Work out the required drainage fall (gradient) for foul and surface water pipes. Enter your pipe length and size to get the gradient as a ratio (1:X), the total fall in millimetres and the fall per metre, with a Building Regulations Part H compliance check.
For educational purposes only. Not a substitute for professional engineering judgment.
Calculator Inputs
Calculation Mode
Choose whether to calculate required gradient or flow capacity
Drainage Type
Select foul or surface water drainage for appropriate Building Regs minimum
Pipe Diameter
Select pipe size in millimetres
Pipe Material & Condition
Select material to determine Manning's roughness coefficient
Manning's n Override (Optional)
Leave blank to use default for selected material
Design Flow Rate
Expected flow at the pipe section in litres per second
UK minimum drainage gradients (Part H)
Indicative minimum falls for gravity drainage. The exact minimum depends on pipe size, the appliances served and the peak flow — always confirm against the current Approved Document H.
| Pipe | Type | Typical minimum gradient | Fall per metre |
|---|---|---|---|
| 110mm | Foul | 1:80 | 12.5 mm/m |
| 150mm | Foul | 1:150 | 6.7 mm/m |
| 110mm | Surface water | 1:100 | 10 mm/m |
| 150mm | Surface water | 1:100 – 1:150 | 6.7 – 10 mm/m |
| 225mm | Foul / sewer | ~1:200 | 5 mm/m |
A useful rule of thumb: foul runs of 1:40 to 1:110 give adequate self-cleansing flow velocities. Too flat risks deposits; too steep can strand solids.
Foul vs surface water drainage
Foul drainage
Carries waste water and solids from WCs, sinks and appliances. Needs a steeper fall (typically 1:80 for 110mm) to keep a self-cleansing velocity of roughly 0.7–1.0 m/s so solids do not settle out.
Surface water drainage
Carries rainwater run-off from roofs and paving. No solids, so shallower gradients (around 1:100) are acceptable. Keep separate from the foul system.
Worked example
A 10 m run of 110 mm foul drain laid at the minimum 1:80 gradient:
- Total fall = length ÷ ratio = 10 ÷ 80 = 0.125 m = 125 mm
- Fall per metre = 1000 ÷ 80 = 12.5 mm/m
- Percentage grade = 100 ÷ 80 = 1.25%
- If the start invert is 30.000 m, the end invert is 30.000 − 0.125 = 29.875 m
What is drainage fall?
Drainage fall (also called gradient or slope) is the vertical drop of a drainage pipe over its horizontal length. It is what makes a gravity drain work. It is expressed as:
- Ratio (1:X): e.g. 1:80 means 1 unit of drop for every 80 units of length
- Fall per metre (mm/m): e.g. 12.5 mm/m
- Percentage (%): e.g. 1.25%
This calculator converts between all three and checks the result against the indicative Building Regulations Part H minimum for the pipe size and drainage type you choose. It uses the Manning equation to relate gradient, pipe size and flow.
Frequently asked questions
What is the minimum fall for drainage?
For a 110mm foul drain carrying at least one WC, a minimum gradient of about 1:80 (12.5mm of fall per metre) is widely used under Building Regulations Part H. 150mm foul drains are typically laid at around 1:150. Surface water drains are commonly laid at a minimum of 1:100. Always confirm against the current Approved Document H and your local water authority, as the exact minimum depends on pipe size and peak flow.
What happens if a drain is laid too steep?
Counter-intuitively, a foul drain that is too steep can be as problematic as one that is too flat. If the water runs away faster than the solids it is carrying, the solids can be left stranded in the pipe and cause blockages. This is why foul runs are usually kept within a sensible range (roughly 1:40 to 1:110) rather than simply made as steep as possible.
What is the difference between foul and surface water drainage falls?
Foul drainage carries solids, so it needs a steeper gradient to maintain a self-cleansing velocity (around 0.7–1.0 m/s) — typically 1:80 for a 110mm pipe. Surface water carries no solids, so shallower gradients such as 1:100 are acceptable. The two systems should be kept separate and each designed to its own minimum fall.
How do I calculate fall per metre?
Fall per metre = pipe length × (1 ÷ gradient ratio). For example, a 10m run at a 1:80 gradient has a total fall of 10 ÷ 80 = 0.125m = 125mm, which is 12.5mm per metre. This calculator does the conversion between ratio (1:X), total fall (mm) and fall per metre automatically.
What fall do I need for a 110mm soil pipe?
A 110mm foul/soil drain is generally laid at a minimum gradient of 1:80 (12.5mm per metre) where it carries at least one WC and there is adequate flow. Steeper than about 1:40 is usually avoided on foul runs to prevent solids being stranded.
How do I measure drainage fall on site?
Set the invert level (the inside bottom of the pipe) at the start and end of the run, then check the drop matches the design fall — for example with a laser level, a dumpy level and staff, or boning rods and a sight rail. Enter a start invert level in the calculator to chain levels along the run.
What fall do I need for a waste pipe?
Internal waste pipes serving sinks, basins, baths and appliances are usually run at a steeper fall than buried foul drains. As a rule of thumb, 32mm, 40mm and 50mm waste pipes are laid at roughly 1:40 to 1:45, which is about 18–22mm of fall per metre. Below about 18mm/m the flow can be too slow to self-cleanse, while much steeper can cause the water to outrun the soap and debris. Buried 110mm foul drains, by contrast, are typically laid at 1:80 (12.5mm per metre).
What fall do I need for a soil pipe?
A 110mm soil pipe (the main below-ground foul drain carrying WC discharge) is generally laid at a minimum gradient of 1:80, which is 12.5mm of fall per metre, where it serves at least one WC and there is adequate flow. A 150mm foul drain is typically laid at around 1:150 (6.7mm per metre). Keep foul runs within roughly 1:40 to 1:110 — too steep risks stranding solids as the water runs away from them.
Standards & related tools
Approved Document H (gov.uk)
Official Building Regulations guidance for drainage and waste disposal.
Manning's n Reference
Roughness coefficients for common pipe materials.
Manning's Pipe Calculator
Full-bore pipe flow capacity when the gradient is known.
Building Regulations Part H Guide
How Approved Document H sets minimum falls for foul and surface water drains.
BS EN 752 Drainage Guide
The European standard for drain and sewer systems outside buildings.
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Last verified: February 2026