Slope calculator — percent grade, angle and ratio
Use any one unit for both rise and run (mm, m, inches or feet). The calculator returns percent grade, angle, ratio and the sloped length, which is how slopes are annotated on site plans, ramps and drainage details.
slope calculator
- Grade
- 8.333%
- Angle
- 4.764°
- Ratio (rise : run)
- 1 : 12
- Slope length
- 12.042same unit
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Formula
grade % = rise ÷ run × 100 · angle = atan(rise ÷ run) · ratio = 1 : (run ÷ rise) · length = √(rise² + run²)Percent grade is not the same as degrees: 100% is 45°, not 90°. Run is the horizontal distance, not the length measured along the slope. Accessibility, drainage and road limits come from the applicable code, not from this calculator.
Worked examples
| Input | Result |
|---|---|
| Rise 1, run 12 | 8.333% · 4.764° · 1 : 12 |
| Rise 0.5 m, run 25 m | 2% · 1.146° · 1 : 50 |
| Rise 3, run 3 | 100% · 45° · 1 : 1 |
Percent grade to degrees
| Grade | Angle / ratio |
|---|---|
| 1% | 0.57° · 1 : 100 |
| 2% | 1.15° · 1 : 50 |
| 5% | 2.86° · 1 : 20 |
| 8.33% | 4.76° · 1 : 12 |
| 10% | 5.71° · 1 : 10 |
| 15% | 8.53° · 1 : 6.67 |
| 20% | 11.31° · 1 : 5 |
| 25% | 14.04° · 1 : 4 |
| 33.3% | 18.42° · 1 : 3 |
| 50% | 26.57° · 1 : 2 |
| 100% | 45° · 1 : 1 |
Questions
How do I calculate slope percentage?
Divide the vertical rise by the horizontal run and multiply by 100. A 1 m rise over 20 m is 5%.
Is a 100% slope vertical?
No. 100% means rise equals run, which is 45°. A vertical face has an undefined percent grade.
What is a 1:12 slope in percent?
1 ÷ 12 × 100 = 8.333%, about 4.76°. It is commonly cited for ramps, but check the code that applies to your project.
Can I mix units?
No. Enter rise and run in the same unit. Convert first with the mm-to-feet or feet-to-mm tools.
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