How the length is worked out
A gradient of 1:12 means one unit of rise for every twelve of run, so the run is twelve times the rise. The same relationship gives the percentage: one in twelve is 8.33 per cent.
The number of flights follows from the maximum rise allowed in one go. Divide the total rise by that maximum and round up, because a part flight still needs its own landing.
Landings are the joints between flights, so one fewer than the number of flights. A single flight needs none.
Worked example
A 24 inch rise at 1:12 needs 288 inches of run, which is 24 feet. The rise is under the 30 inch maximum for a single run, so one run and no intermediate landing is enough.
Halve the gradient to 1:20 and the run doubles, which is why a gentler ramp needs a much longer garden.
What the limits actually say
The ADA Standards limit a ramp to a 1:12 slope, which is 8.33 per cent. A single run may rise no more than 30 inches, so anything taller needs a landing of at least 60 inches between runs. The clear width must be at least 36 inches, and handrails are required on both sides once the rise exceeds 6 inches.
A landing is not optional at the top. It has to be level, and it has to be long enough to stand on while opening a door at the end of the ramp.
Limits of this calculator
It gives the geometry. It does not check handrails, kerbs, crossfall, surface slip resistance, drainage or the door swing at the top.
It does not certify compliance. Building control or an access officer assesses the whole route, including the approach, the landing and the door.
It assumes a straight ramp. A switchback or a curved ramp needs each leg calculated separately.
For the stair that often replaces a ramp, use the US stair calculator.
Inputs for the US ramp calculator
Total rise
Height the ramp has to climb, in inches.
Ramp slope
One unit of rise for every N of run. The steepest allowed is 1:12.
Clear width
Width between handrails. The ADA minimum is 36 in.
CALCULATOR QUESTIONS
US ramp calculator questions
How long does a ramp need to be?
Twelve times the rise at 1:12, twenty times at 1:20. A 500 mm rise needs 6 metres at the steepest legal gradient.
Can a ramp be steeper than 1:12?
Not as an accessible route. Steeper ramps exist for wheelbarrows and vehicles, not for wheelchairs.
When do I need a landing?
Whenever the rise exceeds the maximum for one flight, which is 500 mm in the UK and 30 inches in the US.
Does this check handrails?
No. It gives the geometry, and the handrail and kerb requirements are separate.