What size waste pipe do I need for a basin?

A wash basin requires a minimum 32mm waste pipe with a minimum fall of 18mm per metre (1:56 gradient). The maximum pipe run without a vent is 1.7m for 32mm pipe. If your pipe run is longer than 1.7m, you need to either upgrade to 40mm pipe or fit an air admittance valve (Durgo valve). Most basins are supplied with 32mm fittings as standard.

What is the minimum fall for a waste pipe in the UK?

The minimum fall for waste pipes in the UK is 18mm per metre (a gradient of 1:56) for 32mm, 40mm and 50mm pipes. For 100mm soil pipes (WC), the minimum fall is 18mm per metre (1:40 is the practical standard, giving 25mm/m). These figures are from UK Building Regulations Approved Document H and BS EN 12056. The fall must be consistent along the full pipe run.

Can I connect a washing machine to a 32mm waste pipe?

No. A washing machine requires a minimum 40mm waste pipe. It has a discharge unit rating of 2 DU and pumps water out under pressure, which a 32mm pipe cannot handle. The standpipe (upstand) should be 600mm high and connected to a 40mm trap. Many plumbers fit the washing machine waste alongside the kitchen sink waste into a shared 40mm or 50mm combined waste pipe.

What is a discharge unit (DU)?

A discharge unit (DU) is a measure of the flow rate from a plumbing fixture, used to size waste pipes and stacks. Values are defined in BS EN 12056. A wash basin = 1 DU, a bath or shower = 2 DU, a kitchen sink = 2 DU, and a WC = 7 DU. When multiple fixtures connect to a combined waste pipe, you add up the DU values to determine the pipe size needed.

When do I need an air admittance valve (AAV)?

An air admittance valve (also called a Durgo valve) is needed when your waste pipe run exceeds the maximum unvented length: 1.7m for 32mm pipe, 3.0m for 40mm pipe, or 4.0m for 50mm pipe. The AAV prevents trap siphonage by allowing air into the pipe. It must be installed above the spill-over level of the highest fixture and be accessible for maintenance. AAVs comply with BS EN 12380.

Waste Pipe Sizing Calculator — What Size Waste Pipe Do I Need?

Size waste pipes and calculate the minimum fall for basins, baths, showers, sinks and WCs per UK Building Regs Part H and BS EN 12056.

Select Fixtures

Wash Basins

32mm pipe, 1 DU each

Baths

40mm pipe, 2 DU each

Showers

40mm pipe, 2 DU each

Kitchen Sinks

40mm pipe, 2 DU each

Washing Machines

40mm pipe, 2 DU each

Dishwashers

40mm pipe, 2 DU each

WCs (Toilets)

100mm soil pipe, 7 DU each

Pipe Run Details

Pipe Run Length

Total length from fixture to stack or drain

Number of Bends

Each bend adds ~0.5m equivalent length

How We Calculate This

This calculator sizes waste pipes based on UK Building Regulations Approved Document H and BS EN 12056. It calculates the total discharge units from your selected fixtures, determines the correct pipe diameter, and checks whether venting is required for your pipe run length.

Discharge unit method

Each fixture is assigned discharge units (DU) based on its flow rate:

Wash basin: 32mm pipe, 1 DU
Bath / Shower: 40mm pipe, 2 DU
Kitchen sink: 40mm pipe, 2 DU
Washing machine / Dishwasher: 40mm pipe, 2 DU
WC (toilet): 100mm pipe, 7 DU

Combined waste pipe sizing

Up to 6 DU (no WC): minimum 50mm combined waste
7-12 DU (no WC): minimum 65mm
Any WC connected: 100mm soil pipe required

Maximum unvented pipe runs

32mm: 1.7m maximum
40mm: 3.0m maximum
50mm: 4.0m maximum

Beyond these lengths, an air admittance valve (AAV) or vent pipe is required to prevent trap siphonage.

Minimum fall

All waste pipes require a minimum fall of 18mm per metre (1:56). Soil pipes (100mm) are typically laid at 1:40 (25mm/m). The fall must be consistent — too steep can leave solids behind, too shallow causes blockages.

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Verify with your supplier.