Grease Trap Sizing Calculator

Frequently Asked Questions

Which sizing method should I use?

Indoor under-sink or floor-mounted hydromechanical traps are sized by flow rate from your fixtures (the fixture-capacity method below). Large outdoor in-ground gravity interceptors are sized by volume using the meals-served method. If you're not sure which you have: a steel box near the sinks is hydromechanical; a concrete tank outside with manhole covers is a gravity interceptor.

Is this calculator a substitute for my local plumbing code?

No. It implements the standard industry methods (PDI-style fixture capacity and the UPC-style meals-served formula), which most codes are based on — but your city's plumbing authority has the final word, and some jurisdictions set minimum interceptor sizes (750 or 1,000 gallons is common for new restaurants). Use this to sanity-check a contractor's proposal, then confirm with your local code office.

What happens if my grease trap is undersized?

It hits the 25% FOG threshold quickly, so you pay for cleanings far more often — and between cleanings it passes grease to the sewer, which is what FOG violations and fines are written for. If you're cleaning monthly and still hitting 25%, size is usually the problem. See our frequency benchmarks.

What size do most restaurants end up with?

Indoor hydromechanical traps are commonly rated 20–100 GPM (40–200 lb grease capacity). Outdoor gravity interceptors for full-service restaurants typically land between 750 and 2,000 gallons. Your result depends on fixtures and meal volume — run both methods if you're between options.