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How accurate is Grease Trap Size Calculator?

This tool uses standard, well-tested formulas and algorithms to ensure reliable and accurate results.

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Yes. You can freely use this tool for personal, business, and commercial projects without restrictions.

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Grease Trap Size Calculator

Determine the correct GPM rating for your commercial kitchen interceptor. PDI-G101 compliant sizing based on sink dimensions and drainage load.

Fixture Specifications

Length (inches)

Width (inches)

Depth (inches)

Compartments

Drainage Time

Recommended Size

GPM

Total Volume

0 gal

All fixtures combined

Peak Flow Rate

0 GPM

Multi-fixture adjusted

Note: A 75% capacity factor is applied to account for displacement (dishes, pots, utensils) and avoid overflow, as per standard PDI-G101 plumbing practice. Always select the next larger standard GPM rating if your result falls between sizes.

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How is Grease Trap Size Calculated? (PDI-G101)

Grease trap sizing is based on the peak volume of water that could drain through the unit simultaneously. The recognised industry standard for Hydromechanical Grease Interceptors (HGI) is PDI-G101, which our calculator follows exactly.

Enter your sink dimensions (length × width × depth in inches), the number of compartments, and the expected drainage time. Our calculator applies the displacement factor and multi-fixture logic automatically.

The Sizing Formula

1 Capacity (gal) = (L × W × D × Comp) ÷ 231

2 Actual Load = Capacity × 0.75

3 GPM = Actual Load ÷ Drainage Time

Multiple Fixtures

When multiple fixtures drain into one trap, they rarely all peak at once. The standard engineering approach: 100% of the largest fixture, 50% of the second, and 25% of all remaining fixtures are summed for the design GPM.

1-Minute vs 2-Minute Drainage

A 1-minute drainage time is the conservative standard for high-volume kitchens — it results in a larger GPM requirement. A 2-minute time is allowed for smaller fixtures by some codes and yields a smaller interceptor.

Local Compliance

This tool follows PDI-G101 logic, but local plumbing codes (IPC, UPC) and your local Authority Having Jurisdiction (AHJ) may have additional requirements. Always verify your final selection with a licensed plumber before installation.

Why Does My Kitchen Need a Grease Trap?

Fats, Oils, and Grease (FOG) solidify as they cool, causing blockages in municipal sewer lines and triggering costly sanitary sewer overflows (SSOs). Most jurisdictions legally require commercial kitchens — restaurants, cafeterias, food trucks, and bakeries — to intercept FOG before it reaches the public system. The grease trap rating (GPM) must match the peak discharge flow or the unit will overflow and become non-compliant.

Frequently Asked Questions

GPM stands for Gallons Per Minute. It represents the flow rate capacity of the grease trap — a 20 GPM unit can handle up to 20 gallons of wastewater per minute while separating grease and solids. Undersizing the GPM causes overflow and non-compliance.

A sink is rarely filled to the very brim, and dishes, pots, and utensils displace a significant portion of the water. The PDI-G101 standard accounts for this by using 75% of the calculated sink volume as the actual drainage load.

Many codes require dishwashers to connect to a large external interceptor rather than a small internal trap. High-temperature water and detergents emulsify grease, allowing it to pass through small traps, which defeats the purpose entirely.

The industry standard is the "¼ Rule": pump out when the combined depth of floating grease and settled solids reaches 25% of the trap's total liquid depth. For most restaurants this means every 1–3 months.

"Grease trap" typically refers to small, indoor, under-sink passive devices (this calculator's scope). "Grease interceptor" refers to large, outdoor, in-ground tanks rated in hundreds or thousands of gallons. The terms are often used interchangeably in codes, but the sizing methods differ significantly.

Yes. The sizing logic (volume ÷ 231, × 0.75, ÷ drainage time) and the multi-fixture diversity factors (100% / 50% / 25%) are taken directly from the PDI-G101 standard. However, local codes may impose additional minimums — always confirm with your AHJ.

An undersized trap overflows when peak flow exceeds its rating, sending raw FOG directly into the sewer. This can result in fines, forced closure by health authorities, and liability for sewer blockages downstream. Always round up to the next standard GPM size.

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