Calcady™
Home / Trade / Plumbing / Grease Interceptor Sizing

Grease Interceptor Sizing

Size commercial grease traps and interceptors based on sink compartment volume and required drain time. PDI-G101 compliant plumbing tool.

Sink Configuration

Count
Inches
Inches
Inches
Grease TrapSink to Interceptor Logic (GPM = Vol / Time)

The 75% Fill Rule

PDI (Plumbing and Drainage Institute) sizing standards assume that a sink is rarely filled to the absolute brim. To prevent over-sizing, we calculate based on 75% of the total volume. If you choose a 1-minute drain time, the system discharges faster, requiring a higher GPM rating to properly separate grease before it hits the main sewer.

Recommended Size

50 GPM
Standard PDI-G101 Rating
Calculated Flow Rate
37.9GPM

Required capacity at 1 min drain

Combined Sink Volume50.5 Gallons

Total water capacity of all basins

For estimation purposes only. Always consult a licensed professional before beginning work. Full Trade Safety Notice →
Email LinkText/SMSWhatsApp

What is PDI-G101: Preventing Sewer Blockages?

Grease interceptors (hydro-mechanical grease traps) are mandatory in commercial kitchens to prevent fats, oils, and grease (FOG) from entering the municipal sewer system, where they cool, congeal, and cause massive fatberg blockages. Sizing is a critical exercise in physics: if the interceptor unit is too small, wastewater shoots through at a high velocity. The grease never gets enough 'dwell time' in the baffle plates to cool, float, and separate from the water. Interceptors are sized by calculating the maximum possible flow rate (GPM) dumping out of the sinks.

Mathematical Foundation

Total Compartment Volume
Vgal=L×W×D×C231V_{gal} = \frac{L \times W \times D \times C}{231}
VgalV_{gal}= Total volumetric capacity of all sinks in Gallons.
L,W,DL, W, D= Inside Length, Width, and Depth dimensions of a single bowl in Inches.
CC= Total number of identical compartments/bowls.
231231= Dimensional constant converting cubic inches to U.S. liquid gallons.
Standard Flow Rate Equation (PDI-G101)
GPM=Vgal×0.75Drain Time\text{GPM} = \frac{V_{gal} \times 0.75}{\text{Drain Time}}
GPM\text{GPM}= Required interceptor flow rating in Gallons Per Minute.
0.750.75= Loading factor. It assumes a typical sink is only filled to 75% of its absolute structural brim before dishwashing begins.
Drain Time\text{Drain Time}= Desired clearance duration (1 minute or 2 minutes).

Laws & Principles

  • Standard PDI Ratings: Interceptors are not infinitely customizable. They are manufactured to specific PDI (Plumbing and Drainage Institute) flow ratings, typically 10, 15, 20, 25, 35, 50, 75, and 100 GPM. You must calculate the exact flow rate of your sinks, and then purchase the interceptor rating equal to or immediately larger than your calculation.
  • The 1-Minute vs 2-Minute Rule: Code defines how quickly a sink must drain. A 1-minute drain time is a massive rush of water, requiring a massive, expensive interceptor to slow it down. Permitting a 2-minute drain time cuts the required flow rate exactly in half, letting you use a much smaller interceptor. Always verify with the local AHJ.
  • The 25% Rule for Maintenance: A grease trap only works if there is room for the water to slow down. If uncleaned, the FOG layer thickens until water channels beneath it without separating. PDI standards state an interceptor loses its certified efficiency once grease and solids fill 25% of its wetted operating volume.

Step-by-Step Example Walkthrough

" A plumber is configuring a small commercial diner. The health department mandates a 3-compartment sink setup. Each steel bowl has inside dimensions of 18 by 18 by 12 inches. The inspector requires a 1-minute drain time. "

  1. 1. Calculate volumetric cubic inches: 18 × 18 × 12 = 3,888 cubic inches per bowl.
  2. 2. Multiply by 3 compartments: 3,888 × 3 = 11,664 total cubic inches.
  3. 3. Convert to liquid gallons: 11,664 ÷ 231 = 50.49 total gallons of water.
  4. 4. Apply the 75% operational loading factor: 50.49 × 0.75 = 37.87 actual gallons draining.
  5. 5. Calculate GPM based on 1 minute: 37.87 Gallons ÷ 1 Minute = 37.87 GPM.
Final Result: The sink cluster produces a peak surge flow of 37.87 GPM. Because interceptors are manufactured in discrete sizes, the plumber must round up to the next standard rating: a 50 GPM grease interceptor.

Quick Answer: How are Grease Interceptors Sized?

Commercial grease interceptors are sized by determining the total volumetric gallon capacity of connected sink compartments, multiplying by a 75% operational fill factor, and dividing by the required drain time to establish a Gallons Per Minute (GPM) flow rate. The resulting GPM dictates the required PDI-certified interceptor size to ensure enough physical dwell time for fats, oils, and grease to properly separate from the wastewater stream.

Core Sizing Rules & Equations

PDI-G101 Volume Constraints

Total_Cubic_Inches = Length × Width × Depth × Divider_Count
Max_Gallons = Total_Cubic_Inches / 231
Surge_GPM = (Max_Gallons × 0.75) / Target_Drain_Minutes

To prevent overloading small interceptors, many jurisdictions mandate flow-control valves installed immediately ahead of the interceptor inlet to physically restrict the drain rush.

Real-World Scenarios

✓ The Code-Compliant Kitchen Update

A bakery was expanding operations and installed a massive new 4-compartment prep sink. Using the PDI-G101 calculation, the plumbing engineer determined the sink would dump 65 GPM on a 1-minute drain allowance. Rather than purchasing a massive, expensive 75 GPM interceptor that wouldn't fit under the counter, the engineer worked with the local inspector to approve a 2-minute drain time requirement. This immediately cut the surge flow to 32.5 GPM, allowing the bakery to install a compact, affordable 35 GPM under-sink unit.

✗ The Missing Flow Control Failure

A restaurant owner purchased a correctly sized 20 GPM grease trap online and installed it directly below modern high-flow sink drains without the factory-provided flow-control fitting. Without the restrictor valve choking the water down mechanically at the inlet, the sinks drained with unrestricted gravity force, exceeding 40 GPM. The high-velocity water violently agitated the internal baffles, washing raw grease directly out the tailpiece and resulting in severe municipal sewer fines.

Standard PDI-G101 Interceptor Classifications

PDI Size Classification Max Approved Flow Rate Minimum Grease Retention Limit
Size 10 10 GPM 20 lbs
Size 15 15 GPM 30 lbs
Size 20 20 GPM 40 lbs
Size 25 25 GPM 50 lbs
Size 35 35 GPM 70 lbs
Size 50 50 GPM 100 lbs

Note: Grease retention ratings dictate that the unit must structurally hold exactly double its GPM rating in physical pounds of congealed grease before failing efficiency standards.

Pro Tips & Common Mistakes

Do This

  • ✓Install the Air Intake fitting. The mandatory flow control valve at the inlet must have a vertical air intake (vent) pipe rising above the flood rim. Without this drafted air injection, the fast-moving water creates a siphon that sucks the grease cap violently out of the trap.
  • ✓Specify indirect waste for prep sinks. Wash sinks handle grease, but culinary prep sinks handle raw food and vegetables. Prep sinks must discharge through an indirect air-gap prior to entering an interceptor system to prevent hazardous sewage backup into food zones.

Avoid This

  • ✗Never route a garbage disposal to an internal trap. Food waste grinders create pulverized solids that will instantly fill and destroy the operational volume of an under-sink grease trap. Solids must go to a separate solids-separator before encountering the grease plates.
  • ✗Don't ignore the 140°F temperature limit. Commercial dishwashers dump scalding hot water. High temperatures emulsify grease, turning it liquid so it passes straight through the interceptor walls. Dishwashers typically must bypass under-sink mechanical traps unless explicitly engineered for high heat loads.

Frequently Asked Questions

What is the purpose of the flow control valve on a grease interceptor?

The flow control valve is a mandatory engineered restriction installed just before the interceptor inlet. It physically throttles the rush of draining sink water, ensuring the wastewater velocity never exceeds the GPM rating of the trap. It also injects ambient air into the water stream, assisting grease separation via aeration.

Why do we multiply sink capacity by 0.75 during calculation?

The 0.75 loading factor accounts for displacement. In the real world, a sink compartment is never filled 100% to the absolute brim with water; roughly 25% of the volume is displaced by the physical volume of the dishes, pots, and pans submerged inside the basin.

Can I use enzymes or chemicals instead of cleaning the interceptor?

No. The Uniform Plumbing Code (UPC) and most municipal sewer authorities strictly forbid the introduction of emulsifiers, enzymes, or degreasers into an interceptor. These chemicals break grease down temporarily, allowing it to pass into the city pipes, where it re-congeals blocks miles of public sewer mains.

How often must a commercial grease interceptor be pumped?

Industry standard code dictates the '25 Percent Rule.' An interceptor must be completely pumped and scraped out whenever the combined layer of floating FOG (fats) and settled bottom solids equals 25% of the total wetted volume of the unit. For heavy volume restaurants, this can be weekly.

Related Calculators