Reading ramp slope, and what the ADA actually requires
Slope is written as a ratio of run to rise. At 1:12 you need 12 inches of horizontal run for every 1 inch you climb, so a 24 inch rise needs 24 feet of ramp — a number that surprises most people the first time they work it out. The ADA maximum slope for a new ramp is 1:12, and that is a ceiling, not a target: 1:16 or 1:20 is noticeably easier to self-propel and much safer on wet or icy decking. A run sloped no steeper than 1:20 is treated as a walking surface rather than a ramp, which changes what handrails and edge protection are required. The steeper 1:8 option here exists only for the occupied-use exception in existing buildings, for very short rises, where an attendant is pushing — it is not a design choice for new construction.
Length is limited too. The ADA caps a single ramp run at 30 inches of rise; go higher than that and you must break the ramp with an intermediate landing before the next run. Landings must be at least 60 inches long, at least as wide as the ramp, and level in both directions, with a landing at the top and the bottom of every ramp. Turns need more: a landing where the ramp changes direction must be at least 60 by 60 inches. This calculator applies whichever rule bites first — the 30 inch rise limit or a landing roughly every 30 feet of run — so the landing count is a floor, not a maximum. Also plan for the things the geometry doesn't show: a clear width of at least 36 inches between handrails, handrails on both sides where the rise exceeds 6 inches, edge protection to stop a wheel running off, and a level, unobstructed approach at the bottom.
This is a planning estimate. Exact accessibility requirements depend on the ADA Standards, your local building code, and whether the work is new construction, an alteration, or a private residence — and they differ outside the United States. Confirm the design with your building department, and for public or commercial work with an accessibility consultant or licensed design professional, before you build.