Drainage Fall & Gradient
Convert a drain run and gradient ratio into fall in millimetres.
Total fall75 mm
Total fall0.075 m
Gradient2.5 %
Method: Approved Document H (drainage fall)
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Drainage Fall & Gradient Calculator
Convert a drain run length and gradient ratio into the fall you need at the other end.
How it works
Drainage fall is the vertical drop over a horizontal run. A gradient written as 1 in 40 means the pipe drops 1 unit for every 40 units of horizontal run.
The calculation is fall = run length ÷ ratio. For a run measured in metres, multiply the answer by 1000 to get millimetres. So 3 m at 1 in 40 gives 3 ÷ 40 = 0.075 m, or 75 mm fall.
Calculating a gradient and choosing a suitable gradient are separate tasks. Pipe diameter, expected flow, connected appliances and the type of drainage system decide the design. Approved Document H distinguishes above-ground waste pipes from below-ground drains; the same minimum gradient does not apply to every pipe.
Worked example
A 4.8 m drain run at 1 in 60 needs 4.8 ÷ 60 = 0.08 m fall, so the far end should be about 80 mm lower.
Calculate the gradient from a measured fall
If you already know the run and fall, reverse the calculation: gradient ratio = horizontal run ÷ vertical fall. Put both measurements in the same units first. A 5.4 m run with 90 mm of fall gives 5400 ÷ 90 = 60, so the gradient is 1 in 60.
For a percentage, divide fall by run and multiply by 100: 90 ÷ 5400 × 100 ≈ 1.67%. A 1 in 60 gradient is not 60%. The larger the number after “1 in”, the flatter the drain: 1 in 80 has less fall per metre than 1 in 40.
Use invert levels to check the fall
The invert is the inside bottom of the pipe. Compare upstream and downstream invert levels against the same level datum, rather than measuring from ground that may slope. If the upstream invert level is 10.000 m and the required fall is 90 mm, the downstream invert level is 9.910 m.
Keep the unrounded calculation until the final result. Multiplying a rounded 16.7 mm per metre by a long run introduces an avoidable difference; dividing the full run by 60 preserves the intended gradient.
Use the drainage and sanitation notes to place the gradient calculation in the complete layout: appliance connection, trap, branch pipework and ventilation. Working out the fall is one part of understanding how the system functions.
Drainage gradient conversion chart
These are equivalent ways to express a slope. The table converts units; it does not select a permitted gradient for a particular drain.
| Gradient | Fall per metre | Percentage |
|---|---|---|
| 1 in 40 | 25 mm | 2.5% |
| 1 in 60 | 16.67 mm | 1.67% |
| 1 in 80 | 12.5 mm | 1.25% |
| 1 in 100 | 10 mm | 1% |
Fall per metre = 1000 ÷ ratio. Gradient percentage = 100 ÷ ratio. Recurring decimals are rounded here.
Drainage fall questions
What is a 1 in 40 fall over 1 metre?
A 1 in 40 gradient drops 25 mm for every metre of run: 1 ÷ 40 = 0.025 m. Over 3 m that becomes 75 mm of fall.
How many mm per metre is 1 in 60?
About 16.7 mm of fall per metre of run: 1 ÷ 60 = 0.0167 m. Over a 4.8 m run that is roughly 80 mm.
What gradient does a 100 mm foul drain need?
For below-ground foul drains in England, Approved Document H Table 6 gives a minimum gradient of 1 in 40 for a 100 mm drain with peak flow below 1 litre/second. Its 1 in 80 case is for peak flow above 1 litre/second with at least one WC connected. Check the actual design conditions; these are not universal gradients for above-ground waste pipes.
Is more fall always better for drainage?
No. A steeper gradient is not a substitute for checking pipe size, flow, connections and access for clearing blockages. Use the guidance for the particular drainage system, then calculate the levels needed to achieve that design.
Method: Approved Document H (drainage fall)
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