A Friday night dinner rush. Packed dining room. Then the three-compartment sink stops draining. Within minutes, greasy water backs up across the prep area floor. The health inspector who shows up Monday morning issues a $2,500 fine and a 48-hour deadline to fix the problem. This nightmare scenario plays out in restaurants across the country every week, and the culprit is almost always the same: an undersized or neglected grease trap that could not handle peak kitchen volume. One Midtown fast casual spot was fitted with a trap nearly 40 percent too small for their sink and dishwasher configuration. They did not realize the issue until their third backup in two months, right before a Saturday night rush. By the time they upgraded, they had spent more on emergency pump-outs and plumbing repairs than the replacement would have cost upfront.
Getting grease trap sizing right the first time saves thousands in emergency plumbing calls, prevents health code violations that can shutter your business, and keeps your kitchen running smoothly during your busiest service hours. This calculator takes the guesswork out of sizing by factoring in your actual fixture count, flow rates, and retention time requirements based on industry standards.
How to Use This Calculator
Start by counting every fixture in your kitchen that drains into the grease waste system. This includes your three-compartment sinks, prep sinks, dishwashers, pre-rinse sprayers, floor drains near cooking equipment, and mop sinks. Enter the dimensions for your main sinks since the calculator uses cubic inches converted to gallons to determine drainage load. For dishwashers, use the GPM rating from the manufacturer specs, typically between 15 and 25 GPM for commercial units. The calculator combines flow-based capacity with sink-based capacity and recommends the larger of the two values, then rounds up to the nearest standard trap size available from manufacturers.
How Grease Trap Sizing Works
Grease trap sizing is not about picking the biggest unit you can afford. An oversized trap creates its own problems. When a trap is too large for your actual output, wastewater sits stagnant instead of flowing through at a steady rate. This stagnation leads to anaerobic decomposition, producing hydrogen sulfide gas that creates a rotten-egg smell strong enough to reach your dining room. Worse, that gas is corrosive and will eat away at the trap material over time.
The goal is matching trap capacity to your peak flow rate, which is the maximum gallons per minute draining into the system when multiple fixtures run simultaneously during your busiest service period. A trap that hums along fine during a slow Tuesday lunch can be completely overwhelmed by a packed Saturday night when every sink, dishwasher, and pre-rinse station runs at full capacity.
The Fixture Capacity Method
The most accurate sizing method calculates the capacity of each fixture and adds them together. Measure your sink compartments in inches, multiply length by width by depth, then divide by 231 to convert cubic inches to gallons. Apply a 0.75 displacement factor since the sink is never completely full when draining. For in-ground interceptors, multiply your total GPM by 30 minutes of retention time, which is the minimum period wastewater must sit in the trap for FOG to separate properly and animal fats to harden.
Grease Trap vs Grease Interceptor: Which One Do You Need?
The terms get used interchangeably, but they refer to different equipment with different applications. A grease trap is a smaller unit, typically ranging from 20 to 100 gallons, designed to handle flow rates under 50 GPM. These units install under sinks or nearby within the kitchen and require more frequent cleaning, often weekly or bi-weekly for high-volume operations.
A grease interceptor is a larger system, starting around 100 gallons and ranging up to 2,000 gallons or more, designed for flow rates exceeding 50 GPM. Interceptors install outside the building, typically buried underground in concrete vaults. They capture FOG from all kitchen drains including floor drains and mop sinks. The larger volume means less frequent pumping, usually monthly to quarterly, but the installation cost is significantly higher due to excavation and concrete work.
Your local jurisdiction determines which type you need. Many cities require in-ground interceptors for any commercial food service establishment regardless of size, while others allow under-sink traps for smaller operations like coffee shops and delis. Check with your health department and wastewater authority before purchasing equipment.
The 25% Rule and Cleaning Frequency
The one-quarter rule is an internationally recognized standard that every restaurant owner needs to understand. Once 25 percent of your grease trap volume is filled with FOG and food solids, the trap is no longer working effectively. At this point, grease starts bypassing the trap and flowing directly into the sewer system where it cools, hardens, and creates blockages. Municipalities take this seriously because restaurant grease is a leading cause of sanitary sewer overflows. San Francisco alone spends $3.5 million annually clearing restaurant grease from its underground sewage system.
A properly sized trap should reach that 25 percent threshold every one to three months, giving you a manageable maintenance schedule. If your trap fills faster, it is undersized for your operation. Many cities mandate cleaning every 90 days regardless of fill level, and some jurisdictions require monthly service. High-volume kitchens frying daily may need bi-weekly pump-outs. Keep detailed service records for at least three years because health inspectors will demand documentation during visits, and missing records can result in violations even if your trap is properly maintained.
Common Grease Trap Sizing Mistakes
Sizing for Tuesday Lunch Instead of Saturday Dinner
Your grease trap does not run on averages. It has to survive your absolute busiest, most chaotic service periods. A trap that handles your typical weekday volume will fail spectacularly when every burner is firing and the dish pit is running full speed on a packed Friday night. Calculate based on peak simultaneous fixture use, not average daily flow.
Ignoring Kitchen Growth and Menu Changes
A trap sized correctly for your opening menu may be overwhelmed within six months if you add fryers, expand brunch service, or introduce more high-fat cooking. Fried chicken, bacon-heavy breakfasts, and sauteed dishes put exponentially more pressure on your grease system than grilled proteins and steamed vegetables. Factor in realistic growth when selecting capacity.
Forgetting Floor Drains and Mop Sinks
Restaurant owners often count sinks and dishwashers while completely overlooking floor drains. When your closing crew mops or hoses down the kitchen, that water carries substantial grease loads straight into the trap. Mop sinks are especially problematic because they dump large volumes quickly. Account for every drainage point that connects to your grease waste line.
Assuming Hot Water Solves Grease Problems
Flushing grease down drains with hot water does not make it disappear. The grease liquefies temporarily, travels further into your plumbing, then cools and solidifies in pipes where it is much harder to reach. You end up with a blockage deeper in the system that costs more to clear. Same goes for enzyme additives and chemical drain cleaners, which can damage your trap and interfere with FOG separation.
What Happens When Grease Traps Fail
The consequences of grease trap failure extend far beyond a backed-up sink. A sewage backup during service means immediate shutdown, lost revenue, and a dining room full of customers watching you deal with a plumbing emergency. The smell alone can empty your restaurant in minutes. One industry survey found that 82 percent of customers lose their appetite when they spot grease or grime anywhere in the dining area.
Financial penalties add up fast. EPA rules require minimum fines of $100 per day for first-time violations, with penalties reaching $1,000 daily for repeat offenders. Some jurisdictions impose fines exceeding $100,000 for chronic non-compliance. In New York City, DEP fines can hit $10,000 per day for overflowing or improperly maintained traps. A steakhouse that ignored slow drains and foul odors for two weeks ended up with a Friday night flood requiring emergency grease trap cleaning at $4,500, plus $2,000 in lost revenue and health violations. Regular maintenance would have cost $350.
Beyond your own business, grease discharges into municipal sewers contribute to fatbergs, massive blockages of congealed fat that clog pipes and cause raw sewage overflows into streets and waterways. When authorities trace these blockages back to your establishment, the liability exposure becomes substantial.
Pro Tips from Commercial Plumbers
Test your drains monthly. Fill a sink with water and time how long it takes to empty. Healthy drains clear in under 30 seconds. If drainage exceeds 60 seconds, you have a developing clog that will only worsen without attention.
Train every kitchen employee. New hires need to know where cooking oil recycling bins are located and understand that pouring fryer grease down any drain is a fireable offense. Staff should recognize warning signs like slow drainage, gurgling pipes, and sewer smells, and know to report them immediately.
Never use bleach, emulsifiers, or enzyme additives. These products harm the natural bacteria that help break down grease in your trap. The only additive approved for most systems is bacteria specifically designed to consume FOG and convert it to water and carbon dioxide.
Schedule pump-outs before you need them. Waiting for obvious problems like slow drains or backups means your trap is already overdue. Establish a regular service schedule based on your volume and stick to it. The cost of routine maintenance is always lower than emergency service calls.
Position traps as close to grease sources as possible. The shorter the distance grease travels before reaching the trap, the less buildup occurs in your pipes. In large kitchens, consider multiple smaller traps at key points rather than one distant interceptor trying to handle the entire system.