Gradient Calculator
Gradient between rise and run as 1 in X, percent and degrees, or the fall over a length: for drains, paths, patios, driveways and ramps.
A gradient is a rise or fall over a horizontal distance, written several ways: 150 mm over 6 m is 1 in 40 (1:40), 2.5%, 1.43° or 25 mm per metre. This calculator converts between all of them, works out the fall over a length for a given gradient (a 1:60 fall over 4 m is 67 mm), or the length you need to climb a given rise. It's useful for drains and soakaways, patios and paths that need to shed water, driveways, ramps and landscaping. A table of common gradients, from 1:8 to 1:100, shows each as a percentage, angle and fall per metre or foot.
- Ratio
- 1 in 40
- Percentage
- 2.5%
- Per metre
- 25 mm
- Roof pitch
- 0.3/12
1:40
1.43°
0.3 in per ft
rise per 12 of run
Common gradients
| Ratio | % | Degrees | mm per m | in per ft |
|---|---|---|---|---|
| 1:8 | 12.50 | 7.13 | 125.0 | 1.500 |
| 1:12 | 8.33 | 4.76 | 83.3 | 1.000 |
| 1:14 | 7.14 | 4.09 | 71.4 | 0.857 |
| 1:20 | 5.00 | 2.86 | 50.0 | 0.600 |
| 1:40 | 2.50 | 1.43 | 25.0 | 0.300 |
| 1:60 | 1.67 | 0.95 | 16.7 | 0.200 |
| 1:80 | 1.25 | 0.72 | 12.5 | 0.150 |
| 1:100 | 1.00 | 0.57 | 10.0 | 0.120 |
How it was worked out
- 150 mm ÷ 6 m = 0.025 (rise per unit of run)
- As a percentage: 2.5%; as a ratio: 1 in 40; as an angle: tan⁻¹ = 1.43°
Use the fall your drain designer, code or product specifies: minimum falls for drains depend on the pipe size and what it carries.
Common use cases
- Set the fall on a patio, path or drive so water runs off
- Check a drain run's gradient
- Convert a gradient from a plan into mm per metre or inches per foot
- Work out how long a slope or ramp must be
How to use
- Choose what to work out: the gradient, the fall over a length, or the length for a rise.
- Enter the rise or fall and the horizontal length, or the gradient as 1 in X, a percentage or degrees.
- Read the gradient in every form, and the fall or length.
- Look up common gradients in the table.
FAQ
How do I convert 1:40 to a percentage?
Divide 1 by 40 and multiply by 100: 2.5%. 1:60 is 1.67%, 1:80 is 1.25% and 1:100 is 1%.
How much fall does a patio need?
Paving guides commonly recommend a fall of about 1:60 to 1:80 away from the house, so water drains off rather than pooling. Over 4 m, 1:60 is a fall of about 67 mm.
Is a gradient the same as a slope or pitch?
They describe the same thing. Gradients are usually given as 1 in X or a percentage, slopes as a percentage or angle, and roof pitch in the US as a rise per 12 of run: 1:12 is 1/12, or 8.33%.
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Results are estimates for planning and buying materials, worked out from the figures you enter and typical values (weights, coverage, sizes) that vary by product and supplier. They are not engineering, building or professional advice. Check your local building code, the product maker's instructions and your supplier's figures before you buy or build, and use a qualified professional for structural, electrical and gas work.