Calculate grease interceptor sizing inputs using the drainage fixture unit (DFU) / flow-rate method

Steps

1. Identify every fixture and piece of equipment discharging FOG-laden waste to the interceptor (pot sinks, prerinse sinks, floor sinks/drains receiving kettle drainage, dishwashers without prerinse sinks, etc.) per the applicable plumbing code's fixture list
2. Look up each fixture's assigned drainage fixture unit (DFU) value in the adopted plumbing code's fixture-unit tables (commonly UPC Table 702.1/702.2(a)/702.2(b) or the equivalent IPC table for the adopted edition), confirming the correct table for the code edition in force
3. For continuous or semicontinuous flow sources (e.g., certain equipment drains), convert using the applicable code's conversion factor — commonly 1 GPM of continuous flow being treated as equivalent to a set number of fixture units under many codes — rather than assuming a universal conversion factor without checking the adopted code
4. Sum the DFU load (or convert directly to an estimated peak GPM) for all fixtures connected to the interceptor, and compare this figure against the sizing method actually required locally: some jurisdictions require an HGI sized by PDI-G101/ASME A112.14.3 GPM rating rather than a raw DFU total
5. For hydromechanical interceptors specifically, recognize that DFU-based average-flow sizing can understate peak simultaneous-discharge flow, so confirm whether the plumbing official requires a peak-flow or simultaneous-use calculation method instead of straight DFU totals
6. Document the sizing calculation (fixture list, DFU/GPM per fixture, method used, and final interceptor GPM/gallon size selected) for submission with the plumbing/industrial-waste permit application