Slope Percent and Angle Calculator (Rise over Run)
Slope as a percent is rise divided by run, times 100. A 1-in-12 ramp is 8.33% and about 4.76°. The angle from horizontal is arctan(rise/run) in degrees. Use the same unit for rise and run. This is geometry, not a building-code check for accessibility ramps.
Percent grade8.33%
Angle from horizontal4.76°
Rise : run1:12
How this calculation works
Percent grade = (rise ÷ run) × 100. Run must be positive (horizontal distance).
Angle in degrees = atan(rise/run) × (180/π). A 45° slope is 100% grade (rise = run).
A negative rise is a downhill grade. The ratio is printed as rise:run without reducing.
Road signs that say “8%” mean this percent grade, not 8 degrees. Eight degrees is about 14.1%.
Worked example
Rise 1, run 12 → 8.333% and 4.76°. Rise 1, run 1 → 100% and 45°. Rise 12 on run 100 → 12%.
Frequently asked questions
Percent vs degrees — which do roads use?
Most highway grades are posted in percent. Roof pitch is often rise:run in inches (e.g. 6:12), which is the same ratio.
ADA ramp slope?
US ADA has specific ratios (commonly cited 1:12 max for some ramps). This calculator does not certify a ramp. Check the current standard.
Can run be vertical?
Run is the horizontal leg. A vertical wall has undefined percent (division by zero) and is rejected.
Cross-slope?
Same math: rise of the tilt over the width of the path.
Percent greater than 100%?
Yes — steeper than 45°. A 200% grade is rise = 2 × run, about 63.4°.
Are inputs stored?
No. Browser only.
This tool is provided for general information only. Verify important figures independently. · Last reviewed: August 25, 2026