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A 24-inch rise needs 24 feet of ramp. Know the footprint before you build.
Ramp math surprises everyone the first time: the ADA maximum slope of 1:12 turns even a modest porch rise into a long structure, and the 30-inch rise-per-run rule inserts 60-inch landings that stretch the footprint further. This calculator does the whole layout — run, surface length, run count, landings, handrail triggers — against ADA Section 405 and the IRC residential alternative.
Contractors quoting accessibility work use it to show the client why a two-foot rise is a 24-foot structure — the landings do the talking. Families planning for aging-in-place use it to learn whether the yard has room for 1:12 at all, or whether a switchback or lift is the real answer.
At the ADA maximum slope of 1:12, 24 inches of rise needs 24 feet of horizontal run — plus landings. At the kinder 1:16 it needs 32 feet. That is why porch ramps so often become switchbacks: the footprint, not the slope, is the constraint.
Under the ADA, only on existing sites where 1:12 is infeasible — 1:10 for rises up to 6 inches and 1:8 for rises up to 3 inches. The IRC residential code is looser: R311.8 allows 1:8 generally for ramps serving a dwelling. New public or commercial construction is 1:12, full stop.
Whenever a run rises more than 6 inches (or projects more than 72 inches horizontally) — on both sides, 34–38 inches above the surface, with 12-inch extensions beyond the top and bottom of each run. Almost every real ramp trips this rule.