Article — Ramp Slope Calculator
Ramp slope calculator: ADA 1:12, angle, percent, and length
The maximum ramp slope under the Americans with Disabilities Act is 1:12 — one inch of rise for every twelve inches of run. That equals an 8.33% grade and a 4.76° angle. A 30-inch rise needs at least 30 feet of run, with a landing every 30 feet of continuous travel. Anything steeper than 1:12 is not compliant for new accessibility ramps.
The calculator at the top of this page takes rise and run in either imperial or metric units and returns slope ratio, angle, percent grade, ramp length, and an ADA compliance check. The article below explains where the 1:12 number comes from, when exceptions apply, and how to size a ramp without flunking the inspection.
What is ramp slope?
Ramp slope is the ratio of vertical rise to horizontal run. A ramp that climbs 12 inches over 12 feet of horizontal travel has a slope of 1:12, also written as 1/12 or 8.33%. The steeper the ratio, the harder the ramp is to roll up — wheelchair users need pushing force roughly proportional to slope plus the friction of the surface.
Three numbers describe slope, all equivalent: the ratio (1:12), the percent grade (8.33%), and the angle (4.76°). Building codes, ADA standards, and most architectural plans use the ratio form. Civil engineering and trail design lean on percent. Trigonometry and surveying use the angle.
The ADA 1:12 ramp slope rule
The Americans with Disabilities Act of 1990, and the 2010 ADA Standards that implement it, set the maximum ramp slope at 1:12 for new construction. The U.S. Access Board, the agency that maintains the standards, anchors the rule in section 405 of the 2010 Standards. The 1:12 figure comes from extensive wheelchair-propulsion research showing that steeper grades demand pushing forces beyond what most independent wheelchair users can sustain.
Max slope: 1:12 = 8.33% = 4.76°Max rise per run 30 in over 30 ftMin width 36 in clearA 1:12 ramp at the ADA maximum is already steep for many users. The Access Board recommends gentler slopes — 1:16 or 1:20 where space allows — and the recommended ratio for residential accessibility ramps from agencies like the VA is 1:20. The 1:12 figure is a ceiling, not a target.
The ramp slope formula
Three steps convert rise and run into every form of ramp slope you might need.
First, divide rise by run. That gives the decimal slope. Then invert to express it as 1:N, multiply by 100 for percent, or apply arctan for the angle.
- Ratio (1:N) = run ÷ rise (1:12 means 12 in of run per 1 in of rise)
- Percent = (rise ÷ run) × 100 (1:12 = 8.33%)
- Angle = arctan(rise ÷ run) in degrees (1:12 = 4.76°)
- Required run = 12 × rise (a 30 in rise needs 30 ft of run at 1:12)
- Ramp length = √(rise² + run²) (always greater than run)
Slope, percent grade, angle
The three forms of slope describe the same geometric reality but get used in different contexts. The 1:12 ratio is plain-language and shows up in ADA documentation. Percent grade dominates civil engineering and trail signage. Angle comes from trigonometry and is what a digital level reads off a real surface.
The three diverge at steeper slopes. A 1:4 loading dock is 25% grade and 14.04° — small in degrees, alarming in percent, dangerous as a wheelchair surface.
Ramp length and Pythagoras
The actual length a wheelchair travels — the slanted ramp surface — is the hypotenuse of the right triangle whose legs are rise and run. Pythagoras applies: L = √(rise² + run²). At ADA 1:12, the ramp length is only about 0.35% longer than the run, because the slope is so gentle. The difference grows fast at steeper ratios.
For a 30-inch rise at the 1:12 maximum (run = 30 ft), the ramp length is 30.04 ft. The extra 0.04 ft hardly matters for material take-off, but the ramp surface is what gets sheathed, painted, and walked on, so plan sheets should label the slanted distance not the horizontal run.
The ADA was signed into law on July 26, 1990 by President George H.W. Bush. Section 405 — the ramp requirements — drew on civil rights work going back decades, including the Architectural Barriers Act of 1968 which first required federal buildings to be accessible. The 1:12 figure was already established in earlier ANSI standards (A117.1, 1961) before ADA codified it nationally.
ADA landing and handrail rules
Slope is one of several ADA ramp rules. Landings, handrails, edges, and surface materials all carry requirements that an in-spec slope cannot substitute for.
Plan the landings before you plan the ramp run. A ramp that needs more than 30 feet of total run requires intermediate level landings at least 60 inches long. A direction change requires a 60×60 inch landing. Handrails are required whenever the rise exceeds 6 inches or the projection exceeds 72 inches — practically, that is every accessibility ramp.
Handrails must sit 34 to 38 inches above the ramp surface, be continuous, and extend 12 inches beyond the top and bottom of each ramp segment. Cross slope — the side-to-side tilt — must not exceed 1:48 (about 2%). Surfaces must be firm, stable, and slip-resistant. Edge protection (a curb or barrier) is required wherever a drop-off exists at the ramp edge.
ADA ramp slope exceptions
The 1:12 rule applies to new construction. The 2010 ADA Standards include two exceptions for alterations of existing facilities where space limits make 1:12 impossible.
A slope of 1:10 is allowed for a maximum rise of 6 inches. A slope of 1:8 is allowed for a maximum rise of 3 inches. Both apply only to alterations, only to ramps that cannot meet 1:12 because of existing physical constraints, and require that the design come "as close as feasible" to the standard rules in every other respect.
The 1:10 and 1:8 ramp slope exceptions are not general-purpose. They apply only to alteration work, only where physical limits genuinely block a 1:12 design, and only up to the rise limits shown. New construction has no exception — slopes steeper than 1:12 do not meet ADA. Local building codes sometimes add tighter limits and rarely loosen them.
Common ramp slope mistakes
- Confusing rise per run with rise per length — slope uses horizontal run, not slanted ramp length
- Forgetting the landing requirement — a 36 ft long 1:12 ramp without a mid-landing is not compliant
- Designing the slope right but the width wrong — 36 in clear is the minimum, measured between handrails not at the edge of the deck
- Treating 1:8 as a general exception — it applies only to alterations and only up to 3 inches of rise
- Ignoring cross slope — a flat-looking ramp with 1:36 cross slope already exceeds the 1:48 limit
- Using construction tolerances as a margin — the spec is a ceiling, so target 1:14 or 1:16 to allow for build tolerance