Gradient and fall calculator
Work out a slope as 1 in N, a percentage and an angle, or the fall over a length at a set gradient.
Uses only the sizes you enter.
Result
A 100 mm rise over 4,000 mm is a gradient of 1 in 40, 2.5%, or 1.43°.
| As a percentage | 2.5% |
|---|---|
| As an angle | 1.43° |
| Length along the slope | 4,001.2 mm |
Show the workings
| Step | Sum | Answer |
|---|---|---|
| 1 in | 4,000 run ÷ 100 rise | 40 |
| Percentage | 100 ÷ 4,000 × 100 | 2.5% |
| Angle | tan⁻¹(100 ÷ 4,000) | 1.432 ° |
| Length along the slope | √(100² + 4,000²) | 4,001.25 |
Where the figures come from
This sum uses only the numbers you entered.
Three ways to write a gradient
- 1 in N: for every N along, it goes up or down 1. 1 in 40 is a drop of 1 m every 40 m, or 25 mm a metre.
- Percentage: the rise divided by the length, times 100. 1 in 40 is 2.5%.
- Angle: tan⁻¹ of the rise divided by the length. 1 in 40 is 1.43°.
Working out a fall
To find how much a patio, path or pipe must fall, divide its length by N. A patio 4 m long laid at 1 in 80 falls 4 ÷ 80 = 0.05 m, or 50 mm. The fall a paving maker or drain maker asks for is in their installation guide.