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.
More fall is not automatically better. A pipe laid too steeply can leave solids behind; a pipe too flat can slow the flow and block. Use this as a revision calculator, then follow the design guidance, manufacturer data and Building Control requirements for real work.
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.
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?
A common revision answer is around 1 in 40, with flatter gradients such as 1 in 80 acceptable in defined design conditions (for example minimum flows with a WC connected). The exact case comes from Approved Document H and the design guidance for the pipe size and flow.
Is more fall always better for drainage?
No. Too steep and the liquid can outrun the solids; too flat and the flow slows and risks blockage. The design guidance gives the working band for each pipe size and flow.
Method: Approved Document H (drainage fall)
Training & revision aids — live installations follow the full standard, the manufacturer’s instructions and calibrated instruments.