Drain fall & gradient Calculator: measuring notes and method
Measure levels at two points
Use the vertical fall and horizontal separation along the run. For 100 mm fall over 10 m, enter 0.1 m and 10 m: the result is 1%, equivalent to 1 in 100.
CHECK A PRACTICAL EXAMPLE
Measure fall independently of the pipe length
Both runs have 100 mm of measured vertical fall.
5 m horizontal run
- Gradient
- 2 %
- Slope angle
- 1.146 °
10 m horizontal run
- Gradient
- 1 %
- Slope angle
- 0.573 °
The results describe the measured gradients. They do not establish the required gradient, pipe size or compliance for a drainage installation.
Fixed comparison in metric units. Values are illustrative unless linked to a recorded project; changing the calculator above does not change this example.
Formula
Gradient (%) = rise ÷ horizontal run × 100. Angle = arctan(rise ÷ run).
Worked examples in metric and imperial units
Illustrative values match Reset example and stay fixed when you edit the calculator. Both columns describe the same calculation; converted inputs are rounded.
Metric worked example
- Total vertical fall
- 0.1 m
- Horizontal pipe run
- 10 m
Gradient: 1 %
Imperial worked example
- Total vertical fall
- 0.3281 ft
- Horizontal pipe run
- 32.8084 ft
Gradient: 1 %
Follow the calculation
Lengths in this working are in metres.
- Gradient = 0.1 ÷ 10 × 100 = 1%.
- Angle = arctan(0.1 ÷ 10) = 0.572939°.
Reviewed 19 October 2026. How calculations are checked · Report a correction