The rule
OSHA's construction standard, 29 CFR 1926.1053(b)(5)(i), says a non-self-supporting ladder shall be used at an angle such that the horizontal distance from the top support to the foot is approximately one quarter of the working length, meaning the distance along the ladder between the foot and the top support. Put simply: one out for every four up the ladder, the 4 to 1 rule.
The angle and the math
The ladder, the wall and the ground form a right triangle. The ladder is the hypotenuse, the distance out from the wall is the base, and the height of the top support is the third side.
Base = working length ÷ 4. Height = √(length² − base²). Angle = arccos(¼) ≈ 75.5°.
The Pythagorean theorem calculator gives the height for any length and base, the triangle calculator solves the angle from any two sides, and the angle converter turns degrees into the rise-over-run slope some levels and apps show.
Distances for common ladder lengths
| Working length | Foot out from the wall | Height of the top support |
|---|---|---|
| 8 ft | 2 ft 0 in | 7 ft 9 in |
| 12 ft | 3 ft 0 in | 11 ft 7 in |
| 16 ft | 4 ft 0 in | 15 ft 6 in |
| 20 ft | 5 ft 0 in | 19 ft 4 in |
| 24 ft | 6 ft 0 in | 23 ft 3 in |
| 28 ft | 7 ft 0 in | 27 ft 1 in |
| 32 ft | 8 ft 0 in | 31 ft 0 in |
The working length is the part between the foot and the point where the ladder rests, not the full length of an extension ladder; extended sections overlap, and any part above the support does not count.
Climbing onto a roof or platform
When a portable ladder is used to get onto an upper landing surface, OSHA's rule 1926.1053(b)(1) requires the side rails to extend at least 3 feet (0.9 m) above that surface. So for a gutter 15 feet up, the ladder rests at 15 feet and continues at least 3 feet higher: the working length used for the 4 to 1 rule is the part up to the gutter, and the extra 3 feet comes on top. Plan the ladder length with both numbers in mind.
Setting up, step by step
- Extend the ladder to the length you need, including 3 feet above any landing you will step onto.
- Rest it against the wall and measure the working length up to the support point.
- Move the foot out to a quarter of that length, using the table above or a tape measure.
- Make sure both feet stand on firm, level ground and the top rests evenly on both rails.
Frequently asked questions
What is the 4 to 1 ladder rule?
For every four units of ladder length between the foot and the top support, set the foot one unit out from the wall. It is the plain-language form of the OSHA construction rule, 29 CFR 1926.1053(b)(5)(i), which says a non-self-supporting ladder must be used at an angle where the horizontal distance from the top support to the foot is about one quarter of the working length.
What angle is the 4 to 1 rule?
About 75.5 degrees from the ground. With the foot out a quarter of the ladder's length, the cosine of the angle is 0.25, and the inverse cosine of 0.25 is 75.52 degrees.
How far from the wall should a 20-foot ladder be?
Five feet, a quarter of the working length. The top then rests at about 19.4 feet. If the ladder is used to reach a roof or another upper surface, OSHA requires the side rails to extend at least 3 feet above that landing, so measure the working length to the top support, not to the end of the rails.
Does the rule apply to step ladders?
No. The angle rule is written for non-self-supporting ladders, the kind that lean against a wall or structure. A self-supporting step ladder stands on its own and is not leaned at an angle.
Does the 4 to 1 rule use the full ladder length?
No, the working length: OSHA defines it as the distance along the ladder between the foot and the top support. Any part of an extension ladder above the support, such as the 3 feet required above a roof edge, is not counted.