What is the minimum fall for a 110mm drain?

For foul drainage (toilets, sinks, baths), a 110mm pipe requires a minimum fall of 1:40 (25mm per metre). For surface water (rainwater), the minimum is 1:100 (10mm per metre). These are the minimum gradients specified in UK Building Regulations Approved Document H. This drainage gradient calculator works out the total fall for your pipe run length.

What does a 1:40 fall mean?

A fall (also called gradient, slope or drop) of 1:40 means the pipe drops 1 metre for every 40 metres of horizontal run, or equivalently 25mm per metre. This is the standard minimum gradient for 110mm foul drainage pipes. A steeper gradient (e.g. 1:20) is fine — it is the minimum that must be met.

Can a drain be too steep?

Yes — excessively steep drains (steeper than about 1:10) can cause problems. Water flows too fast and leaves solids behind, which can build up and cause blockages. Building Regs do not specify a maximum fall for short domestic runs, but best practice is to keep between 1:40 and 1:80 for foul drainage.

Where should my drain discharge to?

Foul drainage must connect to the public foul sewer. Surface water can discharge to a soakaway — use our Soakaway Sizing Calculator (/landscaping/soakaway-sizing-calculator) to size one — or to a surface water sewer. A French drain can handle localised ground water — see our French Drain Calculator (/landscaping/french-drain-calculator). Never connect surface water to the foul sewer.

Do I need Building Regulations approval for drainage?

Yes — any new drainage work or modification to existing drainage requires Building Regulations approval under Part H. This includes new connections to the public sewer, changes to pipe runs, and installation of new soakaways. Contact your local council Building Control department before starting work. Ensure patios and paved areas also drain correctly — see our Paving Slab Calculator (/landscaping/paving-slab-calculator).

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Drainage Fall Calculator — UK Building Regs Gradients

Calculate the correct gradient and fall for drainage pipes based on UK Building Regulations Approved Document H.

Pipe Run Length

Total horizontal length of pipe run

Pipe Diameter

Application

Determines minimum gradient required

Start Invert Level (optional)

Invert level at start of run, if known

How We Calculate This

Our drainage fall calculator uses the minimum gradients specified in UK Building Regulations Approved Document H to calculate the required fall for your drainage pipe run.

The formula

Fall per metre (mm) = 1000 ÷ Gradient ratio

Total fall (mm) = Pipe length (m) × Fall per metre (mm)

UK minimum gradients (Building Regs Part H)

110mm foul drainage: 1:40 minimum (25mm per metre)
160mm foul drainage: 1:60 minimum (16.7mm per metre)
110mm surface water: 1:100 minimum (10mm per metre)
160mm surface water: 1:100 minimum (10mm per metre)

Important notes

These are minimum falls — steeper gradients within reason are acceptable
For foul drains serving a single WC, 1:40 minimum applies with adequate flow
Drains must be laid on a firm, even bedding (pea gravel or granular material)
All drainage work requires Building Regulations approval

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Verify with your supplier.