Grease trap 25% rule calculator
By Stew Israil, GreaseTrapRules · Updated Oct 18, 2026
Most city grease programs say you must pump before grease and solids together reach 25% of the liquid depth. Enter your measurements to check.
How it works: (grease + solids) ÷ liquid depth × 100. 3" + 2" in a 30" trap = 16.7%, which is under the limit.
Your city may also set a maximum interval. Look it up on your city page, then set a pump-out reminder.
FAQ
What is the 25% rule?
Many city FOG ordinances require a grease trap or interceptor to be pumped when floating grease plus settled solids reach 25% of the total liquid depth. For example Issaquah, WA (IMC 16.38.060), Olathe, KS (Code 15.08.260) and Corpus Christi, TX (FOG manual) all use it, and most pair it with a maximum interval such as 90 days, whichever comes first.
How do inspectors measure it?
With a clear sampling tube, often called a sludge judge, pushed to the bottom of the tank. They read the grease layer at the top and the solids layer at the bottom and compare the total to the liquid depth. Denton, TX describes inspectors using a measuring device to measure the layers.
Is the rule the same everywhere?
No. Some cities use different thresholds or device-specific schedules. Spokane, WA requires hydromechanical units to be cleaned at least every 30 days and gravity interceptors every 90 days, or at 25%, whichever comes first. Check your city page.
Sources
- Issaquah, WA Municipal Code 16.38.060 · checked 2026-10-18
- Denton, TX Grease Interceptor Maintenance Program · checked 2026-10-18
- Corpus Christi, TX FOG Program Manual · checked 2026-10-18
- Olathe, KS Code 15.08.260 · checked 2026-10-18
- Spokane, WA Code 13.03.0508 · checked 2026-10-18
Not legal or engineering advice. Rules change; verify with your city.