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: September 2026
Formula below · 2 sources (epa.gov, Wikipedia) · Updated Sep 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 gives a planning estimate from your fixtures and meal count: Capacity (gallons) = max(15, (sinks × 15 + dishwashers × 25 + meals per day × 0.75 × kitchen factor) × grease-load factor). It is a rule of thumb, not a code method; the code sizing described below and your local authority govern the final size.
How code sizing works
Hydromechanical (under-sink) grease interceptors are rated by flow: add the drainage rate of every fixture on the line (3-compartment sink 20-45 GPM, dishwasher 4-14 GPM depending on machine size, wok range 15-25 GPM, pre-rinse spray 5-10 GPM) and choose a unit rated at or above that GPM. Gravity grease interceptors are sized by volume, either from the code's fixture-unit table or from a meals-based formula (meals served in the peak hour × gallons of waste per meal × retention time of about 2.5 hours × a storage factor for operating hours). For a busy full-service kitchen those methods can give a considerably larger volume than this quick estimate, so use them for permit design.
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, is a Full Service Restaurant (kitchen factor 1.5), and has High Grease load (factor 1.5). Step 1: sink load = 3 × 15 = 45. Step 2: dishwasher load = 1 × 25 = 25. Step 3: meal load = 200 × 0.75 × 1.5 = 225. Step 4: sum = 45 + 25 + 225 = 295. Step 5: multiply by the grease-load factor = 295 × 1.5 = 442.5 gallons. Step 6: apply the minimum: max(15, 442.5) = 442.5 gallons. You would specify at least the next standard size up (for example 500 gallons), then confirm with the code method your local authority uses.
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 how long wastewater stays inside the interceptor so grease can float to the surface and be held back. Code sizing of gravity interceptors typically assumes around 30 minutes of retention at peak flow, and the meals-based method uses about 2.5 hours for a commercial kitchen. In this calculator the 'Required Retention Time' setting is applied as a simple multiplier (1.0 standard, 1.5 high grease, 2.0 heavy grease) on the estimated volume, so choose a higher factor for fryer-heavy or high-volume menus. Too little retention lets grease carry over 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.