What drain diameter does each bathroom fixture need?

Not all sanitary fixtures discharge the same amount of water, and using the same drain diameter for all of them is one of the commonest mistakes when quoting a bathroom renovation without checking fixture by fixture.

The diameter depends on what it discharges

• 32 mm — washbasin and bidet.
• 40 mm — shower and kitchen sink.
• 50 mm — bath and toilet with horizontal outlet.

In all three cases, the purchase margin over the measured length is 10% for joints and waste. On all runs, the minimum slope is 2% (2 cm per metre) — without it, water doesn't drain well and the drain blocks sooner than normal.

Trap, floor drain and interceptor trap: they're not the same

• Trap: a water seal preventing sewer smells rising up the drain. It is mandatory on every sanitary fixture. The individual one (one per fixture) is the norm in renovations; the bottle trap is used under washbasins for aesthetics and ease of access. The height of the water seal must be at least 50 mm.
• Floor drain: a floor outlet for walk-in shower trays, terraces or utility rooms. The protruding type makes drainage easier with less built slope than a single central point one.
• Interceptor trap: a small accessible chamber under the shower tray or bath that centralises several drains (bath, bidet, washbasin) before joining them to the stack. It is mandatory in many installations that centralise the drain at a single point — always leave it accessible with its access cover, it is the first place blockages build up.

Manholes: three types by function

• Inspection (junction): at changes of direction in the buried network.
• Trapped: mandatory as the last access point before the main connection, to prevent sewer smells returning.
• Access: allows the horizontal network to be inspected and cleared.

Individual stopcock: a detail that saves trouble

Besides the correct diameter, always fit an individual stopcock under each fixture (washbasin, toilet, washing machine), not just the home's main one. That way a fixture can be repaired without shutting off water to the whole house.

Frequently asked questions

What drain diameter does a shower need? 40 mm, the same as a kitchen sink.

Why does my drain block even though the diameter is right? Most often insufficient slope: below the 2% minimum, water doesn't carry waste away well and the run blocks more often.

What's the interceptor trap for if each fixture already has its own trap? It centralises several drains at a single accessible point before joining them to the stack, and is mandatory in many installations that group bath, bidet and washbasin.

Conclusion

Getting the diameter of each run right for the fixture it serves, and not skipping the minimum slope or the access points (interceptor trap, manholes), is what prevents a newly finished bathroom renovation from starting to give blockage problems after a few months. Quoting fixture by fixture gives a clear and technically correct breakdown.