How Grease Trap Sizing Works

The Grease Trap Size Calculator helps commercial kitchen designers, plumbing engineers, and restaurant owners determine the minimum required capacity of a grease interceptor (grease trap) based on the flow rate of connected fixtures and local retention requirements. Proper grease trap sizing is essential for compliance with local plumbing codes, preventing sewer blockages, and reducing environmental impact.

How It Works

Calculation Method (PDI G-101 Adaptation)

The calculator uses the Plumbing and Drainage Institute (PDI) G-101 standard method:

Formula:

Required Capacity (Gallons) = Total Flow Rate (GPM) × Retention Time (Minutes) × Load Factor

Step 1: Aggregate Fixture Flow Rates

  • Add all fixtures that drain into the grease trap (3-compartment sinks, pre-rinse sinks, dishwashers, wok stations, etc.)
  • Each fixture has a flow rate measured in Gallons Per Minute (GPM)

Step 2: Apply Parameters

  • Retention Time: The minimum time wastewater must remain in the grease trap to allow grease to separate and float. Typically 15-30 minutes as per local code.
  • Load Factor (Safety Margin): A multiplier (typically 1.5-2.0) to account for peak flow surges and ensure adequate capacity.

Step 3: Recommend Commercial Size
The calculated capacity is matched to the nearest standard commercial grease trap size:

Standard Sizes (Gallons): 20, 35, 50, 75, 100, 150, 200, 250, 300, 500, 750, 1000

Always select the next size up if your calculated value falls between standard sizes.

Calculation Method (PDI G-101 Adaptation)

The calculator uses the Plumbing and Drainage Institute (PDI) G-101 standard method:

Formula:

Required Capacity (Gallons) = Total Flow Rate (GPM) × Retention Time (Minutes) × Load Factor

Step 1: Aggregate Fixture Flow Rates

  • Add all fixtures that drain into the grease trap (3-compartment sinks, pre-rinse sinks, dishwashers, wok stations, etc.)
  • Each fixture has a flow rate measured in Gallons Per Minute (GPM)

Step 2: Apply Parameters

  • Retention Time: The minimum time wastewater must remain in the grease trap to allow grease to separate and float. Typically 15-30 minutes as per local code.
  • Load Factor (Safety Margin): A multiplier (typically 1.5-2.0) to account for peak flow surges and ensure adequate capacity.

Step 3: Recommend Commercial Size
The calculated capacity is matched to the nearest standard commercial grease trap size:

Standard Sizes (Gallons): 20, 35, 50, 75, 100, 150, 200, 250, 300, 500, 750, 1000

Always select the next size up if your calculated value falls between standard sizes.

ParameterDescriptionDefaultTypical Range
Load FactorSafety margin multiplier for peak flow1.51.0 – 2.5
Retention TimeMinimum time for grease separation (minutes)1510 – 30
Fixture Flow RateGPM for each connected fixtureVaries0 – 50 GPM each
Fixture TypeTypical GPM
3-Compartment Sink7.5 GPM
Pre-Rinse Sink5.0 GPM
Dishwasher (Commercial)15-25 GPM
Wok Station25-40 GPM
Floor Drain (Kitchen)3-5 GPM

FAQ

Q: What size grease trap do I need for a small restaurant?
A: A small restaurant with a 3-compartment sink and dishwasher typically needs a 50-75 gallon grease trap. Use the calculator with your specific fixture GPM values.

Q: What happens if my grease trap is too small?
A: An undersized grease trap will require more frequent cleaning, may overflow, and can cause sewer blockages leading to fines and environmental violations.

Q: How often should a grease trap be cleaned?
A: Typically every 30-90 days depending on usage. The trap should be cleaned when 25% of the capacity is filled with grease and solids.

References

Note: This calculator provides engineering estimates based on standard practices. Final sizing should be verified by a licensed plumbing engineer and approved by local authorities.