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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
32mm pipe, 0.5 DU each (EN 12056-2 System I)
40mm pipe, 0.8 DU each
40mm pipe, 0.6 DU each
40mm pipe, 0.8 DU each
40mm pipe, 0.8 DU each
40mm pipe, 0.8 DU each
100mm soil pipe, 2.0 DU each (6L flush)
Pipe Run Details
Total length from fixture to stack or drain
Each bend adds ~0.5m equivalent length
Equivalent pipe length per bend (default 0.5m)
Override fall rate, applied even with a WC (0 = default: 18mm/m branch, 25mm/m with WC). AD H allows up to 90mm/m on sink, bath and WC branches; a 32mm basin branch is capped lower.
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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) per BS EN 12056-2 Table 2, System I (the UK default):
- Wash basin: 32mm pipe, 0.5 DU
- Bath: 40mm pipe, 0.8 DU
- Shower: 40mm pipe, 0.6 DU
- Kitchen sink: 40mm pipe, 0.8 DU
- Washing machine / Dishwasher: 40mm pipe, 0.8 DU
- WC (6 litre cistern): 100mm pipe, 2.0 DU
Combined waste pipe sizing
BS EN 12056-2 sizes combined branches by converting total discharge units to a flow rate and matching it against pipe hydraulic capacity. As a conservative domestic rule of thumb derived from those capacities:
- Up to 6 DU (no WC): minimum 50mm combined waste
- 7-12 DU (no WC): minimum 65mm
- Over 12 DU (no WC): 75mm or larger
- Any WC connected: 100mm soil pipe required (Approved Document H)
For stack diameters, standard lengths and push-fit vs solvent-weld connections, see our UK soil pipe sizes reference.
Maximum unvented pipe runs
- 32mm: 1.7m maximum
- 40mm: 3.0m maximum
- 50mm: 4.0m maximum
- 100mm WC branch: 6.0m maximum for a single WC (AD H Diagram 3), 15m where more than one WC shares the branch (AD H Table 2)
Beyond these lengths, an air admittance valve (AAV) or vent pipe is required to prevent trap siphonage.
Minimum fall
Small-bore branch wastes (32-50mm) are laid at a minimum of 18mm per metre (1:56). The upper limit varies: Approved Document H Diagram 3 allows 18 to 90mm per metre on 40mm and 50mm sink and bath branches, while a 32mm washbasin branch takes its upper limit from the Diagram 3(b) design curve, which Table 2 puts at 22mm per metre for a full 1.7m run. 100mm WC soil branches sit within the 18 to 90mm per metre limits of Table 2 and are typically laid at 1:40 (25mm per metre) for reliable solids carriage. The fall must be consistent along the run: too steep can leave solids behind or siphon a trap, too shallow causes blockages.
Working figures for this are collected in our Drainage Pipe Sizes and Minimum Fall Gradients reference table.
Frequently Asked Questions
A wash basin requires a minimum 32 mm waste pipe with a minimum fall of 18 mm per metre (1:56 gradient). The maximum pipe run without a vent is 1.7 m for 32 mm pipe. If your pipe run is longer than 1.7 m, you need to either upgrade to 40 mm pipe or fit an air admittance valve (Durgo valve). Most basins are supplied with 32 mm fittings as standard. For supply pipe sizing, see our Pipe Sizing Calculator.
Approved Document H sets a minimum branch discharge pipe gradient of 18mm per metre (a gradient of 1:56) for 32mm, 40mm and 50mm branch wastes, and that is the figure this calculator uses. The upper limit depends on the appliance: Diagram 3 allows 18 to 90mm per metre on the 40mm and 50mm sink and bath branches, while a 32mm washbasin branch follows the design curve in Diagram 3(b), which Table 2 caps at 18 to 22mm per metre once the branch reaches its full 1.7m unventilated length. A 100mm WC branch is separate: the AD H minimum is still 18mm per metre but the practical design fall is 1:40 (25mm per metre), which is the value this calculator uses for soil branches. Too steep a fall can leave solids behind on a soil branch and siphon a basin trap, so stay inside the limit for the appliance. The fall must be consistent along the full pipe run.
No. A washing machine requires a minimum 40mm waste pipe. Under BS EN 12056-2 System I a domestic washing machine (up to 6kg) has a discharge unit rating of 0.8 DU, and it 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.
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 12056-2 Table 2. Under System I (the UK default), a wash basin or bidet = 0.5 DU, a shower (without plug) = 0.6 DU, a bath = 0.8 DU, a kitchen sink = 0.8 DU, a washing machine or dishwasher = 0.8 DU, and a WC with a 6 litre cistern = 2.0 DU. When multiple fixtures connect to a combined waste pipe, you add up the DU values to determine the pipe size needed.
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, 4.0m for 50mm pipe, or 6.0m for a single WC on a 100mm soil branch. 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.
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Last updated: February 2026
Verified against UK standards · estimates only, confirm with your supplier.