Grease Trap Size Calculator
Determine the minimum grease trap capacity (in gallons) required for your commercial kitchen or food service facility based on drainage flow rate, retention time, and connected fixture units.
Total drainage fixture units connected to the grease trap (sinks, dishwashers, floor drains, etc.)
Typical flow rate per fixture unit. Standard value is 2 GPM per DFU (PDI G101).
Time wastewater must remain in the trap for grease separation. Typically 24–30 minutes (PDI G101 uses 30 min).
Percentage of trap volume reserved for accumulated grease. Typically 25% per PDI G101.
Formula
Based on PDI Standard G101 (Plumbing & Drainage Institute):
- Total Flow Rate (GPM) = Fixture Units (DFU) × Flow Rate per Fixture (GPM)
- Working Volume (gal) = Total Flow Rate (GPM) × Retention Time (min)
- Total Capacity (gal) = Working Volume ÷ (1 − Grease Storage Factor)
- Recommended Size = Next standard commercial trap size ≥ Total Capacity
Example: 4 DFU × 2 GPM = 8 GPM → 8 × 30 min = 240 gal working volume → 240 ÷ 0.75 = 320 gal → Recommend 500-gallon trap.
Assumptions & References
- PDI G101: Standard for Hydromechanical Grease Interceptors — the primary sizing standard used in the US.
- Default flow rate of 2 GPM per DFU is per PDI G101 and most local plumbing codes.
- Default retention time of 30 minutes is the PDI G101 standard; some jurisdictions require 24 minutes.
- Default grease storage factor of 25% is per PDI G101; EPA recommends pumping when 25% capacity is reached.
- Standard commercial grease trap sizes: 35, 50, 75, 100, 150, 200, 250, 300, 500, 750, 1000+ gallons.
- Pumping frequency estimate assumes 1% grease accumulation rate and an 8-hour operating day.
- This calculator covers hydromechanical (passive) grease traps. Large facilities may require gravity grease interceptors sized differently per ASME A112.14.3.
- Always consult local municipal codes (FOG — Fats, Oils & Grease ordinances) and a licensed plumber or engineer before installation.
- References: PDI G101, ASME A112.14.3, EPA Grease Management Guidelines, IPC Section 1003.