What Is a Commercial Grease Trap? (Sizing Guide)
A commercial grease trap, also called a grease interceptor, captures fats, oils, and grease from approved kitchen fixtures before wastewater reaches the sanitary sewer. Size it using the locally adopted plumbing code and the unit’s listing, not restaurant size or tank gallons alone. Measure fixtures, calculate the required flow, select a compliant unit, then install and maintain it as specified.
During a kitchen renovation, a sink layout can change late in the job. That may alter which fixtures connect to the interceptor and how much wastewater can reach it at once. I start by checking the approved plumbing plan and measuring the fixtures, rather than relying on a tank size suggested from the room layout.
The term “grease trap” is often used for more than one type of grease interceptor. Rules and sizing methods vary by location and product, so this guide explains the calculation process without treating one formula as universal. Confirm the method with your plumbing authority or qualified plumbing designer before buying or installing a unit.
Identify Fixtures and Prepare the Sizing Data
Sizing starts with a clear list of the fixtures that may legally discharge through the interceptor. Record each fixture’s usable volume, outlet arrangement, and expected use, then verify the adopted code and the interceptor’s listing. These details determine which sizing method applies and help prevent an otherwise suitable unit from being overloaded.
What counts as a commercial grease trap?
A grease interceptor is a plumbing device that slows or separates wastewater so fats, oils, and grease (FOG) can be retained instead of flowing into the sanitary sewer. “Grease trap” is a common general term; product types and code definitions can differ. The unit captures FOG, but it does not replace proper waste handling or regular cleaning.
Hydromechanical grease interceptors are commonly associated with PDI G101. Other listed interceptor types may be covered by ASME A112.14.3 or A112.14.4. Which standard applies depends on the unit and jurisdiction. Check the product’s listing and installation instructions, then confirm local approval.
Which fixtures should be included?
List every sink or other fixture proposed for connection. Then ask the plumbing authority which may connect. A fixture’s presence in a commercial kitchen does not automatically mean it should discharge through the interceptor. Do not route toilets or other prohibited waste through it.
For each approved fixture, record:
- Fixture type and number of basins
- Basin length, width, and usable fill depth
- Outlet location and drain arrangement
- Whether fixtures may drain at the same time
- Any other approved equipment or drains in the proposed connection
The interceptor, seals, flow-control device, and connected piping must suit the wastewater’s temperature and chemistry. Verify the ratings in the product information and approved plan. Do not assume a unit is suitable for every discharge condition.
How do you measure sink volume?
For a rectangular sink basin, estimate volume in U.S. gallons with this formula:
Length × width × usable depth in inches ÷ 231 = gallons
Measure the basin’s usable depth to the intended fill level, not automatically to its rim. This gives a volume estimate for the sizing method; it is not, by itself, the required interceptor size.
| Example basin measurement | Calculation | Estimated volume |
|---|---|---|
| 18 × 18 × 10 inches to intended fill level | 18 × 18 × 10 ÷ 231 | 14.0 gal |
| 20 × 16 × 10 inches to intended fill level | 20 × 16 × 10 ÷ 231 | 13.9 gal |
These are example measurements, not standard sink sizes. For multiple basins, calculate each one and combine volumes only as the governing method directs. Note whether the basins share a drain and whether simultaneous draining is expected. Those details can affect the calculation.
Key next step: Confirm the permitted fixtures and the required local sizing method before calculating flow.
Calculate Capacity and Install the Grease Interceptor
A compliant selection must meet the required flow rating and any applicable grease-retention capacity. Use the sizing method required by the jurisdiction and the unit’s listing or instructions. A tank’s nominal gallon volume alone does not show that it can handle the required flow or meet local rules.
How is interceptor flow calculated?
A common fixture-based method estimates flow as:
Total fixture volume × permitted fill factor ÷ drain time in minutes = flow in gallons per minute (gpm)
The fill factor and drain time must come from the governing code or manufacturer’s approved method. There is no single factor or drain time that applies to every installation. Do not substitute a value found online unless your authority or the product instructions permit it.
For illustration only, suppose the approved method uses 42 gallons of fixture volume, a fill factor of 0.75, and a two-minute drain time:
42 × 0.75 ÷ 2 = 15.75 gpm
Those inputs are examples, not recommended default values. Use the result to identify the required flow rating under the applicable method. Then check whether the selected model also meets any required retention or capacity criteria.
Do not size from restaurant seating, kitchen floor area, or nominal tank gallons alone. These may not reflect fixture volume or peak discharge. If two sizing approaches conflict, ask the plumbing authority or a qualified plumbing designer which one governs.
How do you select and install the unit?
Use this sequence to move from calculation to installation:
- Check the rules and listing. Confirm the adopted code, approved sizing method, required product listing, and manufacturer’s installation instructions.
- Match the performance. Select a model that meets the required gpm rating and any required grease-retention capacity. Confirm its intended installation orientation and flow-control requirements.
- Plan access. Place it where it can be inspected and pumped out. A unit hidden behind fixed construction or blocked by equipment can be difficult to service.
- Follow the approved plan. Install the interceptor, flow-control device, venting, covers, and piping as required by the listing, instructions, and approved plumbing plan.
- Commission the installation. Before discharge, check that connections and covers are secure and the flow-control device is present and installed as specified.
A flow-control device limits the rate of wastewater entering certain interceptor types. Bypassing or removing it can overload the unit and carry FOG into the sewer, even if the tank seems large enough. Never alter the device to solve a drainage problem; have the system checked against its approved design.
| Sizing or installation choice | Why it matters | What to verify |
|---|---|---|
| Flow rating in gpm | Shows the rate the unit is intended to handle | Required rating from the approved method |
| Grease-retention capacity | May be separately required | Code, listing, and manufacturer instructions |
| Tank volume alone | Does not establish compliant sizing | Do not use as the sole selection basis |
| Flow-control device | Helps control incoming flow where required | Correct device, placement, and installation |
| Access location | Supports inspection and pump-out | Practical access to covers and service points |
Key next step: Keep the calculation and product documents with the approved plumbing plan.
Inspect, Pump, and Maintain the Unit
Maintenance keeps retained grease and settled solids from taking up too much interceptor space. Follow local service rules and the manufacturer’s criteria, and keep a record of inspections and pump-outs. The commonly cited 25% rule is a useful benchmark in many settings, but it is not a universal legal limit.
The 25% rule means arranging service when accumulated grease and settled solids occupy about 25% of the interceptor’s liquid capacity. Local code or the manufacturer may set a different trigger, so use the applicable requirement if it is stricter.
Keep a service log with the inspection date, measured buildup if available, service date, and material removed. Follow the service provider’s or manufacturer’s instructions for cleaning and restoring the unit. After service, check that required components are in place and that the designed liquid level is restored when applicable.
To reduce unnecessary FOG entering the system:
- Keep grease and oil out of drains where practicable.
- Scrape food solids into an appropriate waste container rather than into a connected sink.
- Do not use caustic or chemical “grease-dissolving” treatments instead of proper sizing and service.
- Recalculate if fixture count, fixture use, or wastewater flow changes.
An undersized interceptor, an excessive incoming flow, or missed maintenance can allow FOG to pass downstream. A backup or slow drain may have several causes, so do not assume the interceptor alone is at fault. Inspect it and have the plumbing system assessed before making changes.
Key next step: Set the service interval using local requirements and observed buildup, then document each service.
Verify Discharge Performance and Document Compliance
Commissioning confirms that the installed interceptor matches the approved design and can be inspected and serviced. Keep the sizing work, product details, and maintenance history together. These records help a reviewer or service professional check what was installed and identify when a change in fixture use requires a new calculation.
What should the records include?
Keep copies of:
- The approved plumbing plan and connected-fixture list
- Basin measurements and the sizing calculation
- The interceptor’s model, listing, flow rating, and instructions
- Flow-control and venting details required by the approved design
- Inspection and pump-out dates, plus service notes
A change in fixtures or use can change the expected flow. For example, adding a sink or connecting another approved fixture may require a new calculation and approval. Do not assume the existing rating still fits. Ask the authority or plumbing designer to review the change before connecting it.
Illustrative installation scenarios
These examples show how to apply the process, not code-approved designs for a specific site.
Renovated prep sink: A contractor plans to add a basin to an existing kitchen. I would first confirm whether the new fixture may connect, measure its usable volume, and update the approved fixture list. The existing interceptor’s nominal tank size would not prove that it can handle the changed flow.
Existing unit with a flow problem: A kitchen reports slow drainage and considers bypassing the flow-control device. That is not a safe sizing fix. The device may be essential to the unit’s rated performance. Check for obstruction, service needs, and installation issues, then have the design reviewed before changing components.
Key takeaway: Treat a fixture change, device alteration, or repeated drainage issue as a reason to verify the approved design, not to guess at a larger tank.
Frequently Asked Questions
These answers cover common decisions about grease interceptor sizing, installation, and service. Local plumbing rules and the specific product listing remain controlling, so use them to confirm any project-specific requirement.
Is a grease trap the same as a grease interceptor?
People often use the terms interchangeably, but product types and code definitions can vary. Check the terminology and requirements used by your local authority.
How do I size a commercial grease trap?
Measure approved fixtures and apply the sizing method required by the local code and the interceptor’s listing or instructions. Select a unit that meets the required flow rating and any retention criteria.
Can I size it by tank gallons alone?
No. Nominal gallons alone do not establish that a unit meets the required flow rating or other applicable criteria.
Can I use one universal fill factor?
No. Use the fill factor and drain time required by the applicable code or approved manufacturer method.
Should every kitchen drain connect to the interceptor?
No. Connect only fixtures permitted by the plumbing authority and the interceptor listing. Toilets and other prohibited waste must not pass through it.
What does the 25% rule mean?
It is a common service benchmark: pump when grease and settled solids take up about 25% of liquid capacity. It is not a universal legal limit; follow local and manufacturer criteria.
Can I bypass the flow-control device if drainage is slow?
No. Bypassing it can overload the interceptor and allow FOG to reach the sewer. Inspect the system and get qualified help instead.
When should I recalculate?
Recheck sizing when fixture count, use, or wastewater flow changes, or when the approved plumbing design is revised.
Where should the interceptor be installed?
Install it in the approved orientation and location, with practical access for inspection and pump-out. Follow the product instructions and approved plan.
What records should I keep?
Keep the approved plan, fixture measurements, sizing calculation, product details, and inspection and service dates together.
(This article was written by one of our staff writers, Julian R. Hayes. Visit our Meet the Team page.)
