Show or Hide

Drainage Products Support - Helpful Tools and Resources

Grease Interceptor Sizing

General Procedures:

To Determine the Flow Rate of Each Sink:

  1. Calculate the capacity of the sink in cubic inches:
    ___(length) x ___(width) x ___(depth) = ___cu. in.
  2. Convert the capacity from cubic inches to gallons per minute (GPM):
    ___ cu. in. / 231 = ___ GPM.
  3. Adjust for displacement:
    ___ GPM x 0.75 = ___ GPM.
  4. Result is the flow rate required to drain the sink in one minute.*
*Note: If drain down time is not critical, an interceptor with a lesser flow rate up to 1/2 the calculated flow rate may be specified.


Three compartment pot sink, each compartment 12" x 12" x 15"

  1. 12" x 12" x 15" = 2160 cu. in. x 3 comp. = 6480 cu. in.
  2. 6480 cu. in. / 231 = 28 GPM.
  3. 28 GPM x .75 = 21 GPM.

A 20 GPM interceptor would permit the sink to drain in slightly more than one minute.

*Discharge from spray hoods is determined by the flow rate of the hood.
*It is not recommended to pass commercial dishwashers through an interceptor.

Sizing For Multiple Fixtures

  1. Determine the flow rate for each fixture to be serviced by the interceptor.
  2. Add together 100% of the largest flow rate, 50% of the second largest, and 25% of all others.
  3. Result is the recommended flow rate of the interceptor.


  1. Fixture A: 35 GPM Flow Rate
    Fixture B: 26 GPM Flow Rate
    Fixture C: 18 GPM Flow Rate
    Fixture D: 12 GPM Flow Rate
  2. 35 GPM (A) x 100% = 35 GPM
    26 GPM (B) x 50% = 13 GPM
    30 GPM (C + D) x 25% = 7.5 GPM
    Total Flow Rate :55.5 GPM

A 50 GPM interceptor is recommended for this installation.

Typical Interceptor Installations:

Get Adobe Acrobat

Please Note: Watts literature requires Adobe Acrobat Reader to view.

Featured Item

Grease Interceptor Sizing
Find out the general procedures, sizing for multiple fixtures and typical interceptor installations.

For more info, click here.

Additional Information

Specification Drainage Products Catalog and Price List

Get your copy of the latest Watts Drainage Products catalogue. Includes information on Watts' full line of Specification Drainage products.

For more info, click here.