs set a minimum interceptor size for any food-service business, commonly around 750 gallons, regardless of how small the kitchen is. Whether that applies to you depends on your jurisdiction, which is exactly the kind of thing a vetted local installer settles before you spend a dollar.