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

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

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

Minimum fall

Approved Document H sets a branch gradient of 18 to 90mm per metre. Small-bore branch wastes (32-50mm) are laid at a minimum of 18mm per metre (1:56). 100mm WC soil branches are typically laid at 1:40 (25mm per metre) for reliable solids carriage. The fall must be consistent — too steep can leave solids behind, too shallow causes blockages.