How Is Grease Interceptor Sizing Calculated?

Grease interceptor sizing determines the liquid capacity needed for a commercial kitchen's fats, oils, and grease to separate before discharge. The plumbing code offers fixture-unit-based sizing tables, while many jurisdictions use flow-rate methods: total the grease-producing fixtures' flow, apply a retention time (commonly 30 minutes), and select the interceptor volume that provides it. The result must meet the local authority's minimum size and be maintainable. The fixture-based method assigns drainage fixture units to each grease-producing fixture and reads the interceptor size from a code table — quick and conservative. The flow-based method is more precise: sum the actual flow rates, multiply by the required retention time, and you get the minimum liquid volume. I use whichever the jurisdiction accepts, and I verify against the sewer authority's rules too, since they often have their own minimums.

Only grease-producing fixtures route through the interceptor: three-compartment sinks, prep sinks, dishwashers, wok stations, and floor drains in grease areas. Restrooms, mop sinks with only gray water, and non-grease fixtures bypass it — sending clean water through the interceptor just wastes its capacity and stirs up the separated grease. The piping design separates the grease waste line from the rest of the sanitary system and brings it to the interceptor inlet. The interceptor needs venting, an accessible location for the pumper truck, and manholes or cleanouts for inspection and cleaning. Underground interceptors get traffic-rated covers where vehicles pass over. I show the complete grease waste piping on the plumbing drawings with pipe sizes, slopes, and the interceptor schedule — the health department reviewer traces this line carefully.

An interceptor that can't be pumped is a liability, not an asset. The design provides clear access for the vacuum truck, adequate working space around manholes, and a location the service company can actually reach — not buried under the future patio. I note the pumping frequency expectations in the project documents, because interceptors need regular service and the owner should budget for it from day one. Approval typically involves both the building department (plumbing code compliance) and the sewer authority or pretreatment program (their sizing and maintenance rules). Some jurisdictions require a grease management plan with the permit. The submittal shows the sizing calculation, the equipment schedule, the piping layout, and the maintenance provisions — a complete grease story that both reviewers can approve.

• Sizing method: fixture-unit tables or flow-rate with retention time, per jurisdiction
• Fixture separation: only grease waste through the interceptor; everything else bypasses
• Access: truck-reachable location with manholes for pumping and inspection
• Venting and covers: proper venting and traffic-rated covers where needed
• Dual approval: building department plus sewer authority pretreatment requirements