Grease Trap Sizing Calculator
Determine the minimum grease trap size (in gallons) for a commercial kitchen based on sinks, dishwashers, meal volume, kitchen type, and required retention time. Use it when designing or upgrading a kitchen drainage system to meet code.
Last updated: May 2026
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About this calculator
A commercial-kitchen grease trap intercepts fats, oils, and grease (FOG) from wastewater before they enter the municipal sewer, where they solidify and cause blockages. Sizing it wrong is expensive: too small and it saturates in weeks (constant pumping cost, code violation risk); too big and the water sits long enough to break down anaerobically (odor complaints and biological load in the trap itself). This calculator sizes the trap for a real kitchen using the UPC (Uniform Plumbing Code) formula — the reference most jurisdictions use.
The UPC grease trap sizing formula
GPM (gallons per minute) capacity = (drainage rate × fixtures) / retention time. Standard retention time is 30 minutes. Fixture drainage rates from the UPC: 3-compartment sink at 20-45 GPM, dishwasher at 4-14 GPM depending on machine size, wok range at 15-25 GPM, pre-rinse spray at 5-10 GPM. Add all fixtures that share the drain line, multiply by retention time, and you have the minimum tank volume in gallons.
Grease trap size chart (commercial kitchens)
Common sizings by kitchen type: quick-service (single sink, dishwasher, no cooking) 20-50 GPM / 30-100 gal trap. Casual restaurant (dual sink, dishwasher, small line) 50-100 GPM / 250-500 gal. Full-service restaurant (3-comp sink, dishwasher, hood-line cooking) 100-250 GPM / 500-1,500 gal. High-volume institutional or hotel kitchen 250-500 GPM / 1,500-3,000 gal. Above 500 GPM a passive grease trap becomes impractical and code usually requires a gravity grease interceptor (GGI) sized by the wastewater engineer.
Passive trap vs gravity interceptor
Under-sink passive traps handle up to ~50 GPM and are the norm for small kitchens. Above that, code requires an in-ground gravity grease interceptor sized by tank volume rather than GPM — typically 500 to 3,000 gallons for a full kitchen. GGIs need pump-out access, a sample port, and a two- or three-chamber design; the passive under-sink unit is far simpler but hits its limit fast.
Pump-out frequency
A properly sized trap should reach 25% capacity (the point where efficiency drops sharply) every 30-90 days for a normally-loaded kitchen. Monthly service is the safe default for most food-service establishments; some jurisdictions mandate quarterly at minimum. Grease-hauling costs run $200-800 per pump-out depending on tank size and region — a real line item to budget for.
How to use
Suppose a restaurant has 3 sinks, 1 dishwasher, serves 200 meals per day, has a kitchenType factor of 1.25 (full-service), and a retention time of 1.5. Step 1: Sink load = 3 × 15 = 45. Step 2: Dishwasher load = 1 × 25 = 25. Step 3: Meal load = 200 × 0.75 × 1.25 = 187.5. Step 4: Sum = 45 + 25 + 187.5 = 257.5. Step 5: Multiply by retention time = 257.5 × 1.5 = 386.25 gallons. Step 6: Apply minimum: max(15, 386.25) = 386.25 gallons. You would specify a grease trap of at least 400 gallons.
Frequently asked questions
How do I determine the correct grease trap size for my restaurant?
Grease trap sizing depends on the number of sinks and dishwashers, daily meal count, kitchen type, and the required retention time specified by your local plumbing code. Use this calculator to combine all those inputs into a single minimum gallon capacity. Always round up to the next commercially available trap size. Local authorities may impose additional minimums, so confirm with your inspector before purchasing.
What is retention time in grease trap sizing and why does it matter?
Retention time is the number of hours wastewater must remain in the trap for grease to rise to the surface and be retained. Most codes require 1 to 1.5 hours. A longer retention time directly increases the required trap volume, ensuring that even peak-hour flows are adequately treated. Using an insufficient retention time leads to carry-over of grease into the sewer, resulting in fines and clogs.
When is a grease trap required by plumbing code?
Most commercial kitchens—including restaurants, cafeterias, hotels, and food-processing facilities—are required by local plumbing and FOG ordinances to install a grease trap or interceptor. The threshold is typically any establishment that discharges grease-laden wastewater into a municipal sewer. Residential kitchens are generally exempt, but catering businesses operating from home may fall under commercial rules. Always verify requirements with your local authority having jurisdiction (AHJ).
What size grease trap do I need for a restaurant?
A full-service restaurant with a 3-compartment sink, dishwasher, and cooking line typically needs 100-250 GPM capacity, corresponding to a 500-1,500 gallon trap. Quick-service (single sink, dishwasher only) needs 20-50 GPM. Use the UPC formula in the calculator above with your specific fixture list for the exact figure.
What is the retention time for a grease trap?
Standard code is 30 minutes minimum. This is the time water must sit in the trap for grease to separate to the top and settle out. Faster flow-through means grease bypasses the trap; longer sitting time can lead to anaerobic breakdown and odor complaints.