How to Size a Grease Trap for Commercial Kitchens
A grease interceptor that is too small does not fail quietly. It slows drains during a rush, sends grease farther into the plumbing system, creates unpleasant cleanup work, and can put an operation on the wrong side of local wastewater requirements. Knowing how to size grease trap equipment before finalizing a kitchen layout protects service speed, plumbing capacity, and maintenance costs.
For most foodservice operations, sizing is not a matter of choosing the largest unit that fits under a sink. The right capacity depends on what enters the drain, how quickly water is discharged, the number and type of connected fixtures, and the rules set by the local authority having jurisdiction. A high-volume BBQ kitchen, a full-service restaurant, and a small bakery may all have different grease-control needs even when their floor plans look similar.
Start With the Type of Grease Interceptor
The term grease trap is commonly used for several different systems. A hydromechanical grease interceptor is typically a compact indoor unit installed near the fixtures it serves. It separates grease by slowing the water flow and is often rated in gallons per minute, or GPM. These units are common for three-compartment sinks, prep sinks, dishwashing areas, and limited groups of fixtures.
A gravity grease interceptor is a larger tank, usually installed outdoors or below grade. It retains wastewater longer, allowing grease, oils, and solids to separate before water leaves the tank. Its capacity is generally expressed in gallons. Larger restaurants, commissaries, food processors, and operations with heavy cooking volume may require this style.
Do not treat these systems as interchangeable. A 50 GPM hydromechanical interceptor is not simply the same thing as a 1,000-gallon gravity interceptor in a smaller package. They use different sizing methods, maintenance schedules, installation requirements, and code criteria.
How to Size Grease Trap Flow Rate
For a hydromechanical unit, the central question is how much wastewater the connected fixtures can release at one time. The interceptor must be able to process that peak discharge without allowing grease to pass through the outlet.
A common field method begins with sink volume. Measure the inside length, width, and usable water depth of each compartment in inches. Multiply those three figures, then divide by 231 to convert cubic inches to gallons. If a three-compartment sink has compartments measuring 18 by 18 by 12 inches, each compartment holds about 16.8 gallons. All three hold roughly 50 gallons.
Next, estimate the drain period. A sink that discharges 50 gallons in one minute produces a flow rate near 50 GPM. If it takes two minutes to drain, the calculated rate is 25 GPM. The interceptor's rated flow capacity must meet or exceed the required rate, subject to the sizing method mandated by the local plumbing code or sewer authority.
That calculation is only a starting point. Connected equipment matters. A pre-rinse sink, mop sink, food prep sink, wok station, floor drain, dishwasher, and automatic grease-producing equipment can change the load substantially. Some jurisdictions prohibit certain fixtures from connecting to a grease interceptor, while others require them. Commercial dishmachines can also create high-temperature discharge and detergent conditions that affect grease separation. Verify the approved connection plan before selecting equipment.
Use the Peak, Not the Average
A kitchen may use only a modest amount of water across an entire day, yet discharge a large volume within several minutes after lunch service. Grease interceptor sizing needs to account for that peak condition. This is especially relevant for restaurants that wash cookware, hotel pans, sheet trays, and smoker racks in batches.
For example, a butcher shop may have lower overall sink use than a busy diner but still create concentrated loads from fat trimming, sausage preparation, and heavy washdown. A BBQ operator may produce grease-rich runoff from pans, utensils, and smoker-related cleanup. The water volume alone does not tell the whole story.
Account for Grease Load and Food Volume
Flow rate establishes the hydraulic demand. Grease and solids establish the separation demand. A kitchen producing fried foods, bacon, burgers, rendered meats, rich sauces, or large volumes of roasted protein will generally need more grease-control capacity than a beverage counter with limited food preparation.
Look at the menu and production process, not just the seating count. Consider the following operational factors:
- Fryers, griddles, charbroilers, smokers, and high-fat cooking processes
- Meat trimming, grinding, mixing, sausage production, and protein-heavy prep
- Pot washing, sheet-pan washing, and frequent equipment cleanup
- Hours of operation, meal-period peaks, and catering or commissary production
Follow Local Code Before Ordering Equipment
There is no single grease trap size that applies across the United States. Plumbing code adoption, municipal sewer regulations, health department requirements, and wastewater utility standards vary by jurisdiction. The local authority may specify a minimum GPM rating, tank capacity, retention time, installation location, venting method, sampling port, or cleaning frequency.
Some municipalities provide a sizing worksheet based on meals served, seating capacity, fixture units, food grinder use, dishwasher flow, or gallons per meal. Others require an engineer, plumber, or approved interceptor manufacturer to document the calculation. If there is a conflict between a general sizing formula and local requirements, local requirements control.
This step should happen early, ideally before concrete is poured or a kitchen line is finalized. An outdoor gravity interceptor requires excavation, access clearance for pumping, and a practical route for service vehicles. An indoor interceptor needs adequate clearance for lid removal and grease removal. A correctly sized unit that cannot be serviced properly will still underperform.
Build in a Practical Capacity Margin
A larger unit is not automatically better. Oversizing can increase upfront cost, floor-space demands, and installation complexity. It may also create long retention conditions that are not appropriate for every hydromechanical application. The goal is an appropriately rated, code-compliant system with enough capacity for the operation's real peak load.
Still, avoid sizing strictly around a temporary low-volume menu if growth is planned. A cafe adding breakfast sandwiches, a restaurant adding fryers, or a meat market expanding prepared-food production can outgrow its original drainage design quickly. If the menu, seating, or production schedule will change within the next few years, discuss the future load with the plumber and local inspector before installation.
The same principle applies when replacing kitchen equipment. A new high-capacity pot sink, faster dishmachine, additional prep sink, or expanded hot line may require a review of the grease interceptor. Commercial equipment upgrades should improve production, not overload the waste system that supports it.
Plan Maintenance Into the Sizing Decision
Grease interceptor performance depends on cleaning. Grease, oils, food solids, and settled material reduce the working volume of the unit. As the retained material builds up, separation performance declines and the risk of downstream blockage rises.
Your service schedule should match actual production, not a calendar date chosen at random. Heavy grease operations may need frequent pumping or cleaning, while lower-volume facilities may require less frequent service. Many jurisdictions apply a 25 percent rule, meaning the interceptor should be serviced before grease and solids occupy more than roughly one-quarter of its liquid depth or capacity. Confirm the threshold required in your area.
Keep cleaning records, disposal manifests when applicable, and service dates available for inspection. Train staff to dry-wipe cookware and food pans before washing, use sink strainers, and keep cooking oil out of drains. No interceptor can compensate for sending buckets of fryer oil, meat scraps, or sauce solids directly into the plumbing.
Get the Final Selection Right
A dependable grease-control system begins with accurate fixture data, realistic peak-flow assumptions, menu-based grease loading, and local code verification. Provide your plumber, engineer, or interceptor supplier with sink dimensions, drain times, equipment schedule, number of meals or production volume, planned fixture connections, and site constraints.
For a commercial kitchen, grease trap sizing is part of the production system, not a last-minute plumbing accessory. Select capacity that matches the way your team actually cooks, washes, and grows, then make service access and routine cleaning part of the operating plan. That approach keeps drains moving and lets the kitchen stay focused on food production.