Shower Flow Rate vs Waste Capacity: Build a Drainage Budget

Shower Trays · Drainage · Specification

Short answer: approve the shower outlets, waste fitting, pipe route and tray as one hydraulic chain. The design input is the maximum flow that can actually enter the tray. The system limit is not the largest number on a waste datasheet; it is the weakest verified part of the complete installed route.

A 90 mm waste hole, a 40 mm pipe and a “high-flow” product name are dimensions and descriptions—not a drainage budget.

Start with four different numbers

Number	What it describes	What it does not prove
Outlet flow	Water delivered by the showerhead, hand shower, body jets or filler at stated pressure and operating mode	That two outlets cannot run together, or that site pressure produces the catalogue value
Waste-fitting capacity	Flow through a named waste under its stated test condition, often including a stated water head	Capacity of the connected branch pipe or a partially obstructed in-service assembly
Pipework design capacity	What the applicable plumbing design allows for the selected diameter, gradient, length, bends and ventilation	That the tray-to-waste joint is sealed or the strainer remains clean
Installed test result	Observed behaviour of the assembled tray, waste and pipe route under a controlled flow	Every future condition unless the test flow, duration and maintenance state are recorded

Keep these values separate. Converting them to the same unit helps comparison, but it does not make their test conditions equivalent.