Grease Trap Size Calculator 🍳

UPC Appendix H Formula (Gravity Interceptors)

Example: 120 seats, restaurant, no dishwasher, 12 hours: 120 × 1.0 × 5 × 2.5 × 1.5 = 2,250 gal (8,517 L). Many authorities set a minimum size (often 500–1,000 gal) and a maximum single unit, so check the local FOG program.

PDI Method (Hydromechanical Grease Traps)

1. Sink volume = length × width × depth of each compartment (in³ ÷ 231 = gallons).
2. Displacement: multiply by 0.75, since a sink is never filled to the rim.
3. Flow rate = volume ÷ drain period (1 minute recommended, 2 minutes allowed).
4. Select the next standard PDI rating: 4, 7, 10, 15, 20, 25, 35, 50, 75 or 100 gpm. Its grease capacity is at least 2 lb per gpm of rating.

Dishwashers are usually not connected to small hydromechanical traps because hot water and detergents carry the grease through. Size the drain from the trap with the Fixture Unit Calculator and check its fall with the Drainage Slope Calculator.

FAQ

How do you calculate grease trap size?

For a gravity interceptor use the UPC formula: meals per peak hour × waste flow per meal × retention time × storage factor, in gallons. For a hydromechanical trap, size by the sink volume × 0.75 drained in one minute, giving the flow rating in gpm.

What size grease trap does a 100-seat restaurant need?

With no dishwasher and 12 hours of operation: 100 × 1.0 × 5 × 2.5 × 1.5 = 1,875 gallons, so a 2,000-gallon gravity interceptor. Local minimums may change this.

What is the difference between a grease trap and a grease interceptor?

A grease trap usually means a small hydromechanical unit rated in gpm, installed near the sinks. A grease interceptor usually means a large gravity tank rated in gallons, installed outside, that also handles dishwashers and floor drains.

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