How Often Should You Pump a Grease Trap? The 25% Rule
A grease trap that overflows doesn’t warn you: it blocks your drains in the middle of service. Yet the right pumping frequency isn’t a mystery — it’s a simple rule used across the industry: the 25% rule.
The 25% rule in one sentence
Pump your grease trap as soon as the combined thickness of the grease layer on top and the solids at the bottom reaches 25% of the trap’s liquid depth.
That’s it. No magic calendar, no “once a season”: the right timing depends on what actually goes into your trap. Two restaurants of the same size can have completely different schedules.
Why 25% and not 50%?
A grease trap works by gravity: water slows down, grease rises, solids sink, and clarified water exits through an outlet at mid-height.
When more than a quarter of the volume is taken up by grease and solids, retention time drops and the separation zone shrinks. The result: grease “leaks” into your lines with the outgoing water. The trap still looks fine — it rarely overflows — but it has stopped separating. That’s why 25% became the industry reference, and several municipalities use it as the benchmark in their maintenance requirements.
Estimating your frequency: the 3 factors
- Cooking volume and type. A fryer and a grill produce far more grease than a salad counter. The greasier the menu, the faster the trap fills.
- Trap size vs. real flow. A trap undersized for your dish pit hits 25% in a few weeks, sometimes less. A properly sized trap gives you a predictable margin.
- Kitchen habits. Scraping plates into the garbage, collecting used oil in a recovery container (never down the drain) and keeping strainer baskets in place can easily double the interval between pump-outs.
For a number tailored to your kitchen type, use our grease trap calculator — it applies the 25% rule to your meal volume.
Signs you’ve waited too long
- Odours near the dish pit or floor drains — the first sign, and the most ignored.
- Slow drains in the kitchen, especially at peak hours.
- Visible grease at the inspection port or in the first downstream manhole.
- Repeated backups — at that point the grease is already in your lines and a commercial drain cleaning is needed before it happens again mid-service.
Keep a log (it protects you)
Every pump-out should leave a paper trail: date, volume removed, trap condition, provider. That log is your proof of due diligence for the city during an inspection, for your insurer after water damage, and for your landlord if your lease makes you responsible for the equipment.
The simplest way to never think about it: fold trap checks and pump-outs into a commercial plumbing maintenance contract with a frequency set to your real volume — the log keeps itself.
In short
| Type of establishment | Reasonable starting point* |
|---|---|
| Heavy frying (fast food, fried chicken) | Monthly check, frequent pump-outs |
| Full-service restaurant with frying | Check every 1–2 months |
| Light kitchen (café, sandwich shop) | Quarterly check |
| Caterer / seasonal production | Check before and after peak season |
* Starting points only: the one reliable rule is measuring against 25%. Adjust the frequency after two or three measurement cycles.
Want to know where your kitchen stands? Run the grease trap calculator, or request a quote for scheduled maintenance.