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.
About this calculator
Grease traps prevent fats, oils, and grease (FOG) from entering municipal sewer systems. Sizing is based on the total flow load from all fixtures plus the volume of grease generated per meal service. The formula is: TrapSize = max(15, ((sinks × 15) + (dishwashers × 25) + (meals × 0.75 × kitchenTypeFactor)) × retentionTime). Each sink contributes 15 GPM of flow, each dishwasher 25 GPM, and each meal adds 0.75 gallons adjusted by kitchen type (e.g., fast food vs. full-service). Retention time—typically 1 or 1.5—reflects how long water must sit for grease to separate. The minimum trap size is 15 gallons, as required by most plumbing codes. Selecting an undersized trap leads to FOG blockages and regulatory violations.
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).