OSHA and ladder manufacturers recommend the 4-to-1 rule: for every 4 ft of height to the support point, move the base out 1 ft. That gives an angle of about 75.5°. Too steep and the ladder can tip backwards; too shallow and the base can slide out. When accessing a roof, the ladder should extend 3 ft above the edge.
How to use the Ladder Angle Calculator
- Ladder length (working)
- Base distance from wall
- Every result field is editable: type into Ladder angle and Base distance from wall plus every other field update to match.
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the ladder length and the distance of its foot from the wall. The calculator returns the height reached, the angle and the ideal base distance. Type a target angle or height to find the base distance.
Inputs explained
| Field | What to enter |
|---|---|
| Ladder length (working) | Units: ft, m, in, yd, cm |
| Base distance from wall | Units: ft, m, in, yd, cm |
What each result means
- Height reached = sqrt(max([Ladder length (working)]^2 – [Base distance from wall]^2, 0))
- Ladder angle = deg(acos(min([Base distance from wall]÷[Ladder length (working)], 1)))
- Ideal base distance (75.5°) = [Ladder length (working)]÷4
- Height at ideal angle = [Ladder length (working)]×sqrt(15)÷4
Example values
How the main result changes when Base distance from wall changes and everything else stays at the example values above:
| Base distance from wall (ft) | Ladder angle (°) |
|---|---|
| 2.5 | 82.8 |
| 3.75 | 79.2 |
| 5 | 75.5 |
| 6.25 | 71.8 |
| 7.5 | 68 |
| 10 | 60 |
Formula
Height = √(L² − base²)
Angle = acos(base ÷ L)
Ideal base = L ÷ 4 (75.5°)
Worked example
Inputs: Ladder length (working): 20 ft; Base distance from wall: 5 ft.
Results: Height reached = 19.36 ft; Ladder angle = 75.5 °; Ideal base distance (75.5°) = 5 ft; Height at ideal angle = 19.36 ft.
Frequently asked questions
What is the safe angle for a ladder?
About 75.5°, which is the 4-to-1 rule: 1 ft out for every 4 ft up.
How tall a ladder do I need for a 16 ft roof edge?
About 20 ft for the angle plus 3 ft above the edge, so a 24 ft extension ladder.
Does the overlap of an extension ladder reduce its length?
Yes, a 24 ft extension ladder with 3 ft overlap has about 21 ft of working length.