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Grease interceptor sizing calculator

For estimating only. Your city's FOG ordinance and plumbing official can require a larger minimum (many set 500–1,500 gallon minimums for gravity interceptors; see your city page). Have a licensed plumber or engineer size the final unit.

1. Gravity interceptor: UPC drainage fixture unit (DFU) method

Uniform Plumbing Code Table 1014.3.6 sizes gravity grease interceptors by the total DFUs of the kitchen drain lines connected to it (30-minute retention basis).

Total drainage fixture units (DFUs)

Source: UPC (IAPMO) §1014.3.6 and Table 1014.3.6, up.codes/s/sizing-criteria. Where a connected fixture has no DFU value, the UPC adds its flow rate (gpm) × 30 minutes. Above 2,640 DFU, consult the code/engineer.

2. Gravity interceptor: IPC peak-flow × 30-minute retention

The International Plumbing Code sizes gravity grease interceptors by multiplying the peak drain flow into the interceptor (gpm) by a 30-minute retention time.

Peak drain flow into the interceptor (gallons per minute)

Source: IPC §1003.3.7 (2024 IPC numbering), up.codes.

3. Hydromechanical interceptor (under-sink / in-line): PDI G-101 fixture method

Add each sink compartment. Volume (gal) = L × W × D (inches) ÷ 231; drainage load = volume × 0.75 fill factor; flow rate = load ÷ drainage period (1 or 2 minutes); pick the next larger PDI-rated size.

Drainage period Other fixtures/appliances load (gallons, optional)

Source: Plumbing & Drainage Institute Standard PDI-G101 (rev. 2017), §8.3.2 sizing procedure, PDI-G101 PDF; UPC Table 1014.2.1; grease retention capacity per IPC Table 1003.3.4.1 (2 lb per rated gpm).

Grease interceptor sizing calculator

1. Gravity interceptor: UPC drainage fixture unit (DFU) method

2. Gravity interceptor: IPC peak-flow × 30-minute retention

3. Hydromechanical interceptor (under-sink / in-line): PDI G-101 fixture method

Grease rule changes, once a week