Ladder Angle Calculator (1-in-4 Rule)
How far to set the foot of a ladder from the wall for the safe 1-in-4 angle, and the ladder length needed to reach a given height.
Results
What this tool does
The 1-in-4 rule says to set the foot one unit out for every four units of height to the support. The angle is the same whatever the height — tan⁻¹ 4, about 76°, which safety guidance rounds to 75° — and a ladder set much steeper tends to tip backwards, while a flatter one is likely to slip at the foot. For access to a roof or a landing, the ladder should also stand at least about a metre above the point where it rests, so there is something to hold when stepping off. Check the ladder's rating, and tie or foot it, before relying on the geometry alone.
Formula
foot distance = height ÷ 4 (1-in-4 rule) · angle = atan(4) ≈ 76° · length = √(h² + (h ÷ 4)²) + extension
Variables
| Symbol | Meaning | Unit |
|---|---|---|
h | Height where the ladder rests | m |
r | Ratio: 1 out for every … up | — |
ex | Extension above the support | m |
BD | Distance of the foot from the wall | m |
AN | Angle with the ground | ° |
LL | Ladder length up to the support | m |
LT | Ladder length needed, with the extension | m |
Worked example
- Height where the ladder rests4 m
- Ratio: 1 out for every … up4
- Extension above the support1 m
- Distance of the foot from the wall1.00 m
- Angle with the ground76.0 °
- Ladder length up to the support4.12 m
- Ladder length needed, with the extension5.12 m
Limitations
- For work that must comply with a standard or be signed off, check the result against the applicable code and have it reviewed by a qualified engineer.
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.