ADA Ramp Length Calculator
The ADA's ramp guideline is 1:12: one foot of ramp for every inch of rise. Measure the total rise from the ground to the door threshold in inches, and this does the rest, landings included.
At a 1:12 slope, 21 inches of rise needs about 21 feet of ramp run. Plan roughly 31 feet of total straight-line space once you add 5-foot level landings at the top and bottom. Many installs fold the run with switchbacks to fit the yard; the run length stays the same.
Honest notes: homes are not usually required to meet ADA, but 1:12 is the benchmark the industry designs to for occupied use, and gentler is always safer. Ramps with a rise over 6 inches should have handrails. Permanent ramps can need a permit; your local building department is the authority. The full technical rules live in the 2010 ADA Standards, section 405, at ada.gov.