What Is the 25% Rule for Grease Traps?

Short answer: when the floating grease on top and the settled solids on the bottom together reach 25% of the trap's liquid depth, the trap must be cleaned. Past that point it stops doing its job. The rule is written into council FOG ordinances across Australia, so it is a compliance line, not just a guideline.

What the 25% rule means

The rule sets a clear trigger. When the combined depth of grease and solids hits 25% of the trap's total liquid depth, cleaning is required. It takes the guesswork out. You are not cleaning on a hunch or a calendar date, you are cleaning when the trap is actually a quarter full of waste.

Why 25% is the line

A grease trap works by giving wastewater time and space to separate, with grease floating up and solids sinking down. Once waste fills 25% of the trap, that separation breaks down and untreated grease starts flowing out into the sewer. The 25% mark is the point where the trap quietly stops protecting you.

How to measure it

You check it with a dipstick, often called a sludge judge. Lower it in, and it shows the depth of the floating grease layer and the settled solids against the total liquid depth. If the two together are at or near a quarter, book a clean. The check takes a minute and tells you exactly where you stand.

How it ties to your cleaning schedule

The 25% rule and your service interval work together. Most kitchens are told to clean every one to three months or at 25% capacity, whichever comes first. If you keep hitting 25% before your scheduled visit, your cycle is too long and you tighten it. If you are nowhere near it, your trap may be sized well for your output.

The bottom line

The 25% rule is the line where your trap stops separating grease from water. Measure with a dipstick, clean at or before a quarter full, and let it set your schedule. Stick to it and you stay compliant and out of trouble.

Hitting 25% too fast? Compare quotes from licensed operators. Related: how often to clean and signs it needs cleaning.