Last updated: 2026-06-16

Rise, Run, and Slope Ratio

A ramp is described by three quantities. The rise is the vertical height it has to climb — the height of a step, porch, or threshold. The run is the horizontal distance the ramp covers along the ground. The slope ratio, written as 1:N, states how many units of run accompany each unit of rise. A 1:12 ratio means twelve units along for every one unit up.

Given the rise and the ratio, the run follows directly:

The 1:12 ADA Rule

For new construction, the Americans with Disabilities Act sets a maximum running slope of 1:12. Expressed as a grade, that is a rise divided by run of about 8.3%, or roughly 4.76 degrees from level. Where there is room, a gentler 1:16 to 1:20 slope is preferred, because it is easier to climb and to descend under control.

Ramp Length by Pythagoras

The ramp surface is the hypotenuse of a right triangle whose legs are the rise and the run, so its length is

For a 45 cm (0.45 m) rise at 1:12, the run is $0.45 \times 12 = 5.4$ m, and the sloped ramp length is $\sqrt{0.45^2 + 5.4^2} = 5.42$ m. The slope is $0.45 \div 5.4 = 8.3\%$.

Gentler Slopes for Self-Propelled Users

A 1:12 slope assumes an assistant or a power-assist drive. A person propelling a manual chair without help finds 1:16 to 1:20 far more manageable, at the cost of a longer ramp. Halving the slope roughly doubles the run for the same rise.

The ADA also requires level landings at the top and bottom of a ramp and at turns, along with handrails and edge protection once the rise passes modest limits. Local accessibility codes add their own dimensions, so confirm any design against the governing rules before building.