How to size a ramp
A ramp's slope is given as a ratio of run to rise. At 1:12 — the ADA standard — the ramp runs 12 inches horizontally for every 1 inch of vertical rise: run = rise × ratio. A 24-inch rise needs 24 × 12 = 288 inches, or a 24-foot run, which works out to an 8.33% grade.
How to use this calculator
Enter the rise in inches (how high you need to reach) and the slope ratio (12 for 1:12). The result shows the required run length in feet and the grade. A gentler ratio like 1:16 or 1:20 is easier to use but needs more length. Where a ramp will not fit, the stair calculator works out risers and treads.
Accessibility guidelines
The ADA caps ramp slope at 1:12 (8.33%) with a maximum 30-inch rise per run before a landing, and level landings top and bottom. For occupied or icy areas a gentler 1:16–1:20 is safer. For a slope expressed as a percentage, see the elevation grade calculator.
Surface and edges matter as much as slope. Use a non-slip finish, and add edge protection such as a raised curb so a wheel cannot roll off the side. Handrails are generally expected once the rise passes 6 inches. Roofs express the same idea in twelfths, as the roof pitch calculator shows.
Frequently asked questions
- How long should a wheelchair ramp be?
- At the ADA 1:12 ratio, allow 1 foot of ramp for every inch of rise. A 24-inch rise needs a 24-foot ramp.
- What is the maximum ADA ramp slope?
- 1:12, or 8.33% grade, with a maximum 30-inch rise before a level landing.
- What slope ratio should I use?
- 1:12 is the steepest allowed. Gentler ratios like 1:16 or 1:20 are easier and safer where there's room.
- How do I convert ramp ratio to a percent grade?
- Divide 100 by the ratio. 1:12 is 100 ÷ 12 = 8.33%.
- How much landing space does a ramp need?
- Allow a level landing at least 5 feet long at the top and bottom, and another wherever the ramp changes direction. Landings are part of the total length you need, not extras.