PLUMBCRATE
Waste pipe sizes explained: 32mm, 40mm or 50mm?
Quick answer: in a typical UK home, use 32mm (1¼") waste for basins, 40mm (1½") for kitchen sinks, baths, showers, washing machines and dishwashers, and 50mm for longer runs or where several appliances share one pipe. Sizes are the nominal outside diameter.
Common sizes and where they go
| Size | Imperial | Typical use |
|---|---|---|
| 21.5mm | — | Overflows and boiler condensate |
| 32mm | 1¼" | Wash basins, small hand basins |
| 40mm | 1½" | Kitchen sinks, baths, showers, washing machine and dishwasher standpipes |
| 50mm | 2" | Long runs, multiple appliances on one branch, some commercial uses |
| 110mm | 4" | Soil pipe from toilets (not covered here) |
Why run length matters
Building Regulations guidance (Approved Document H) gives maximum unvented branch lengths. As a rule of thumb, 32mm basin branches are kept short (around 1.7m), and 40mm bath or sink branches around 3m. Longer runs often move up a size, for example 32mm to 40mm, or 40mm to 50mm, and need a steady fall. Check the current guidance or your building control for your exact layout.
Fall (gradient)
Waste pipe needs a continuous fall towards the stack or gully. Between about 18mm and 90mm per metre is commonly used for small-bore waste. Clip pipe regularly so it does not sag.
Joining systems
- Solvent weld: permanent glued joints, neat and compact. See white solvent-weld waste and black solvent-weld waste.
- Compression waste: push-on with a nut and seal ring, can be undone, handy for repairs and connecting to traps. See compression waste fittings.
- Push-fit waste: ring-seal joints, quick to assemble.
Not all brands are compatible: make sure fittings match the pipe system you are using.
Traps
Match the trap to the waste size: 32mm traps for basins, 40mm for sinks, baths and showers. A 75mm seal is standard for most appliances. Shallower seals (38mm or 50mm) are used where space is tight, typically on baths and showers. Browse traps and wastes.
FAQs
Can I connect 32mm to 40mm? Yes, with a reducer. Always reduce in the direction of flow from smaller to larger, never the other way.
Is 1½" the same as 40mm? In UK waste systems they are used interchangeably for the same nominal size.