Calculate ramp slope, angle, and rise, run, or surface length from your measurements and target ratio for accessible ramps using ft, in, cm, or m.
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Ramp Slope Formula
Slope % = (Rise / Run) × 100 Ratio = 1 : (Run / Rise) Angle = arctan(Rise / Run) Surface length = √(Run² + Rise²)
- Rise: vertical height from the lower landing to the upper landing.
- Run: horizontal distance covered by the ramp (not the surface length).
- Surface length: the length along the sloped ramp surface itself.
- Ratio: written as 1:x, where x is run divided by rise.
Rise and run must be in the same unit before dividing. The calculator converts automatically. If you measured the ramp surface instead of the run, the run is back-calculated as √(Surface² − Rise²), so the surface length must be greater than the rise.
Slope Reference Tables
Common ramp slopes and what each one means in practice:
| Ratio | Slope % | Angle | Typical use |
|---|---|---|---|
| 1:20 | 5% | 2.86° | Walkway, no handrail required under ADA |
| 1:16 | 6.25% | 3.58° | Preferred accessible ramp slope |
| 1:12 | 8.33% | 4.76° | ADA maximum for new construction |
| 1:10 | 10% | 5.71° | Existing sites, rise up to 6 in |
| 1:8 | 12.5% | 7.13° | Existing sites, rise up to 3 in; loading ramps |
| 1:6 | 16.7% | 9.46° | Occupied scooter, with assistance |
| 1:4 | 25% | 14.0° | Unoccupied equipment loading only |
Quick lookup for ramp length at a 1:12 slope:
| Rise | Horizontal run (1:12) | Horizontal run (1:16) | Horizontal run (1:20) |
|---|---|---|---|
| 6 in | 6 ft | 8 ft | 10 ft |
| 12 in | 12 ft | 16 ft | 20 ft |
| 24 in | 24 ft | 32 ft | 40 ft |
| 30 in | 30 ft | 40 ft | 50 ft |
| 36 in | 36 ft | 48 ft | 60 ft |
Worked Example and FAQ
Example. Your porch sits 22 inches above the walkway. For a 1:12 ramp:
- Run = 22 in × 12 = 264 in = 22 ft
- Surface length = √(264² + 22²) ≈ 264.9 in ≈ 22.08 ft
- Slope = 8.33%, angle ≈ 4.76°
ADA also requires intermediate landings every 30 in of rise, so this ramp would need one landing partway up.
Is rise the same as ramp length? No. Rise is vertical only. Run is horizontal only. Surface length is the diagonal you walk on.
What slope should I use? 1:12 is the maximum for new accessible ramps under ADA. 1:16 to 1:20 is easier to push a wheelchair up and is preferred when you have the room.
How do I convert a percent to a ratio? Divide 100 by the percent. A 5% slope is 100 ÷ 5 = 1:20. A 10% slope is 1:10.
