A correctly sized grease interceptor is the difference between passing a health department inspection and a sewer backup. This calculator follows the IAPMO Z1001 and PDI-G101 approach: it converts the connected kitchen fixtures into a peak drainage flow rate, then selects the smallest standard hydromechanical interceptor rated to handle that flow.
How it works
Each fixture contributes a number of drainage fixture units (DFU). The DFU total is converted to a probable peak flow rate, and the result is matched to the standard PDI ladder of interceptor ratings:
total DFU = Σ (fixture count × fixture DFU value)
peak GPM = DFU total mapped onto the PDI-G101 flow curve
selected = smallest PDI rating (4, 7, 10, 15, 20, 25, 35, 50, 75, 100 GPM)
whose rating ≥ peak GPM
retention = grease capacity paired with that rating (2 lb per GPM nominal)Standard PDI interceptors guarantee a grease retention capacity of about two pounds per rated GPM, so a 20 GPM unit holds roughly 40 pounds of grease before it must be cleaned.
Example and notes
A kitchen with one three-compartment sink (3 DFU), one mop/floor sink (3 DFU), two floor drains (2 DFU each), and a pot sink (2 DFU) totals 12 DFU, which maps to roughly a 15 GPM peak. The next standard rating that meets this is the 15 GPM interceptor with about 30 pounds of retention. Always confirm the final selection against your local authority having jurisdiction, because some jurisdictions mandate larger gravity interceptors for food-service occupancies regardless of the hydromechanical calculation.