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Drainage Fall Calculator

Drain, Gutter and Patio Fall From a Gradient, With Slope Percentage and Angle

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How the Drainage Fall Calculator works

This drainage fall calculator works out how far a pipe, channel, patio or gutter must drop over its length so water runs away. Enter the length of the run and a gradient such as 1 in 40 to get the total fall, or enter a fall you have measured to get the gradient, along with the slope as a percentage, the fall per metre and the angle.

Gradients are written as 1 in X, meaning one unit of drop for every X units along. A smaller X is steeper: 1 in 40 falls 25 mm per metre, 1 in 80 falls 12.5 mm per metre. Too shallow and water sits and solids settle; too steep and water outruns the solids in a foul drain.

How the drainage fall calculator works

The formula is fall = length ÷ X, and X = length ÷ fall, with slope % = 100 ÷ X. For example, a 10 m run at 1 in 40 needs a 250 mm fall, a 2.5% slope and 25 mm per metre. A second example: a 6 m patio with a measured 100 mm fall has a gradient of 1 in 60, 1.67% and 16.7 mm per metre.

Typical gradients (check local building regulations and the pipe maker's data):

To see how many millimetres a run needs, lay a string line from the top invert to the bottom one and measure down at intervals.

Fall over 1 m, 5 m and 10 m by drainage gradient

Gradient Slope Fall over 1 m Fall over 5 m Fall over 10 m
1 in 10 10.00% 100.0 mm 500 mm 1000 mm
1 in 15 6.67% 66.7 mm 333 mm 667 mm
1 in 20 5.00% 50.0 mm 250 mm 500 mm
1 in 25 4.00% 40.0 mm 200 mm 400 mm
1 in 30 3.33% 33.3 mm 167 mm 333 mm
1 in 40 2.50% 25.0 mm 125 mm 250 mm
1 in 50 2.00% 20.0 mm 100 mm 200 mm
1 in 60 1.67% 16.7 mm 83 mm 167 mm
1 in 70 1.43% 14.3 mm 71 mm 143 mm
1 in 75 1.33% 13.3 mm 67 mm 133 mm
1 in 80 1.25% 12.5 mm 62 mm 125 mm
1 in 90 1.11% 11.1 mm 56 mm 111 mm
1 in 100 1.00% 10.0 mm 50 mm 100 mm
1 in 110 0.91% 9.1 mm 45 mm 91 mm
1 in 120 0.83% 8.3 mm 42 mm 83 mm
1 in 125 0.80% 8.0 mm 40 mm 80 mm
1 in 150 0.67% 6.7 mm 33 mm 67 mm
1 in 160 0.62% 6.2 mm 31 mm 62 mm
1 in 175 0.57% 5.7 mm 29 mm 57 mm
1 in 200 0.50% 5.0 mm 25 mm 50 mm

FAQ

What fall does a drain pipe need?

A 100 mm underground foul drain is usually laid between 1 in 40 and 1 in 80, which is 25 to 12.5 mm of fall per metre. Larger pipes can run flatter. Your local building regulations set the minimums, and building control will check them before the trench is backfilled.

What is a 1 in 60 fall?

It means the surface drops 1 unit for every 60 units of length, which is about 16.7 mm per metre or a 1.67% slope. It is a common fall for patios and paths, enough to shed rain without the slope being noticeable underfoot.

How do I convert a gradient to a percentage?

Divide 100 by the X in 1 in X. So 1 in 40 is 2.5%, 1 in 80 is 1.25% and 1 in 100 is 1%. To go back, divide 100 by the percentage: a 2% slope is 1 in 50.

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