Ramp Slope Calculator

Background.

The quick answer to how long a wheelchair ramp must be is one foot of ramp for every inch of rise. That is the 1:12 slope the 2010 ADA Standards set as the steepest allowed for a new ramp, and it makes a good first check: a porch 24 inches up needs at least 24 feet of sloped run.

The quick answer is also too short. ADA 405.6 limits any single run to 30 inches of rise, and every run needs a level landing at its top and bottom, at least 60 inches long. A taller rise therefore breaks into several runs with landings between them, and a 36-inch rise becomes 36 feet of slope but 51 feet of ramp. Entering the rise with either a target slope or the run you have gives all of these figures: run, slope, runs, landings and total length.

The slope is then checked against the standard you pick. Under the ADA, slopes between 1:12 and 1:8 are allowed only in existing buildings short of space, and only for a few inches of rise; anything steeper than 1:8 is prohibited outright; and 1:20 or gentler is not a ramp at all but a sloped walk. Houses fall under section R311.8 of the International Residential Code, which is looser. In one widely reproduced wording it requires 1:12 only for the ramp at the required exit door and accepts 1:8 elsewhere, but a second wording in circulation holds every residential ramp to 1:12.

Slope and length are only part of a compliant ramp. Width, cross slope, surface, handrails and edge protection are not checked here, local amendments can change any figure, and a licensed architect, engineer or accessibility specialist should confirm the design before it is built.

What is ramp slope calculator?

Ramp slope is vertical rise divided by horizontal run, written as a ratio (1:12), a percentage (8.33%) or an angle (4.76°). Codes use the ratio, where a larger second number means a gentler ramp. Run is measured level along the ground, not along the sloping surface; the surface length is the longer hypotenuse, and it is what you buy decking and handrail by. A ramp run is one continuous sloped section between landings. Running slope is the slope in the direction of travel; cross slope, across the ramp, is limited separately by the ADA to 1:48.

How to use this calculator.

1. 01Under "What are you working out?" choose the length needed for a target slope, or the slope you get from the run you have.
2. 02Choose the standard: the 2010 ADA Standards for businesses and public facilities, or IRC R311.8 for a house, using the egress option only for the ramp at the required exit door.
3. 03Measure the total rise in inches between the finished surfaces, over the door threshold; if the ground falls away, measure where the bottom landing will sit.
4. 04Enter the target as the N in 1:N, or the horizontal run available in feet for the sloped part only.
5. 05For ADA work keep 30 in as the maximum rise per run and 60 in as the landing length.

The formula.

With the rise converted to feet, the run for a 1:N slope is run = rise × N, and the slope from a known run is N = run ÷ rise. The other forms follow: slope % = 100 ÷ N, angle θ = arctan(1 ÷ N), and surface length = √(rise² + run²).

For the ADA, the number of runs is the rise divided by the maximum rise per run, rounded up, and landings = runs + 1: one at the bottom, one at the top and one between each pair of runs, as 405.7 requires. The IRC sets no rise limit per run, so its options always show one run and two landings. Total length = run + landings × landing length. At 1:20 or flatter the surface is not a ramp under 403.3, so the calculator reports one run, no landings and a total length equal to the run.

The verdict depends only on N. Under the ADA: N of 20 or more is a walking surface under 403.3; 12 up to 20 complies with 405.2; 10 up to 12 is allowed only in existing facilities and for no more than 6 inches of rise; 8 up to 10 likewise for no more than 3 inches; below 8 is prohibited by footnote 1 to Table 405.2. Because the code says "not steeper than 1:12", exactly 1:12 passes. For the IRC egress ramp, 12 or more complies, 8 up to 12 is available only under the exception for technical infeasibility, and below 8 fails; for other IRC ramps the line is 8.

A worked example.

Frequently asked questions.

References& sources.

1. [1]United States Department of Justice — 2010 ADA Standards for Accessible Design (28 CFR Part 36, revised as of 2010). Sections 403.3 (walking-surface slope 1:20 and cross slope 1:48), 405.2 and Table 405.2 with footnote 1 (ramp running slope 1:12; existing-facility allowances of 6 in of rise between 1:12 and 1:10 and 3 in between 1:10 and 1:8; steeper than 1:8 prohibited), 405.3, 405.5, 405.6, 405.7, 405.7.3, 405.7.4, 405.8 and 405.9.
2. [2]United States Access Board — ADA Accessibility Standards, Chapter 4: Accessible Routes, Section 405 Ramps. The Access Board is the agency that writes the underlying guidelines the DOJ adopts, and is used here as an independent copy of the same clause text.
3. [3]International Code Council — International Residential Code, Section R311.8 Ramps (R311.8.1 Maximum slope, R311.8.2 Landings required, R311.8.3 Handrails required, R311.8.3.1 Height). Named as the governing residential section. ICC's reader is access-gated and returns HTTP 403 to automated retrieval, so the clause text used on this page was taken from the two municipal reproductions cited below rather than from here.
4. [4]Town of Essex, Connecticut, Building Department — Residential Stairways and Ramps handout reproducing the 2015 International Residential Code portion of the 2018 Connecticut State Building Code. Verbatim source for the two-tier R311.8.1 slope limit (1:12 for the egress-door ramp, 1:8 for all others, exception for technical infeasibility), for the R311.8.2 wording that gives a 36 in landing width perpendicular to the ramp and no landing length, and for R311.8.3 through R311.8.3.3 including the 34 to 38 in handrail height.
5. [5]City of Richmond, Kentucky, Codes Enforcement — Section R311.8 Ramps handout. Verbatim source for the second, incompatible reproduction of R311.8 that is also in circulation: a single-tier R311.8.1 holding every ramp to one unit vertical in 12 units horizontal with no 1:8 allowance, and an R311.8.2 requiring a minimum 3 ft by 3 ft landing at the top and bottom of ramps, where doors open onto ramps, and where ramps change direction.