Ladder Angle Calculator: 1 in 4 Rule, Base Distance and Length Needed
How far to stand a leaning ladder from the wall, the angle it makes, and how long a ladder you need.
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How this calculator works
A leaning ladder is safest at about 76 degrees, a slope of 1 out for every 4 up. Too steep and it can tip backwards, too flat and the foot can slide out. The base distance is the height divided by 4, and the angle is the angle whose tangent is 4.
The length of the ladder up to the point it rests on is the hypotenuse of the triangle formed by the height and the base distance. The ladder should also extend about 1 m above the step-off point, so the least ladder length is that hypotenuse plus the overhang.
The calculator compares that with the length of the ladder you have. A ladder is for access or for short light work, and where a platform, scaffold or elevating work platform is reasonably practicable it is a better choice.
Formulas used
- b = H / r
- \theta = \tan^{-1}(r)
- L_c = \sqrt{H^2 + b^2}
- L_{min} = L_c + \text{overhang}
Worked example
With the default numbers in the calculator above:
- Base distance from the wall: 0.75 m
- Ladder angle from the ground: 75.96 degrees
- Ladder length up to the contact point: 3.09 m
- Least ladder length with the overhang: 4.09 m
- Spare length on the ladder you have: 0.41 m
Change the numbers in the calculator to see the full working, every step, for your own case.
A second worked example
A different case, with these numbers: Height of the point the ladder rests against 5 m; Ladder length above the step-off point 0.9 m.
- Base distance from the wall: 1.25 m
- Ladder angle from the ground: 75.96 degrees
- Ladder length up to the contact point: 5.15 m
- Least ladder length with the overhang: 6.05 m
- Spare length on the ladder you have: -1.55 m
Common questions
What is the 1 in 4 rule?
The foot of the ladder is placed 1 unit out from the wall for every 4 units of height to where it rests. For a height of 4 m the foot is 1 m out.
How far above the step-off should a ladder extend?
About 1 m, or there should be a firm handhold at the top, so a person can step on and off safely.
Is the height measured to the top of the ladder?
No. It is measured to the point the ladder rests against, such as the roof edge or platform.
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Got a question about this? Ask an engineering question and get a plain-language answer with references.
Further reading: Safe Work Australia and Queensland guidance on portable ladders; AS/NZS 1892 (portable ladders, licensed); the Managing the risk of falls at workplaces Code of Practice (Queensland).
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