How Grease Interceptors Are Sized for a Commercial Kitchen

Sizing a grease interceptor is not a matter of picking a tank that “looks big enough.” It is a calculation tied to how much greasy water your kitchen sends down the drain and how long that water needs to sit still so grease can separate out. Get the number wrong in either direction and the unit fails its one job. This guide walks through the two sizing methods used in practice, the kitchen details that actually move the number, and why an interceptor that is too large can be just as much of a problem as one that is too small.

A quick boundary before the numbers. Sizing and installation are code-regulated design work, not a do-it-yourself task. The point of understanding the method is so you can read a proposal, ask the right questions, and recognize when a unit is mismatched to your kitchen. The final size and the installation belong to a licensed plumber or engineer working from your locally adopted code. Treat everything below as the logic behind their math, not a substitute for it.

Two Sizing Methods: Fixture-Unit vs. Flow-Rate

There are two families of grease interceptor, and each is sized by a different method. Knowing which one your project uses is the first fork in the road.

The first is the hydromechanical grease interceptor, sometimes called a grease trap. It is a smaller, often indoor unit installed close to the fixtures it serves, and it is sized by flow rate in gallons per minute (GPM). Under the International Plumbing Code, these units are sized and tested against recognized standards such as PDI G101 or the ASME A112.14 series. The rating answers a single question: how many gallons per minute of grease-laden water can pass through while the unit still captures most of the grease.

The second is the gravity grease interceptor, a larger in-ground or below-slab tank, usually outside the building. It is sized by volume, in total gallons, based on the drainage load from the kitchen and a required retention time. The Uniform Plumbing Code and the International Plumbing Code both carry provisions for these larger interceptors, and many of the gallon-based formulas in use trace back to a method published in the Uniform Plumbing Code appendix.

These methods are not interchangeable. A flow-rate (GPM) rating describes a hydromechanical unit; a volume (gallon) rating describes a gravity interceptor. Mixing the two is a common source of confusion when comparing proposals. Which type your kitchen needs is usually driven by your local sewer authority and the amount of FOG (fats, oils, and grease) your operation generates, a requirement covered in our guide on why restaurants are required to have grease interceptors (220).

Inputs That Drive the Size: Fixtures, Dishwasher, Seats, and Hours

Whichever method applies, the size is built from real details about how your kitchen runs. The same blueprint and equipment list that an inspector reviews are the inputs to the math.

For a hydromechanical (flow-rate) unit, the central input is the drainage flow rate of the fixtures connected to it. A three-compartment sink, a prep sink, a mop sink, and similar fixtures each contribute flow. The plumbing code assigns flow values to fixtures, and the connected total, often adjusted by a simultaneity factor because not every fixture runs at once, gives the GPM the interceptor must handle. The unit is then selected at the next standard size above that calculated flow.

For a gravity (volume) interceptor, the inputs widen. A commonly used method builds the number from four things:

• Seating or meals served at peak, often adjusted by a factor for the type of establishment (a fast-food counter turns tables faster than a leisure-dining room)
• A waste flow value per fixture type, which is higher for a kitchen with a dishwasher than one without
• A retention time, which is the minimum the formula assumes water must stay in the tank to separate grease
• A storage factor tied to hours of operation, since a 24-hour kitchen accumulates far more grease between cleanings than an 8-hour one

The dishwasher deserves a flag of its own. It adds both volume and hot water, and hot water is the enemy of grease separation, because warm grease stays emulsified and floats less readily. That is why dishwasher presence raises the assumed waste flow in the gravity formula. Garbage disposers, where allowed, also change the load, and some jurisdictions restrict or prohibit them ahead of an interceptor.

Hydromechanical Trap Ratings (GPM and Pound Capacity)

A hydromechanical interceptor carries two paired numbers: a flow rating in GPM and a grease storage capacity in pounds. The two are linked, and reading them together is what tells you whether a unit fits.

Recognized standards rate these units across a common set of flow sizes. Industry references and code-referenced tables list flows in steps such as 10, 15, 20, 25, 35, 50, 75, and 100 GPM. Each flow rating comes with a minimum grease retention capacity. In the table that the International Plumbing Code references, the storage capacity tracks the flow at roughly two pounds of grease per GPM. A 20 GPM unit is paired with a minimum of about 40 pounds of grease capacity, and a 100 GPM unit with about 200 pounds, with the exact figures set by the standard and the manufacturer’s certified rating.

To select a unit, you calculate the expected peak drainage flow from the connected fixtures and choose the next standard GPM size at or above that figure. A frequent mistake is sizing on flow alone while ignoring the paired grease capacity. A unit can be rated for enough GPM yet hold so little grease that it needs cleaning constantly. The capacity number is what sets how long the unit runs between pump-outs, a maintenance question covered in our guide on how often a grease trap needs cleaning (221).

Because these are certified, tested ratings, the authoritative source for a specific model’s GPM and pound capacity is the manufacturer’s data sheet for the unit listed to the standard your code adopts. Verify the listing rather than trusting a round number on a spec line.

Sizing an In-Ground Interceptor by Volume and Retention Time

A gravity interceptor is sized in total gallons, and the gallon figure exists to deliver one thing: enough retention time for grease to rise and solids to settle before water moves on to the sewer.

The simplest way to see the logic is the flow-and-retention relationship. Volume in gallons equals the peak flow rate in gallons per minute multiplied by the retention time in minutes. Many gravity interceptors are designed around a minimum retention time, with thirty minutes a figure cited in the ASME standard for larger interceptors and adopted by numerous local authorities. Hold the flow steady and increase the required retention, and the required tank volume rises in step.

The more detailed method, drawn from the Uniform Plumbing Code appendix, multiplies a calculated drainage load by a retention factor and a storage factor. Its building blocks are the inputs from the section above: meals or seats at peak adjusted by an establishment factor, a per-fixture waste flow that is higher with a dishwasher, a retention time on the order of a couple of hours for a fully equipped kitchen, and a storage multiplier that climbs with hours of operation. The output is a target volume, which is then rounded up to the nearest standard tank size offered.

Local authorities frequently set their own floor and their own version of the formula. Some require a minimum tank size regardless of what the calculation returns, and the minimum varies by jurisdiction. This is exactly why sizing cannot be finalized from a general article. The drainage fixture unit method, where the code converts your fixture count into a load value and reads the interceptor size from a table, is another accepted path that some jurisdictions require instead. Your engineer or plumber works from the method your adopted code names, which is why the same restaurant can be assigned a different tank size in two different cities.

Too Small Overflows, Too Large Goes Septic: The Retention-Time Window

Both undersizing and oversizing are failures, and the reason is the same variable seen from two ends: retention time.

Undersize the interceptor and water moves through too fast. There is not enough dwell time for grease to separate and float, so a slug of grease passes straight through to the sewer lateral during a busy service. The unit also fills to capacity sooner, overflows its stored grease, and demands constant cleaning to keep up. Grease that breaks through reaches the very sewer the device exists to protect, which is the discharge problem that drives the requirement in the first place. The downstream consequences of a failing interceptor are covered in our guide on what happens when a grease trap is neglected (224).

Oversizing causes the opposite and less obvious failure. When a tank is far larger than the kitchen’s flow, water sits for too long with too little turnover. Oxygen in the trapped water gets used up, anaerobic bacteria take over, and the contents go septic. That process generates hydrogen sulfide gas, which is the rotten-egg odor associated with a stagnant interceptor and is corrosive to concrete and metal over time. An oversized tank can also let already-separated material re-disturb, sending solids and odor toward the sewer. Wastewater authorities have documented oversized interceptors producing strong odors and elevated discharge strength for exactly this reason.

The target is the window between those extremes: large enough to give grease time to separate, small enough that water keeps turning over before it goes anaerobic. That window is why sizing is a calculation rather than a guess, and why “bigger to be safe” is the wrong instinct with a grease interceptor.

Frequently Asked Questions

What size grease interceptor does my restaurant need?
There is no single answer, because the size depends on your fixtures, whether you have a dishwasher, your seating or meals at peak, your hours of operation, and the method your local code requires. A licensed plumber or engineer runs the calculation against your adopted code and your sewer authority’s rules, then rounds up to the nearest standard size.

What is the difference between a grease trap and a grease interceptor?
The terms are often used loosely. In sizing practice, a hydromechanical grease interceptor (the smaller, often indoor “trap”) is rated by flow in gallons per minute, while a gravity grease interceptor is a larger in-ground or below-slab tank rated by total gallons. They are sized by different methods.

Why does retention time matter so much?
Grease separates by floating, and floating takes time. Retention time is the minimum the water must sit still for that separation to happen. Too little retention and grease passes through; too much retention and the water goes septic and produces odor. The required volume is built around hitting that window.

Can I just install a larger interceptor to be safe?
No. An oversized interceptor lets water stagnate, which can turn the contents septic, generate hydrogen sulfide odor, and corrode the tank. Bigger is not safer with a grease interceptor; the right size is the one matched to your actual flow and retention requirement.

Does a dishwasher change the sizing?
Yes. A dishwasher adds flow and hot water, and warm water keeps grease emulsified so it separates less readily. Sizing methods assign a higher waste flow value to kitchens with a dishwasher, which increases the required interceptor size.

This guide is general information, not professional engineering or plumbing advice. Grease interceptor sizing and installation are code-regulated work; confirm the method and final size with a licensed plumber or engineer and your local sewer authority.

Sources

• International Code Council, IPC Section 1003.3.4 Hydromechanical grease interceptors: https://codes.iccsafe.org/s/IPC2015NY/chapter-10-traps-interceptors-and-separators/IPC2015-Ch10-Sec1003.3.4
• UpCodes, IPC Hydromechanical Grease Interceptors and Automatic Grease Removal Devices (sizing standards and flow/grease-capacity table): https://up.codes/s/hydromechanical-grease-interceptors-and-automatic-grease-removal-devices
• UpCodes, Interceptor Sizing (IPC/UPC sizing provisions): https://up.codes/s/interceptor-sizing
• Plumbing & Drainage Institute, Standard PDI-G101, Testing and Rating Procedure for Hydromechanical Grease Interceptors: http://static1.1.sqspcdn.com/static/f/1395156/27911530/1526888927933/StandardPDI-G101revised2017.pdf
• National Precast Concrete Association, Gravity Grease Interceptor Design (UPC Appendix H sizing formula and factors): https://precast.org/blog/gravity-grease-interceptor-design/
• Plumbing & Mechanical, Sizing Grease Interceptors: https://www.pmmag.com/articles/91758-sizing-grease-interceptors
• U.S. Environmental Protection Agency, Fats, Oils, and Grease (FOG) and sanitary sewer overflows: https://www.epa.gov/npdes/national-pollutant-discharge-elimination-system-npdes