Restaurant Grease Trap Sizing Calculator
Determine peak wastewater flow rates in GPM, grease holding capacity (lbs), and pumping service intervals per PDI G-101 and Uniform Plumbing Code (UPC).
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How Commercial Grease Traps Are Sized
Grease interceptors are sized by certified peak flow rate (Gallons Per Minute, GPM) using standards defined by the Plumbing and Drainage Institute (PDI G-101) and Uniform Plumbing Code (UPC):
Fixture Volume (Gallons) = (Length × Width × Depth × Compartments) / 231 cu in/gal
Actual Fill Volume = Fixture Volume × 0.75 (Displacement factor)
Sink Flow Rate = Actual Fill Volume / Drain-Down Minutes (1 or 2 min)
Minimum Grease Retention (lbs) = 2 × Certified GPM Flow Rate
The 25% Rule: Municipal sewer ordinances require grease traps to be pumped out whenever the combined layer of floating fats, oils, and grease (FOG) plus settled bottom solids occupies 25% of the total liquid depth. Exceeding 25% allows emulsified grease to bypass into the city sewer line.
Frequently Asked Questions
What is the difference between an indoor hydromechanical grease interceptor and an outdoor gravity interceptor?
A point-of-use hydromechanical grease trap (HGI) sits inside the kitchen near the sinks, typically handling 20 to 50 GPM (40 to 100 lbs grease), requiring manual or pump-out cleaning every 1 to 4 weeks. An outdoor gravity grease interceptor (GGI) is a large concrete or fiberglass underground vault (500 to 2,000+ gallons) pumped by a vacuum truck every 3 months.
Why is a flow control fitting required on a grease trap?
PDI-certified grease interceptors must have an air-vented flow control device installed on the inlet line. The flow restrictor prevents sink discharge from exceeding the rated GPM capacity of the trap, ensuring water slows down sufficiently for grease to separate and float to the surface.
Can commercial dishwashers drain directly into a point-of-use grease trap?
Most local plumbing codes prohibit connecting high-temperature dishwashers (which discharge 140°F to 180°F wash water and heavy detergents) into small indoor grease traps. The intense heat liquefies and emulsifies collected grease, carrying it straight out of the trap and into the building plumbing where it solidifies downstream.