How to Size a Wheelchair Ramp Slope to the ADA 1:12 Rule

A ramp that looks fine on paper can still be the thing that traps someone at their own front door. The slope is the whole story. Too steep and a self-propelled wheelchair user stalls on the way up or loses the brakes on the way down. Too gentle and the ramp swallows your entire yard. The ADA gives you a single number to aim at, and once you understand it, sizing a ramp becomes arithmetic instead of guesswork.

This guide walks through what the 1:12 rule actually requires, how to turn a measured rise into a ramp length, where landings come in, and how to tell a safe ramp from one that is quietly too steep.

The one number that matters: 1:12

The Americans with Disabilities Act sets the maximum running slope for an accessible ramp at 1:12. Read that as rise to run: for every 1 inch of vertical rise, you need at least 12 inches of horizontal ramp. A 1-inch lip needs a foot of ramp. A 10-inch rise needs 10 feet.

That ratio shows up in three forms, and they all describe the same slope:

Ratio: 1:12, the shorthand builders use.
Percent grade: 8.33%, which is 1 divided by 12, times 100.
Angle: about 4.76 degrees, the arctangent of rise over run.

The 5-degree ceiling is worth memorizing. An accessible ramp never tilts more than roughly 5 degrees, which is far shallower than it feels when you picture it. That shallowness is the point. The whole reason these ramps get long is that they are deliberately gentle. If your design reads steeper than 8.33% or 4.76 degrees, it is not ADA compliant, full stop.

The single most common mistake here is reading 1:12 as 12%. It is not. The grade is 8.33%, and treating it as 12% lets you talk yourself into a ramp that is genuinely too steep. Whenever you check a number, look at the ratio, the percent, and the angle together so they can never drift apart. The wheelchair ramp slope calculator shows all three side by side for exactly this reason.

Turn a rise into a ramp length

The math is simple once you have a clean rise measurement. Measure the vertical distance from the lower surface (the walkway, the driveway) up to the landing surface (the threshold, the deck). That vertical number is your rise. Multiply it by 12 and you have the minimum run at 1:12.

The rule of thumb people remember is one foot of ramp per inch of rise. A 7-inch step needs 7 feet of ramp. A 22-inch porch needs 22 feet.

Worked example: a 24-inch porch

Say you measure 24 inches from the front walkway up to the door threshold. At 1:12:

Run needed: 24 inches × 12 = 288 inches = 24 feet of ramp.
Slope: 8.33%.
Angle: about 4.76 degrees.

Twenty-four feet is longer than most front yards are deep. That single number tells you, before you have spent a dollar on lumber, that a straight shot is not going to work. You will need a switchback with a landing, or you will need to rethink where the ramp lands. This is the most valuable thing the calculation does: it kills bad layouts early.

One unit trap to watch: keep the rise unit and the run unit consistent. If the rise is in inches, the run comes out in inches too. A 24-inch rise gives 288 inches of run, which is 24 feet, not 288 feet. Mislabel that and you will think a small porch needs a ramp the length of a city block.

When you need a landing

ADA caps a single ramp run at 30 inches of vertical rise. Climb more than that and you cannot use one continuous ramp. You have to break the climb into runs separated by a level landing at least 60 inches long, which gives a user room to rest, brake, and turn.

So a 36-inch deck is not a single 36-foot ramp. It is two runs with a landing between them. A clean way to split it:

Lower run: 30-inch rise at 1:12 = 30 feet.
A level landing, at least 60 inches long.
Upper run: 6-inch rise at 1:12 = 6 feet.

Landings also belong at the top and bottom, and at any point where the ramp changes direction, like the turn in a switchback. People routinely forget this limit, size a single 36-foot run, find it doesn't fit, and redesign from scratch. Size each run separately from the start and lay the whole assembly out on paper before you dig.

Safe, too steep, and the residential exception

Here is where a number turns into a yes-or-no answer. Anything at or below 8.33% passes the 1:12 check. Anything above it is too steep for an unassisted manual chair.

Reverse the math to test a ramp you are thinking of buying. A vendor sells a 6-foot folding ramp, and your van floor sits 14 inches off the ground. A 14-inch rise over a 72-inch run works out to about 19.4%, more than double the limit. That ramp is dangerous for an unassisted user, and now you know to look at an 8 or 10 foot model instead.

A few reference points:

1:16 (6.25%) is gentler than required and is often the better call for heavy power chairs and long runs.
1:12 (8.33%) is the commercial maximum.
1:8 (12.5%) is steep and only allowed under a narrow residential exception, and only over tiny rises: 1:8 for a rise up to about 3 inches, 1:10 up to about 6 inches.

That residential exception is a last resort for existing homes where space is genuinely tight, not a shortcut for new construction. A 1:8 ramp is hard work for a manual chair even over three inches. Plan new builds at 1:12 or gentler, and always confirm the final design against your local building code, which can be stricter than the federal floor.

How I run a real measurement

When I size a ramp for someone, I never trust a single tape pull. I measure the rise three times at the points the ramp will actually touch, because a "level" walkway often slopes a little and a deck board can sit a quarter inch proud. I take the largest rise of the three and design to that, then add the landings before I touch the length math, because the landings change where the runs can physically go. Only then do I work out the run. Doing it in that order, landings first and length second, has saved me from at least one layout that fit the yard on paper and not in real life. The arithmetic is trivial; the measuring discipline is the part that actually keeps people safe.

Quick checklist

Measure the vertical rise, largest of a few readings, in one consistent unit.
Multiply by 12 for the minimum run at 1:12 (one foot of ramp per inch of rise).
Confirm the slope reads 8.33% / 4.76 degrees or gentler.
If the rise exceeds 30 inches, split it into runs with a 60-inch landing between.
Treat anything steeper than 1:12 as non-compliant unless a documented small-rise exception applies.
Check the result against your local code.

If your ramp replaces a flight of stairs, it helps to know the original rise and going of those steps before you design around them; the stair calculator gives you that rise figure to feed straight into the slope math. Measure once, run the numbers, and you will know whether your ramp is safe long before the first board gets cut.

Made by Toolora · Updated 2026-06-13