Plumbing codes do not size pipes on raw flow, because fixtures almost never run simultaneously — they size on fixture units, a probabilistic load score assigned to each fixture type. Drainage fixture units (DFU) rate the waste side and water supply fixture units (WSFU) the pressure side; a system total is just each fixture’s value times its count. Two WCs at 3 DFU, three lavatories at 1, and a shower at 2 add up to 11 DFU.
The unit total then becomes a design flow through the Hunter curve — Q = 0.0084·DFU^0.613 for drainage, Q = 0.013·WSFU^0.555 for supply — and the pipe diameter is read from the standard’s fixture-unit capacity table: the 11 DFU example lands in a 75 mm drain at a minimum gradient of 1:40. European practice under EN 12056 takes a different route to the same destination, converting per-fixture design units to flow by a root-sum-of-squares method with a frequency factor.
Sizing does not end with capacity: the selected drain must also pass a self-cleansing check, with the flow velocity confirmed between 0.6 and 3.0 m/s so solids keep moving without eroding the pipe.