About this calculator
Total drop for a drain or waste pipe at a given gradient, so you can set invert levels at each end.
How it’s calculated
Fall = length ÷ gradient ratio. 1:40 = 2.5%, 25 mm per metre.
Worked example
With these inputs:
- Pipe run length
- 12 m
- Gradient
- 1:40
the result is:
- Total fall
- 300 mm
- Gradient
- 1:40 = 2.5%
- Angle
- 1.43°
- Fall per metre
- 25 mm
Tips from the trade
- Approved Document H: 100 mm drains usually 1:40, or down to 1:80 with at least one WC connected.
- Too steep can be as bad as too flat: water outruns the solids.
Results are estimates for planning and pricing. Check quantities against your drawings, the manufacturer’s data and the Building Regulations before ordering.