Free calculator

Slope & fall

Height difference, percent and 1:X at once.

Height difference90 mm
Fall15 mm/m
Slope1.5 %
Ratio1:67

Guide values: drain pipes often 10–20 mm/m, ground away from the foundation at least 15 mm/m (1:50) for the first few metres, fall to a floor drain in a shower 1:150 (7 mm/m) to 1:50 (20 mm/m). Follow the applicable industry rules for your application.

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An estimate, not a finished calculation – check against drawings, regulations and supplier data before pricing or ordering.

Preview of slope & fall
This is how slope & fall looks

How to work out the fall

Enter the length and the fall, and the height difference comes straight back – plus the slope in both percent and as a ratio (1:X). Handy for drainage, ground and roofs, where fall is often given in different ways.

Diagram: fall as length, height difference and ratio 1:X
Fall is expressed as mm/m, percent and 1:X. 20 mm/m = 2% = 1:50.
  1. Enter the length in metres.
  2. Enter the desired fall in mm per metre.
  3. See the height difference, the slope in percent and as 1:X.

Guide values

The fall you need depends on the application and is usually set out in a drawing or industry rule. The same value can be given as mm per metre, percent or ratio (1:X) – the calculator converts between all three, so you can match the requirement without doing the maths by hand.

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Frequently asked questions

How do I work out the fall?
Fall is often given in mm per metre. The height difference is the length times the fall: 6 m at 15 mm/m gives 90 mm. The calculator also shows percent and ratio.
What fall should the drain have?
Guide values are often 10–20 mm per metre for drain pipes, but it depends on pipe dimension and application. Follow the applicable industry rules.
How much fall away from the foundation?
The ground should slope away from the house, often at least 15 mm per metre (1:50) for the first few metres, to drain water away.
Does it cost anything?
No, the calculator is free and requires no account.