What is the minimum gradient for a 100mm foul drain?

Building Regulations Part H specifies a minimum gradient of 1:40 for a 100mm foul drain serving a single dwelling. This means 25mm fall per metre of pipe. For drains serving more than one dwelling, a reduced gradient of 1:80 may be acceptable if peak flow is sufficient for self-cleansing.

What gradient do I need for surface water drainage?

Surface water drains should have a minimum gradient of 1:100 for 100mm pipe and 1:150 for 150mm pipe. Unlike foul drains, surface water contains no solids so steeper gradients are less critical. However, the pipe must still achieve self-cleansing velocity of at least 0.75 m/s.

What happens if the gradient is too steep?

Excessively steep gradients (steeper than 1:10) can cause problems: water flows too fast and leaves solids behind in the pipe, leading to blockages. If a steep gradient is unavoidable, a backdrop manhole should be used to reduce the gradient in stages.

How do I calculate invert levels?

The invert level is the internal bottom of the pipe, measured from a datum (usually Ordnance Datum). To find the downstream invert: Downstream IL = Upstream IL - (Gradient × Distance). For example, 1:40 gradient over 10m means 0.25m fall, so if start IL is 99.50, end IL is 99.25.

Can I use a gradient less than minimum?

In some cases, reduced gradients are accepted where building control agrees. For 150mm foul drains at 1:80 or even 1:150, flow from multiple dwellings provides sufficient self-cleansing. This must be agreed with building control and is based on the Maguire Rule or flow calculations.

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Drainage Gradient Calculator — Fall, Invert Levels & Compliance

Check or calculate drainage gradients for foul and surface water pipes against Building Regulations Part H minimum and maximum falls.

Pipe Run Length

Horizontal distance of pipe run

Drain Type

Start Invert Level

Invert level at start (upstream)

End Invert Level

Invert level at end (downstream)

How We Calculate This

This calculator checks drainage gradients against Part H minimum requirements or calculates the required end invert level for a compliant gradient.

The formula

Gradient = (Start Invert - End Invert) / Pipe Length

Fall per metre (mm/m) = Gradient × 1000

Part H minimum gradients

100mm foul: 1:40 (25mm/m)
150mm foul: 1:80 (12.5mm/m)
100mm surface: 1:100 (10mm/m)
150mm surface: 1:150 (6.7mm/m)

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.