The two limits that shape every accessible ramp
Ramp geometry is set by two independent rules in the 2010 ADA Standards, and both bite. Section 405.2 caps the running slope of a ramp at 1:12. Section 405.6 caps the rise of any single run at 30 inches. The first fixes how long the ramp is; the second fixes how many pieces it comes in.
The first rule is the one everyone knows: one unit of run for every unit of rise, twelve times over. A 30 inch rise needs 360 inches - 30 feet - of ramp. It is a surprising amount of space the first time you calculate it, and it is the reason accessible ramps switch back, wrap around corners, or run along a building rather than straight out from a door.
The second rule is the one people miss. Once the rise exceeds 30 inches, the ramp must be broken by a level landing, because a continuous 40 foot climb with nowhere to stop is exhausting and unsafe for someone propelling a chair by hand. A 48 inch rise therefore becomes two runs with a landing between them, and that landing adds 5 feet to the footprint on top of the 48 feet of ramp.
Landings are also required at the top and bottom of every ramp, at least 60 inches long and at least as wide as the ramp itself. Where a ramp changes direction, the landing must be at least 60 by 60 inches so a chair can turn. Those landings are structure you have to build and space you have to find, so they belong in the footprint from the start.
Run, slope, angle and how many runs
The run follows directly from the rise: run = rise × k, where k is the slope denominator. At 1:12 a 30 inch rise gives 360 inches. At 1:16 the same rise gives 480 inches, and at 1:20 it gives 600 inches - and a 1:20 slope is not classed as a ramp at all under the ADA, which is why long gentle approaches avoid the handrail and landing requirements entirely.
Slope as a percentage is 100 ÷ k. A 1:12 ramp is 8.33 percent, 1:16 is 6.25 percent, and 1:20 is 5.00 percent. The angle is arctan(1/k), which for 1:12 is 4.764 degrees. That angle is small enough to be worth noting: an accessible ramp is far shallower than most people picture, and a slope that looks gentle to a walker can be at the limit of what someone can push unassisted.
The number of runs is ceil(rise ÷ 30). One run up to 30 inches of rise, two up to 60, three up to 90. Intermediate landings are one fewer than the number of runs, since the top and bottom landings are counted separately.
Total occupied length is the sum: the sloped run, plus one landing at the bottom, one at the top, and one for each intermediate break. That figure assumes the ramp runs straight. A switchback halves the length but roughly doubles the width and requires 60 by 60 inch turning landings, so it trades one dimension for another rather than saving area.
Worked example: a 30 inch rise to a front door
A front door sits 30 inches above the path. You want a straight ADA-compliant ramp at the maximum slope, 36 inches wide, with 60 inch landings.
- Ramp run. 30 in × 12 = 360 in = 30.00 ft of sloped surface.
- Slope. 100 ÷ 12 = 8.333 percent, an angle of arctan(1/12) = 4.764 degrees.
- Number of runs. ceil(30 ÷ 30) = 1, so no intermediate landing is required - the rise is exactly at the limit.
- Landings. One at the bottom and one at the top, 60 in each, so 120 in = 10 ft.
- Total length. 360 + 120 = 480 in = 40.00 ft from the start of the bottom landing to the door.
- Decking area. 36 in × 360 in = 12,960 in² ÷ 144 = 90.0 ft² of ramp surface, plus 2 × (36 × 60) ÷ 144 = 30.0 ft² of landings, so 120 ft² in all.
- Handrails. The run is 30 ft, plus a 12 in extension at each end - 2 ft per side - so 32 ft per side and 64 lineal feet for both.
Now add one inch to the rise. At 31 inches the run grows only to 31 ft, but the run count goes to two, an intermediate landing appears, and the total length jumps from 41 ft to 46 ft. That step is worth knowing before the site is surveyed, because a single inch of grading at the bottom can remove a whole landing from the design.
Choosing a slope and reading the footprint
Treat 1:12 as a ceiling, not a target. It is the steepest slope the standard permits, and it is genuinely hard work for someone propelling a manual chair without help. Where space allows, 1:16 or 1:20 is markedly easier, and a 1:20 approach is not classified as a ramp at all - which means no handrails and no landing rule, and often a better result for the same money.
Compare the total footprint against the site before anything else. A 40 foot straight ramp does not fit in most front gardens, and the usual answers are to run the ramp along the face of the building, to switch back with a 60 by 60 inch turning landing, or to regrade so the rise the ramp has to climb is smaller. Regrading is often the cheapest of the three, because every inch removed takes a foot of ramp with it.
The steeper slopes in the list - 1:10 and 1:8 - exist for a narrow purpose. The ADA permits steeper slopes only for specific alterations to existing buildings where space is genuinely constrained, and residential ramps under IRC R311.8 follow a different rule set. Neither is available for new construction on an accessible route, which is why the calculator flags them as an error rather than a warning.
Cross slope matters as much as running slope and is easier to get wrong. The ADA limits ramp cross slope to 1:48, so a 36 inch wide ramp may fall no more than 0.75 inches across its width. On an outdoor timber ramp that is roughly the tolerance of the framing itself, so it has to be built to, not hoped for.
For the stair alongside the ramp, the stair rise and run calculator handles the geometry, and the deck footing size calculator sizes the footings under a timber ramp's posts.
Ramp run and footprint by rise
| Rise (in) | 1:12 run (ft) | 1:12 runs | 1:12 total (ft) | 1:16 run (ft) | 1:20 run (ft) |
|---|---|---|---|---|---|
| 6 | 6.00 | 1 | 16.00 | 8.00 | 10.00 |
| 12 | 12.00 | 1 | 22.00 | 16.00 | 20.00 |
| 18 | 18.00 | 1 | 28.00 | 24.00 | 30.00 |
| 24 | 24.00 | 1 | 34.00 | 32.00 | 40.00 |
| 30 | 30.00 | 1 | 40.00 | 40.00 | 50.00 |
| 36 | 36.00 | 2 | 51.00 | 48.00 | 60.00 |
| 48 | 48.00 | 2 | 63.00 | 64.00 | 80.00 |
| 60 | 60.00 | 2 | 75.00 | 80.00 | 100.00 |
| 72 | 72.00 | 3 | 92.00 | 96.00 | 120.00 |
Total length = run + (intermediate landings + 2) × 5 ft. The jump between 30 and 36 in of rise is 11 ft, not 6, because the second run brings an intermediate landing with it.
What goes wrong on ramp layouts
- Forgetting the landings. Two 5 ft landings are non-negotiable and they are 10 ft of the footprint before the ramp starts.
- Measuring rise to the threshold rather than the finished landing. The rise must be measured to the surface a chair will actually sit on at the top, including any decking build-up.
- Missing the 30 inch run limit. A 36 in rise is not a single 36 ft ramp; it is two runs and a landing.
- Ignoring cross slope. 1:48 across the width is easy to exceed on a timber deck built to normal carpentry tolerances.
- Building a 34 inch clear width. Handrails, posts and edge protection all eat into the 36 in minimum, which is measured between the rails, not between the framing.
- No edge protection. ADA 405.9 requires a curb, rail, wall or projecting surface that stops a wheel or a cane tip running off the side.
- Overlooking the landing at a direction change. A switchback landing must be at least 60 by 60 in so a chair can turn.
Which rules apply where
The 2010 ADA Standards for Accessible Design govern ramps on accessible routes in public accommodations and commercial facilities: Section 405.2 for slope, 405.5 for width, 405.6 for the 30 in rise limit, 405.7 for landings, 405.8 for handrails and 405.9 for edge protection. Private single-family homes are not covered by the ADA; their ramps fall under IRC R311.8, which permits a maximum slope of 1:8 for ramps that are not the required egress route and 1:12 for those that are. Where a residential ramp is being built for a specific person, the ADA geometry is still the better target - it is what makes a ramp usable rather than merely legal.
When a ramp is the wrong answer
A ramp is the default solution, not the only one. Above roughly 60 inches of rise the footprint becomes hard to justify - 75 feet of structure for a 5 foot climb - and a platform lift, an inclined lift or a small elevator often costs less than the ramp, the landings and the ground they occupy. Lifts also work in weather that makes a long outdoor ramp unpleasant, though they introduce maintenance and a power supply that a ramp does not need.
Regrading deserves a look before either. Rise is the only input in this calculation, and it is often the most movable one. Bringing a path up 6 inches removes 6 feet of ramp at 1:12 and can remove a whole landing if it takes the total under 30 inches. On a sloping site, a ramp that runs across the fall rather than against it can shorten dramatically.
Where the ramp is part of a deck, size the structure with the deck footing size calculator and the deck board calculator, and remember that ramp decking runs across the direction of travel so wheels do not drop into the gaps. Where the same level change is also served by steps, the stair rise and run calculator and the concrete stairs calculator handle those, and the baluster spacing calculator covers the guard between them.