Slope percentage calculator
Slope as a percentage is rise divided by run, multiplied by 100. Put the two distances in the boxes above; the notes below cover units, degrees and the one measurement people get wrong.
12 × 12 = 144
Adding 12 to itself 12 times gives the same answer.
The formula
Slope % = rise ÷ run × 100. Rise is the vertical change, run is the horizontal distance travelled. Both must be in the same unit before you divide: metres with metres, feet with feet.
A path that climbs 8 m over 200 m of horizontal distance is 8 ÷ 200 = 0.04, then × 100, so 4%.
Run is horizontal, not along the surface
This is the mistake that quietly shrinks every answer. The run is the flat distance from start to finish as seen from above, not the length you walk along the slope. On gentle grades the two are close enough not to notice; on steep ones the gap is large. If all you have is the sloped length, the horizontal distance is the third side of a right triangle.
Percent, degrees and ratios
- 100% is 45 degrees, not vertical. A percentage above 100 simply means the rise is bigger than the run.
- 1 in 12 means one unit up for twelve along, which is 1 ÷ 12 × 100 = 8.33%.
- Roof pitch written as 4/12 works the same way: 33.3%.
- Millimetres per metre is the same number as a tenth of a percent: 20 mm/m is 2%.
When the answer looks wrong
A figure in the thousands means one distance was in millimetres and the other in metres. A figure near zero on a slope you can clearly see means rise and run went in the wrong way round: dividing run by rise gives the reciprocal, not the grade. Estimate first, one decimal place is enough, and compare.
The multiply-by-100 step is the same one covered in how to find a percentage on a calculator, and the reverse conversion sits in the decimal to percentage calculator. Everything else is on the calculators page.