How to Calculate a Wheelchair Ramp Slope

The short version

The ADA rule is 1:12 — one inch of ramp length for every inch of vertical rise, the steepest slope allowed. Measure the rise straight down from the top surface, multiply by 12 to get the run in inches, divide by 12 for feet. A 24-inch porch step needs 24 feet of ramp. If the rise tops 30 inches, split the ramp with a level landing. That is the whole calculation — the ramp slope calculator runs it with a full 1″–36″ reference table, but the hand method here works on any job site.

Why ramps are so much longer than people expect

Almost everyone underestimates ramp length by a factor of three or four, because the brain pictures the slope, not the ratio. A 24-inch step feels like a 6-foot board would do it. It will not: at 1:12, that step needs 24 feet — longer than most driveways are wide. The ratio is about physics: pushing 200–300+ pounds up a 1:8 slope demands force most people cannot sustain, and on the way down gravity steers. At 1:12 the effort is manageable and a chair will not tip backward. Every inch steeper than 1:12 trades a little length for a lot of danger — which is why the number is a maximum, never a target.

Step 1: measure the rise (vertically)

Rise is the vertical distance from the top walking surface — the porch floor, the doorway threshold — straight down to the ground where the ramp will land. Hold a 4-foot level out from the top edge and run the tape from the level down. The classic mistake is measuring along the slope of the ground and calling that the rise; on a sloped yard that reads short, and a short rise designs a short ramp — which is exactly how too-steep ramps get built. On uneven ground, measure at the highest point of the landing area: the ramp must clear the worst case. Write the number down in inches. A typical porch step is 18–24 inches; a full flight of three steps is 24–30.

Step 2: multiply by 12 — that is your run

Run (inches) = rise (inches) × 12. Then divide by 12 for feet — which, at exactly 1:12, is a party trick: the run in feet equals the rise in inches. 18″ rise → 18 ft of ramp. 24″ → 24 ft. 30″ → 30 ft. In percent terms the grade is 100 ÷ 12 = 8.33%; the angle is arctan(1/12) = 4.8°. If you are using a gentler ratio, multiply by 16 or 20 instead — a 24″ rise at 1:16 needs 32 feet. The trade is always the same: comfort costs length.

Step 3: check the 30-inch rule

ADA caps any single ramp run at 30 inches of rise. At 1:12 that is 30 feet of ramp — and it is the reason long ramps zigzag. A 36″ rise is not one 36-foot ramp; it is two runs (say 18″ + 18″) joined by a level landing at least 60 inches long (60″×60″ where the ramp changes direction). The landing is a rest stop and a turning platform in one. Every ramp — even a short one — also needs a level 60″×60″ landing at the top and bottom, so the chair is fully level before the slope begins and after it ends.

Step 4: width, handrails, and edges

The slope gets the attention, but these three finish the ramp. Width: 36 inches clear between handrails is the ADA minimum; 42–48″ is noticeably more comfortable and worth it on a home build. Handrails: required on both sides when the rise exceeds 6″ or the run exceeds 72″ — i.e., nearly always — continuous, 34–38″ high, grippable. Edge protection: a curb, rail, or barrier so a wheel cannot slip off the side. Skip any of these and the ramp meets the slope rule while failing the safety one.

The three mistakes that make ramps unusable

1. Designing to the slope instead of the rise. "We have 12 feet of yard, so the ramp is 12 feet" — that sizes the ramp to the space, then hopes the slope works out. It never does: 12 feet of ramp on a 24″ rise is 1:6, twice the legal steepness. Measure the rise first; let the rise dictate the length; then find the space (switchbacks, L-shapes) to fit it.

2. Forgetting the landings in the length budget. The 24-foot run for a 24″ rise is ramp surface only — the top and bottom 5-foot landings add ~10 more feet of footprint. A 36″ rise with a mid-landing needs roughly 36 + 5 + 5 + 5 = 51 feet of linear space. Budget the whole footprint before you buy lumber.

3. Building at exactly 1:12 on a bouncy structure. Wood flexes; a ramp that measures 1:12 empty can hit 1:11 under a 300-pound load. Build a hair gentler than the maximum — or verify the slope under load with a digital level. The code maximum is a ceiling to stay under, not a line to hug.

Practice: three rises

Cover the answers, then check against the calculator and table.

When 1:12 does not apply

Two honest exceptions. Portable threshold ramps for 1–2 inch lips (doorway saddles) run steeper — they are bridging a bump, not a staircase, and the chair crosses in one push. Existing buildings sometimes qualify for ADA exceptions where space makes 1:12 genuinely impossible — but "impossible" is a high bar, and a home build never qualifies. A ramp built to the rule works for every chair, every pusher, every time.

The bottom line: measure the rise vertically, multiply by 12, split at 30 inches, land top/middle/bottom, rail both sides. Five steps, one safe ramp — and the calculator plus the 36-row table do the arithmetic while you measure.