Grease Trap Size Guide: What Size Does Your Restaurant Need?

Choosing the right grease trap size is one of the most important decisions a restaurant owner can make — and one of the most commonly botched. Install a trap that's too small and you'll be paying for constant pump-outs, dealing with foul odors, and risking code violations. Go too big and you're throwing money away on equipment and installation costs you didn't need.

This guide breaks down exactly how to determine the right grease trap size for your kitchen, what the different size categories mean, and how much you should expect to spend.

Why Grease Trap Size Matters More Than You Think

Your grease trap's job is simple: catch fats, oils, and grease (FOG) before they enter the municipal sewer system. But a trap can only do that job if it's properly sized for your kitchen's wastewater output.

Too small:

• FOG passes through the trap and into your plumbing, causing blockages and sewer backups
• You'll need cleanings every 2-4 weeks instead of every 2-3 months
• Health inspectors will flag it — many jurisdictions require traps to be sized to local plumbing code specifications
• You're at higher risk of fines, shutdowns, and expensive emergency plumbing calls

Too big:

• Higher upfront cost for the unit itself (sometimes thousands more than necessary)
• Larger installation footprint — in-ground interceptors require excavation, which adds cost
• Cleaning costs more per service because of the larger capacity
• Wastewater may not flow through quickly enough to maintain proper separation

The goal is to match your trap size to your actual kitchen output — not to guess, and not to just buy whatever the plumber suggests without understanding why.

Common Grease Trap Sizes at a Glance

Grease traps are measured in gallons (or sometimes pounds of grease capacity) and fall into four general categories:

Understanding the difference between a grease trap and a grease interceptor is important — they serve the same function but at very different scales. For a deeper comparison, see our guide on grease traps vs. interceptors.

How to Calculate the Right Grease Trap Size

The most widely accepted method for sizing a grease trap is the flow rate method, based on gallons per minute (GPM) of wastewater your kitchen produces. This is the method required by most plumbing codes and municipalities.

The GPM Calculation Formula

The Plumbing and Drainage Institute (PDI) standard formula works like this:

1. Determine your fixture capacity. Add up the cubic capacity of all sinks, dishwashers, and other fixtures that drain into the grease trap. Measure each basin in inches (length x width x depth) and divide by 231 to convert to gallons.
2. Multiply by 75%. Fixtures don't fill to the absolute brim, so you use 75% of total capacity as the working volume.
3. Divide by drainage time. Most sinks drain in about 1 minute, giving you your GPM flow rate.
4. Select a trap rated for that GPM. Grease traps are rated in GPM — a 20 GPM trap, a 35 GPM trap, a 50 GPM trap, etc.

Example: A restaurant has a 3-compartment sink (each basin 24" x 24" x 12") and a prep sink (18" x 18" x 10").

• 3-compartment sink: 3 x (24 x 24 x 12) = 20,736 cubic inches = 89.7 gallons
• Prep sink: 18 x 18 x 10 = 3,240 cubic inches = 14.0 gallons
• Total: 103.7 gallons x 0.75 = 77.8 gallons
• At 1 minute drain time: ~78 GPM

In this case, you'd need a grease trap rated for at least 75-100 GPM — likely a 500-750 gallon in-ground unit.

Quick GPM-to-Trap-Size Reference

Important: Always round up. If your calculation says 22 GPM, go with a 25 GPM trap, not a 20. A slightly oversized trap is far better than an undersized one.

Recommended Grease Trap Size by Restaurant Type

If you want a quick starting point before running the full GPM calculation, here's what most restaurants of each type typically need:

These are guidelines, not guarantees. Your local plumbing inspector will have the final say, and they'll base their decision on your specific fixture count and layout, not on the type of food you serve.

Local Code Requirements: They Vary More Than You'd Expect

There is no single national standard for grease trap sizing. Instead, requirements are set at the state, county, or city level — and they can differ dramatically.

Here are some of the key ways local codes vary:

• Minimum trap size: Some cities require a minimum of 500 gallons for any commercial kitchen, regardless of output. Others allow 20-gallon under-sink units for low-volume operations.
• Interior vs. exterior: Many jurisdictions no longer allow indoor grease traps for new construction and require in-ground exterior interceptors instead.
• Sizing method: Most follow the PDI flow rate calculation, but some cities have their own formulas that factor in seating capacity, number of meals served per day, or menu type.
• Permit requirements: You'll almost always need a plumbing permit for installation. Some areas also require a separate grease trap permit or FOG discharge permit.
• Inspection frequency: Certain municipalities mandate monthly inspections for larger interceptors and quarterly inspections for smaller traps.

Bottom line: Before you buy anything, call your local water authority or plumbing department. Ask specifically what size and type of grease trap is required for your type of establishment. Getting this wrong means paying to rip it out and start over. Check our state compliance guide for an overview of requirements by state.

What Happens If Your Grease Trap Is Undersized

An undersized grease trap isn't just an inconvenience — it creates a cascade of problems that get worse over time:

1. FOG bypass: When wastewater moves through the trap too quickly, grease doesn't have time to separate and float to the top. Instead, it flows straight into your drain lines and the municipal sewer.
2. Frequent pump-outs: A trap that fills up every 2 weeks instead of every 2 months means 6x the cleaning costs. At $175 to $500+ per cleaning, that adds up fast.
3. Code violations and fines: If an inspector determines your trap is undersized, you'll face fines and a mandatory upgrade — on their timeline, not yours.
4. Sewer line blockages: FOG that escapes your trap solidifies in pipes downstream. You'll be responsible for clearing those blockages, and if they affect the municipal system, you could face significant liability.
5. Business interruption: A backed-up sewer line can shut down your kitchen for days. Lost revenue from a closure often exceeds the cost of the trap upgrade itself.

When to Upgrade Your Grease Trap

You should consider upgrading your grease trap if any of the following apply:

• You're cleaning it more than once a month. This is a strong signal that your current trap can't keep up with your kitchen's output.
• You've expanded your menu or hours. Adding a fryer station, extending brunch service, or increasing catering operations all increase FOG output.
• You've failed an inspection. If the inspector says your trap is undersized, you don't have a choice — upgrade or face escalating fines.
• You're renovating anyway. If you're already tearing up the kitchen, it's the cheapest time to upsize your trap since excavation and plumbing work is already happening.
• You're experiencing drain backups. Recurring slow drains or backups in the kitchen are often caused by an overwhelmed grease trap, not just pipe issues.
• Your trap is more than 15-20 years old. Older traps may not meet current code requirements and may have degraded in efficiency. Upgrading gives you better performance and compliance.

Grease Trap Cost Comparison by Size

Here's what you should expect to pay for the trap itself, installation, and ongoing maintenance:

The upfront cost difference between a 50-gallon under-sink trap and a 1,500-gallon interceptor is significant. But remember: you're not just choosing based on budget. You're choosing based on what your kitchen actually requires. Installing a cheaper, undersized trap just means paying more in maintenance, fines, and eventually replacing it anyway.

For a more detailed breakdown, see our full grease trap cleaning cost guide.

Tips for Getting the Size Right

1. Do the GPM calculation yourself before talking to vendors. It takes 10 minutes and gives you a baseline so you won't be oversold.
2. Call your local water authority first. They'll tell you the minimum size required for your type of operation. Don't skip this step.
3. Get quotes from at least 3 installers. Sizing recommendations can vary between companies. If one suggests a size dramatically different from the others, ask why.
4. Factor in future growth. If you plan to add equipment or expand your kitchen within the next few years, it's cheaper to install a slightly larger trap now than to upgrade later.
5. Ask about automatic grease removal units (AGRUs). For high-volume operations, these self-cleaning systems can reduce pump-out frequency and may allow a smaller physical trap.

The Bottom Line

The right grease trap size depends on your kitchen's flow rate, your local code requirements, and the type of food service you operate. For most independent restaurants, a trap in the 100-500 gallon range will be the sweet spot. Coffee shops and food trucks can often get by with a 20-50 gallon under-sink unit, while large-scale commercial kitchens will need 1,500+ gallon interceptors.

Don't guess. Run the GPM calculation, check your local codes, and get multiple quotes. The cost of getting it right the first time is always less than the cost of getting it wrong.

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Not sure where to start? Request a free quote and let local providers help you determine the right size for your kitchen.

Related articles:

• Grease Trap vs. Interceptor: What's the Difference?
• Grease Trap Cleaning Cost in 2026: What Restaurant Owners Should Actually Expect to Pay