How to size a grease interceptor
Fats, oils, and grease congeal inside drain lines, cause backups and odors, and overload the treatment plant downstream. A grease interceptor holds kitchen drainage long enough for grease to float and solids to settle, so an undersized unit simply passes grease through. This calculator works from sink compartment dimensions, the way UPC and IPC sizing practice does, and reports both a flow class and a tank volume.
The sequence is straightforward: compartment volume in gallons, multiplied by the 75 percent usable factor and the number of compartments, divided by the drainage period (normally one minute) to get gpm. That flow is rounded up to the next PDI G101 class, and grease retention capacity is taken as 2 lb per rated gpm. A three-compartment sink at 24 by 24 by 12 inches gives about 67 gpm, which rounds to a 75 GPM class unit with 150 lb of grease capacity.
For in-ground gravity interceptors, the tank is usually sized on retention time instead of a flow class, so the calculator also multiplies the design flow by the retention minutes you enter. Required sizes, sampling boxes, and pump-out intervals are set by the local plumbing authority and the sewer utility, so confirm the adopted code edition and any city amendments before ordering, and have a plumbing engineer review larger installations.
Frequently asked questions
Compartments are rarely filled to the rim, and dishes and cookware take up part of the volume. The Uniform Plumbing Code sizing practice treats 75 percent of the compartment volume as the usable drainage load.
Under-sink hydromechanical interceptors are commonly sized on a 2 to 5 minute basis, while in-ground gravity grease interceptors are usually sized for about 30 minutes of retention. Your local plumbing authority may require a specific value, so confirm before ordering.