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Grease Trap Sizing Information

What size is right for you?

It is critical that a grease interceptor of sufficient size is installed in commercial kitchens. If the grease trap is too small, back flows and blockages can occur. The process of estimating the required capacity for a grease trap is called sizing or specifying.

Generally, a plumber or engineer determines the correct size based on the fixtures and layout of the kitchen, and any specific regulations that might apply.

Thermaco recommends a sizing method based on real world data and real world conditions. The company’s products are installed in thousands of commercial kitchens around the world and this sizing method is based on that experience.

Grease trap sizing is based on peak wastewater flow. There are two common scenarios:

1. Grease traps or grease interceptors serving a single fixture (such as a three-compartment sink), or

2. Traps serving multiple fixtures, including sinks, kitchen drains and other fixtures.

1. Single fixture configurations

For single fixture configurations, sizing is relatively simple. Use the following data sheets to calculate the maximum potential flow rate based on the dimensions of the fixture:

After determining the peak flow rate, use the next largest size for the Trapzilla or Big Dipper system in question.

2. Multi-fixture configurations

For kitchens with multiple fixtures, often a grease trap will be plumbed to a wastewater line that serves all the fixtures.

Because multiple fixtures never operate at peak capacity simultaneously in the real world, it would be a mistake to simply add up the individual peak flow rates for each fixture and then size based on that.

Thermaco has developed a method that lets you develop a realistic peak flow number on which to base grease trap sizing. For each fixture, use the manufacturer’s peak flow and then apply the Thermaco multiplier to it.

The sum of all the adjusted peak flows should be used to size the grease trap.

Dealing with solids

If there are a lot of solids flowing into the grease trap (ex. rice, coleslaw, etc.) Thermaco recommend a solids strainer or separator be installed between the effluent source and the grease trap. Otherwise, solids can interfere with the proper operation of a grease trap.

Thermaco offers a number of solutions for food solids, from point-source strainers to Trapzilla Solids Separators.


In addition to peak flow rates, it’s important that you consider the distance kitchen wastewater will travel from its source (such as a sink or drain) to the grease trap. If that distance is greater than six feet (1.95 meters), we recommend a Vented Flow Control Assembly to control flow rates at higher pressures.