How long does a ramp need to be?
Multiply the rise by the slope ratio. A 1:12 ramp needs 12 in of horizontal run for every inch of rise, which is 1 ft of run per inch. Measure the rise from the ground where the ramp will start to the top landing or floor.
Run (in) = rise (in) × 12 for a 1:12 slope. Ramp runs = rise ÷ 30 in, rounded up.
A porch 24 in above the ground needs 24 ft of sloped run at 1:12. That's one run, with a 60 in landing at the top and bottom, so plan on 34 ft overall in a straight line. At 36 in of rise, the run is 36 ft. That's more than 30 in of rise, so it splits into two runs with a 5 ft landing between them: 41 ft of ramp, or 51 ft with the top and bottom landings.
ADA ramp slope requirements
The 2010 ADA Standards for Accessible Design cover ramps in section 405. These are the main numbers for new construction:
| Requirement | ADA 2010 | Section |
|---|---|---|
| Maximum running slope | 1:12 (8.33%) | 405.2 |
| Maximum cross slope | 1:48 (about 2%) | 405.3 |
| Minimum clear width | 36 in between handrails | 405.5 |
| Maximum rise per run | 30 in | 405.6 |
| Landing length | 60 in minimum | 405.7.3 |
| Landing where the ramp turns | 60 × 60 in minimum | 405.7.4 |
| Handrails | Both sides when rise is more than 6 in | 405.8 |
| Handrail height | 34 to 38 in | 505.4 |
Existing buildings with tight space may use steeper slopes for very short ramps (Table 405.2), and some states and cities add their own rules. Check with your building department. This calculator helps you plan. It doesn't certify that a ramp complies.
Wheelchair ramp slope calculator for homes
Home ramps fall under the residential code, not the ADA. The IRC (R311.8) also limits ramps to 1:12 in most cases, allows a steeper slope only in limited situations, and requires landings at the top and bottom and handrails on steeper ramps. Your local code may differ, so check the edition your area adopted.
Many people who push their own wheelchair find 1:16 or 1:20 easier than 1:12, if the yard has room. A 24 in rise at 1:16 needs 32 ft of run.
Slope as a ratio, percent grade and degrees
The same slope can be written three ways. Percent grade is rise ÷ run × 100. The angle is arctan(rise ÷ run). Use the same unit for rise and run.
| Ratio | Percent grade | Degrees | Rise per foot |
|---|---|---|---|
| 1:8 | 12.5% | 7.13° | 1-1/2 in |
| 1:10 | 10% | 5.71° | 1.2 in |
| 1:12 | 8.33% | 4.76° | 1 in |
| 1:16 | 6.25% | 3.58° | 3/4 in |
| 1:20 | 5% | 2.86° | 0.6 in |
| 1:48 | 2.08% | 1.19° | 1/4 in |
To check a ramp you already have, switch to "Check the slope of a run" and enter the rise in inches and the run in feet. A 24 in rise over 20 ft is 1:10, a 10% grade and 5.71°. That's too steep for a new ADA ramp, which would need 24 ft.
Slope calculator for driveways and drainage
The check mode works for any slope. A driveway that drops 2 ft over 30 ft is 24 ÷ 360 = 6.67%, or about 3.8°. Local rules on driveway grades vary, so check yours before you pour.
For drainage, the IRC (R401.3) calls for the ground to fall at least 6 in within the first 10 ft from the foundation, a 5% grade. Plumbing codes typically call for 1/4 in per foot (2.08%) on small drain lines and 1/8 in per foot on larger ones.
Pricing a ramp job
Once you know the length, price the build like a low deck: posts, framing, decking, handrails and any concrete landing. Use the deck calculator for the surface, the board foot calculator for framing lumber and the concrete calculator for a landing pad, then send it as an estimate.
Questions
How long does a ramp need to be for 24 inches of rise?
24 feet of horizontal run at a 1:12 slope, plus a 60 inch landing at the top and bottom under ADA. That's about 34 feet overall in a straight line.
What is the ADA ramp slope?
A maximum of 1:12 for new construction under the 2010 ADA Standards, section 405.2. That's 1 inch of rise per foot of run, an 8.33 percent grade or about 4.76 degrees.
How often does a ramp need a landing?
Under ADA, a single ramp run can rise no more than 30 inches. After that you need a level landing at least 60 inches long, and a 60 x 60 inch landing wherever the ramp turns.
How do you calculate percent slope?
Divide the rise by the run, using the same unit, and multiply by 100. A 6 inch drop over 10 feet is 6 divided by 120, times 100, or 5 percent.