A ramp slope calculator that converts between rise, run, ramp length, angle and percent grade in one step — and checks your result against ADA §405.2 and UK BS 8300 accessibility standards. Whether you are designing a wheelchair ramp for a commercial building, calculating the gradient of a loading bay, checking a kerb drop, or verifying a driveway incline, every representation you need appears instantly in your browser with nothing sent to a server.
How it works
A ramp is a right triangle. The rise is the vertical height gained, the run is the horizontal distance covered, and the ramp length is the slant surface — the hypotenuse. Given any two of these three values (or the angle, or the percent grade), every other dimension is fully determined by trigonometry.
The core formulae are:
slope % = (rise ÷ run) × 100
angle θ = arctan(rise ÷ run)
ramp length = √(rise² + run²)
The calculator supports four input modes so you can start from whichever two values you already have:
- Rise + Run — the most common field measurement
- Rise + Ramp length — useful when you have measured along the surface itself
- Run + Angle — useful in CAD or when a transit gives you the angle
- Slope % + Run — start from a target grade and find the resulting rise and length
Results are shown in six representations simultaneously: angle in degrees, slope percent, 1:n ratio, decimal slope, per-mille grade, and either mm/m (metric) or in/ft (imperial).
ADA and BS 8300 compliance
The ADA Standards for Accessible Design §405.2 cap running slope at 1 in 12 (8.33 %) for any ramp serving accessibility. The slant length of a single ramp run may not exceed 9 m (30 ft); longer rises must include level landings. Cross-slope is separately limited to 1:48 (2.08 %).
UK BS 8300:2018 recommends a preferred gradient of 1:20 (5 %) where space permits and sets an absolute maximum of 1:12 for short ramps with a maximum rise of 500 mm. The calculator flags both thresholds with coloured badges. If the slope fails ADA, an inline hint tells you precisely what rise or run to change to bring it into compliance.
Worked example
A shop entrance has a step of 150 mm (0.15 m) that needs to be ramped. What length of ramp is needed to meet ADA 1:12?
- Required run = rise × 12 = 0.15 × 12 = 1.80 m
- Ramp length = √(0.15² + 1.80²) = √(0.0225 + 3.24) = 1.806 m
- Slope % = (0.15 ÷ 1.80) × 100 = 8.33 % — exactly 1:12, ADA compliant
Now imagine the available space is only 1.50 m. Enter Rise = 0.15 m, Run = 1.50 m:
- Slope % = (0.15 ÷ 1.50) × 100 = 10 % — exceeds 1:12, fails ADA
- The calculator will tell you: reduce rise to 0.125 m OR extend run to 1.80 m
| Rise | Run | Slope | Angle | ADA 1:12? |
|---|---|---|---|---|
| 0.15 m | 1.80 m | 8.33 % | 4.76° | Yes |
| 0.15 m | 1.50 m | 10.00 % | 5.71° | No |
| 0.30 m | 3.60 m | 8.33 % | 4.76° | Yes |
| 0.50 m | 10.0 m | 5.00 % | 2.86° | Yes (BS pref.) |
Formula note
All computations use double-precision floating-point arithmetic. The angle is calculated as atan2(rise, run) to correctly handle edge cases. The “1:n” ratio divides 1 by the decimal slope (rise ÷ run), so a 5 % slope gives a ratio of 1:20. Per-mille is slope % × 10, matching the convention used in road and railway engineering standards (e.g. EN 13803 track geometry).