How do fixture units determine pipe size?

Each sanitary fitting has a discharge unit (DU) value: WC=7, basin=1, bath=7, shower=2, kitchen sink=6, washing machine=3, dishwasher=3. The total DU determines the required pipe diameter and flow capacity. Up to 10 DU (no WC): 50mm. Up to 18 DU (1 WC): 82mm. Up to 100 DU: 110mm. Over 100: 160mm.

What gradient should drainage pipes be laid at?

110mm foul drains: 1:40 (25mm fall per metre) is recommended. 1:80 is the absolute minimum for a single WC. Steeper is acceptable. The gradient must be consistent with no low points. Design the gradient to be self-cleansing — water must flow fast enough to carry solid waste.

How far apart should inspection chambers be?

For 110mm pipes, access points (inspection chambers or rodding eyes) should be no more than 12m apart. For 160mm pipes, 22m maximum. Access is needed at every change of direction, every junction and at the head of each drain run. This allows blockages to be cleared with drain rods or jetting equipment.

What size inspection chamber do I need?

For drain depths up to 600mm: 450mm diameter (shallow access chamber) is sufficient. For depths 600mm-1200mm: 475mm diameter or larger. Over 1200mm: 1050mm diameter manhole with step irons. All chambers need suitable covers rated for the expected traffic loading (pedestrian, light vehicle or heavy vehicle).

Do drainage pipes need testing?

Yes. All new drains must be tested before backfilling. The standard test is an air test: plug both ends, pressurise to 100mm water gauge, and confirm the pressure drop does not exceed 25mm in 5 minutes. Alternatively, a water test can be used. Building Control will inspect drainage at this stage.

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Part H Drainage Calculator — Pipe Sizing & Gradient

Calculate below-ground drainage pipe diameter from fixture units, determine gradient requirements, and plan inspection chamber positions per Part H.

WCs

Basins

Baths

Showers

Kitchen sinks

Drain run length

How We Calculate This

This calculator sizes drainage pipes using the fixture unit method from BS EN 12056 and Approved Document H.

Fixture unit method

Each fitting contributes discharge units (DU). Total DU determines minimum pipe diameter. Gradients ensure self-cleansing velocity.

Access points

Inspection chambers at direction changes, junctions and maximum intervals. Rodding eyes at upstream ends of runs.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.