Commercial Kitchen Hood CFM Calculator
Exhaust CFM for a kitchen hood.
Special
Grease Trap Size Calculator
Size a hydromechanical grease trap for a commercial kitchen sink: enter the compartment dimensions and the calculator works out the drain flow rate in gallons per minute and the matching interceptor rating.
How it works
- Measure one compartment’s inside length, width and depth in inches and enter the number of compartments.
- Enter how many identical sinks drain to the trap, and any other fixtures’ flow in gpm.
- Keep the 75% fill factor and one-minute drain period unless your code specifies otherwise.
- Read the design flow rate and the interceptor rating to look for.
Formula
Sink capacity (gal) = L × W × D ÷ 231 × compartments × sinks Design flow (gpm) = capacity × fill factor ÷ drain minutes + other fixtures Interceptor = next standard rating (4, 7, 10, 15, 20, 25, 35, 50, 75, 100 gpm) Grease capacity ≈ 2 lb per gpm of rating
This is the Plumbing and Drainage Institute (PDI G-101) method for hydromechanical grease interceptors: there are 231 cubic inches in a U.S. gallon, sinks are assumed 75% full, and the full sink drains in one minute. A two-minute drain period halves the flow and is used where a flow control restricts drainage. The gravity interceptor line is only a rough 30-minute retention reference.
Example
A café with one three-compartment pot sink (each bowl 24 × 20 × 14 in) and a 4 gpm prep sink draining to the same interceptor.
Inputs
- Compartment length
- 24 in
- Compartment width
- 20 in
- Compartment depth
- 14 in
- Compartments
- 3
- Identical sinks
- 1
- Fill factor
- 75 %
- Drain period
- 1 minute (standard)
- Other connected fixtures
- 4 gpm
Result
Hydromechanical interceptor75 gpm ratingDesign flow 69.5 gpm · grease capacity ≈ 150 lb
Design flow69.5 gpm
Sink capacity87.3 gal
Water drained (at fill)65.5 gal
Hydromechanical vs. gravity interceptors
Hydromechanical grease interceptors (the “grease traps” under or beside sinks) are sized by flow rate and must have a flow control fitting. Gravity grease interceptors are large in-ground tanks sized by volume and retention time, and many municipalities require them for full-service restaurants.
Which type you need, where it goes and how often it is pumped are set by your local plumbing code and sewer utility. Use this estimate to understand the flow involved before talking to your plumber or engineer.
Frequently asked questions
What size grease trap do I need for a 3-compartment sink?
With 18 × 18 × 12 in. bowls, the sink holds about 50 gallons; at 75% fill drained in one minute that is about 38 gpm, so a 50 gpm interceptor under the PDI method.
How do I convert sink size to gallons?
Multiply length × width × depth in inches and divide by 231. An 18 × 18 × 12 in. bowl holds about 16.8 gallons.
How much grease does an interceptor hold?
PDI-rated hydromechanical interceptors must hold at least twice their flow rating in pounds of grease — a 20 gpm unit holds about 40 lb.
Is this approved for permits?
No. Local codes and pretreatment programs have their own rules. This is a planning estimate to be confirmed by a plumbing engineer and the local authority.