Free Commercial Grease Interceptor Sizing Calculator
Calculate total kitchen fixture drainage flow (GPM), recommended PDI certified trap size, and minimum required grease retention capacity in pounds.
🚰 Kitchen Fixture Specifications
Certified Grease Capacity: ≥ 100 lbs (2× GPM Rating)
• Standard Sizes: 10, 15, 20, 25, 35, 50, 75, or 100 GPM.
• Vented Flow Control Fitting: Must be installed on inlet with air intake to prevent hydraulic surge from bypassing separation baffles.
Code Warning (The 25% Rule): Plumbing inspectors require grease interceptor cleaning before the combined grease layer and settled sludge layer exceed 25% of total liquid depth. Operating past 25% causes FOG bypass into city sewers.
Recommended Tools & Equipment
Tested hardware and components for high reliability
PDI G-101 Sizing Methodology
The Plumbing and Drainage Institute standard PDI G-101 governs the testing and certification of hydromechanical grease interceptors (HGIs). Sizing proceeds through four logical steps:
Calculation Steps
2. Liquid Capacity: Gallons = V_cuin / 231 cu in/gal
3. Total Sink Flow: GPM_sink = (Gallons × Comps × 0.75) / Drain_Mins
4. Total Kitchen Flow: GPM_total = GPM_sink + GPM_dishwasher + GPM_floordrains
5. PDI Selection: Round UP to next standard PDI size (20, 25, 35, 50, 75, 100)
6. Grease Retention: Minimum Capacity (lbs) = 2 × GPM_rating
Hydromechanical (HGI) vs. Gravity (GGI) Interceptors
Indoor compact stainless steel or polyethylene units are Hydromechanical Grease Interceptors (HGIs). They utilize internal baffles, laminar fluid dynamics, and air entrainment to separate grease in a compact footprint. For large banquet facilities or heavy cooking operations, local authorities may mandate an outdoor underground Gravity Grease Interceptor (GGI) with a retention volume of 500 to 1,500+ gallons.
Frequently Asked Questions
Why does the calculation use a 75% fill factor?
Commercial sinks are rarely filled to the absolute brim. Pots, pans, and dish racks displace water, and faucets are shut off well before overflowing. PDI G-101 standardizes on 75% usable volume to represent realistic maximum sink drainage.
Can a commercial dishwasher be connected to a grease trap?
Yes, but high-temperature discharge (140°F-180°F) and chemical sanitizers can emulsify grease, temporarily melting it so it bypasses separation baffles. Ensure the interceptor is adequately sized with a flow restrictor, or plumb the dishwasher to a downstream dedicated interceptor per local code.
What is the purpose of the vented flow control tee?
The flow control tee limits discharge flow to the interceptor certified GPM rating. Its air vent introduces ambient air into the stream, aerating the mixture. The rising air bubbles help float microscopic grease globules to the water surface.