How to size a grease trap for a commercial kitchen


Kitchen Drain CloseupWhether it’s a small, neighborhood pizza place or a large, institutional cafeteria that runs 24/7, installing the correctly sized grease trap is critical.

Install a unit that’s too small, and you risk overflows and messy back-ups in the kitchen. Install a unit (or units) that are too large, though, and you’ll end up with a different kind of waste – money flushed down the drain.

However, the methods used in many plumbing codes to estimate grease trap size requirements tend to overestimate peak flow, and therefore can overestimate the unit capacity needed. That’s because those codes often assume that all fixtures in a kitchen will simultaneously experience peak flow; that doesn’t happen in the real world.

In the real world:

-       The building’s potable water supply simply can’t support all those simultaneous peak flow rates.

-       All the fixtures, therefore, can’t be used at peak rates at the same time.

-       Plus, only store-and-release fixtures, such as deep, multi-compartment sinks, ever produce peak flows.


A sizing method based on real-world data

With Thermaco’s experience in thousands of commercial kitchens, and real-world data collected from those facilities, we’ve developed a method to safely and accurately determine grease trap size requirements. 

-       Estimate the real peak flow from fixtures that need a grease trap or grease interceptor.

-       Install the next larger size based on the real peak.

There are two scenarios we typically see. Either a grease interceptor is plumbed to a single unit (usually in smaller kitchens), or a grease trap is plumbed in line to handle multiple fixtures (for larger facilities).


Single-fixture plumbing

W-250-IS Undersink 40k

For kitchens where the grease trap only needs to intercept flows from one fixture, such as a multi-compartment sink, simply use the manufacturer’s peak flow rate to determine the grease trap capacity.

Our Trapzilla sizing sheet contains flow rate data for common single-fixture installation scenarios. For single-fixture point-source installations, our Big Dipper sizing sheet (metric version) has common flow rates.

In all cases, the next largest unit should be chosen based on those flow rates. Never install a grease interceptor rated for a lower rate than the peak flow rate you’ve calculated.


Multi-fixture sizing

Where it gets complicated are mult-fixture installations in larger commercial kitchens and food-service establishments.

Here, the key is to account for all the fixtures that could send effluent to the grease trap, and then adjust their combined peak flow rates in a realistic fashion. Because these fixtures will never simultaneously reach peak flows, simply adding up peak flow rates results can result in oversized grease traps.

The equation works as follows:

   Number of the particular type of fixture (ex. floor drains)  

x Manufacturer’s peak flow rate per fixture

x Averaging multiplier

= Peak flow for that fixture


Job Progress Correctional Facility

This calculation should be performed for each type of fixture in the kitchen that needs to be served, and the sum of the peak flows used to calculate the capacity needed in the grease trap.

For examples of this calculation, including averaging multipliers, see our Big Dipper sizing sheet (metric) or Trapzilla sizing sheet.


Managing solids and longer distances

In situations where there are a lot of solids entering the wastewater, we recommend a solids strainer be used to ensure the grease interceptor operates at maximum efficiency.

If the grease trap or point-source grease interceptor is installed more than 6 feet (1.95 m) from the fixture, we also usually recommend a Vented Flow Control Assembly be used to control flow rates at higher pressures.


If you have questions, please contact us. Our experienced technicians and engineers are always willing to consult with you.