Commercial Drain Sizing Calculator

This calculator sizes drain, vent, and cleanout requirements from an IPC drainage fixture unit (DFU) table (Table 710.1) with eight brackets, from 2" pipe at 3 DFU or fewer up to 8" pipe above 620 DFU. Total Fixture Units is the primary driver of Drain Pipe Size and Vent Pipe Size: across its full range it moves the drain recommendation from 2" up to 8", spanning the entire bracket table, though because the brackets are wide (33 through 160 DFU all map to the same 4" drain, for example) a modest change in fixture count often will not move the sizing at all -- only a change large enough to cross into the next bracket does. Building Stories has a much narrower effect across its full range, shifting the drain size by only one bracket, but it does apply a sharp, threshold-based bump at the default 3-story setting specifically: buildings over 3 stories require the drain to bump up one full standard size for stack capacity, so crossing that 3-story threshold changes the recommendation even when Total Fixture Units stays fixed.

Cleanouts Required is driven almost entirely by Pipe Run Length, estimated at one cleanout per 75 feet of run, plus a smaller allowance of one per two building stories; Total Fixture Units never enters the cleanout formula at all. Required Slope is a fixed code value (1/8" per foot for 4"-and-larger drains, 1/4" per foot for smaller) that only changes when the drain size itself crosses the 4" boundary. This tool applies simplified DFU tables and does not replace a licensed plumbing engineer's stamped drainage design.