Wetrooms
Shower drain capacity against shower flow
- where
- total shower flow (l/s)
- shower heads running together
- flow per head (l/min)
- other outlets running at once (l/min)
- drain utilisation (%)
- drain capacity at the water head available (l/s)
- spare drain capacity (l/s)
Explain this equation
Use this when choosing a linear drain or gully for a wetroom or level-access shower, to make sure it can clear all the water the shower can deliver.
q is the flow of each head in litres per minute, n how many run together, qb any body jets or hand set. Qd is the drain’s rated flow in litres per second at the depth of water your floor can hold, often 10 mm at a flush threshold.
Shower flow is quoted per minute and drains per second, so the total is divided by 60. Utilisation is the shower flow as a percentage of what the drain can take; under 100% means it keeps up.
A standard 15 l/min head is 0.25 l/s, so a 0.4 l/s drain runs at about 63%. A 30 l/min rain head is 0.5 l/s and needs a drain rated at least that at 10 mm.
Watch out: published capacities are often at a 20 mm head. Hair and soap build-up reduce capacity over time, so leave a margin.
Checks that a linear drain or gully can take all the water the shower can deliver.
Enter your values to see the result.
Good to know
Divide litres per minute by 60 to get litres per second. EN 1253-1 sets minimum flows at a 20 mm water head (0.8 l/s for DN50, 0.6 l/s for DN40, 0.4 l/s for single-head shower drains without side inlets), but at flush thresholds you want the capacity at 10 mm, which many makers also publish. Kessel’s guidance: a standard head 12–15 l/min (about 0.25 l/s), a large rain head 20–30 l/min (about 0.5 l/s). Linear drain length is chosen for the layout and fall; capacity is set mostly by the outlet and trap, so a longer channel does not by itself carry more.
Common mistake
Comparing the 20 mm-head figure with a level-access shower that can only hold 10 mm of water before it spills.
Limits
Steady-state check only. Hair traps, partial blockage and a long or poorly vented waste reduce capacity. Pipe sizing is not covered.
Sources
Results are calculations from the published methods and manufacturer figures shown, and depend on the data you enter. Always follow the tile, adhesive and grout manufacturers’ instructions, BS 5385 and the Tile Association guidance. Coverage rates vary by product and substrate. Check prices and VAT rules with your own records and HMRC.