You’ve just signed a lease on a new restaurant space in Seattle, Spokane, or Yakima. You’re excited to start cooking — but even before the first fryer heats up, there’s a question you can’t afford to ignore: how big should your grease trap be?

Choose one too small, and you risk backups, plumbing damage, regulatory fines, and unpleasant odors. Go too large, and you tie up capital and space unnecessarily.

In this article, we’ll walk through how to size a grease trap (or interceptor) correctly for your commercial kitchen in Washington. By the end, you’ll know what rules to follow, what calculations to perform, and how to work with a trusted grease management partner like Grease Guys to ensure your system is functional and compliant.

You’ll learn about:

• What a grease trap or interceptor does

• Key Washington regulations to watch

• The steps and formulas for sizing

• Common pitfalls and FAQs

• How Grease Guys supports you with installation, maintenance, and service
  

Understanding Grease Traps and Why Sizing Matters

A grease trap or interceptor is a device installed in the wastewater line to capture fats, oils, and grease (FOG) before they enter municipal sewers. Over time, FOG solidifies, clogs pipes, and leads to backups or costly repairs.

The trap’s job is simple but critical: slow the flow of wastewater, allow grease to rise, and release cleaner water downstream. The size — in terms of capacity and flow rate — determines how well it performs.

Why correct sizing is critical:

• Ensures compliance with Washington and municipal codes

• Prevents plumbing failures and restaurant downtime

• Reduces long-term maintenance costs

• Helps schedule cleanings efficiently

• Optimizes use of kitchen space

In Washington, state and city codes require specific design standards for grease traps and interceptors. Local rules may vary between Tacoma, Lynnwood, Bothell, and other cities, so always verify your jurisdiction’s requirements before installation.

How to Determine the Right Grease Trap Size

Before you purchase or install your grease trap, follow a systematic approach. These steps will help you size the trap appropriately and meet Washington’s compliance standards.

Step 1: Identify all grease-generating fixtures

Make a list of all sinks, dishwashers, pot sinks, mop sinks, floor drains, and other fixtures that send greasy wastewater into your plumbing system. Exclude hand-wash sinks and toilets unless your local code requires otherwise.

Step 2: Calculate total flow rate

Find the flow rate (GPM) or drainage fixture units (DFU) for each appliance. Use manufacturer specifications or standard plumbing code tables. Add them together to determine the total flow entering the trap.

Step 3: Apply the retention time formula

Multiply the total flow rate by the required retention time (commonly 30 minutes). This gives you the total volume your grease trap should hold.
Example: 80 GPM × 30 minutes = 2,400 gallons.

Step 4: Check minimum and maximum limits

Most cities set minimum and maximum grease trap sizes. For instance, some require at least 750 gallons for commercial kitchens, while others limit installations to 4,000 gallons without special approval.

Step 5: Add safety margin

Round up your calculation to the nearest standard trap size and include a 10–20% safety buffer. This accounts for peak surges or future expansion.

Step 6: Plan for access and maintenance

Ensure your grease trap is accessible for pumping, inspection, and maintenance. Covers and risers should be easily reachable, and the trap should be within range for service vehicles.

Key Considerations and Common Pitfalls

Sizing a grease trap involves more than math. Local rules and practical challenges often influence the final decision. Here are the most common pitfalls to avoid:

1. Ignoring local jurisdiction requirements
Each Washington city may have unique plumbing or FOG program standards. Always consult your local utilities department before installation.

2. Undersizing the trap
A smaller trap fills faster, causing grease to pass through into the sewer system and leading to code violations and clogs.

3. Oversizing unnecessarily
While it’s tempting to go big, an oversized trap increases installation costs and can hold stagnant waste for too long.

4. Poor access or placement
Traps buried too deep or with small covers are difficult to service. Ensure easy access for vacuum trucks and technicians.

5. Lack of maintenance schedule
Even a correctly sized trap will fail if not cleaned regularly. A buildup of more than 25% grease and solids requires pumping to maintain efficiency.

6. Forgetting future expansion
If you plan to add sinks, dishwashers, or fryers, factor that into your design. Upsizing slightly now can save thousands later.

Frequently Asked Questions

How often should a grease trap be cleaned?
Most restaurants in Washington clean their traps every 90 days or when grease and solids reach 25% of total volume. Frequency depends on kitchen size, menu type, and usage.

Can I install multiple grease traps in series?
Yes, if your calculated volume exceeds local limits, you can install two or more traps in series. This is common in larger facilities.

What’s the difference between a hydromechanical and a gravity interceptor?
Hydromechanical traps are compact indoor units designed for smaller kitchens. Gravity interceptors are large, outdoor tanks suited for high-flow or multi-fixture systems.

Do all Washington cities follow the same sizing rules?
No. While state codes provide general guidance, each municipality can impose its own requirements. Always confirm sizing standards with your city’s building or utilities department.

What happens if I install the wrong size?
An undersized trap can cause clogs and regulatory issues. Oversized traps are inefficient and expensive. Working with a professional ensures compliance and proper sizing.

Conclusion

Choosing the right grease trap size for your commercial kitchen in Washington isn’t just about compliance — it’s about protecting your business, your plumbing, and your reputation. A properly sized system ensures smooth kitchen operations, fewer service disruptions, and easier maintenance.

Grease Guys helps restaurants, food processors, and facilities throughout Washington with used cooking oil collection, grease trap cleaning, and full-service FOG management. Whether you’re building a new kitchen or upgrading an existing one, our experts can help you size, install, and maintain your grease interceptor correctly from the start.

Contact Grease Guys today to schedule a consultation or grease trap service.