How the 4-to-1 rule works
- OSHA's construction standard: a non-self-supporting ladder is used at an angle where the horizontal distance from the top support to the foot is about one-quarter of the working length (29 CFR 1926.1053(b)(5)(i)).
- The working length is measured along the ladder from the foot to the top support, not the wall height. With a 1:4 ratio the height is √15 ÷ 4 ≈ 0.968 of the working length, so working length = height ÷ 0.968.
- The angle is arccos(¼) ≈ 75.5°. Wood job-made ladders with spliced side rails use one-eighth instead, about 82.8° ((b)(5)(ii)).
- When you climb onto a roof or landing, the side rails must extend at least 3 feet above that surface ((b)(1)). The calculator measures the 3 ft vertically, which takes about 3 ft 1 in of rail at 75.5°.
Ladder setup chart (1:4)
| Height of top support | Foot out from support | Working length | Ladder to step off (+3 ft) |
|---|---|---|---|
| 6 ft | 1 ft 7 in | 6 ft 2 in | 9 ft 4 in |
| 8 ft | 2 ft 1 in | 8 ft 3 in | 11 ft 4 in |
| 10 ft | 2 ft 7 in | 10 ft 4 in | 13 ft 5 in |
| 12 ft | 3 ft 1 in | 12 ft 5 in | 15 ft 6 in |
| 14 ft | 3 ft 7 in | 14 ft 6 in | 17 ft 7 in |
| 16 ft | 4 ft 2 in | 16 ft 6 in | 19 ft 7 in |
| 18 ft | 4 ft 8 in | 18 ft 7 in | 21 ft 8 in |
| 20 ft | 5 ft 2 in | 20 ft 8 in | 23 ft 9 in |
| 24 ft | 6 ft 2 in | 24 ft 9 in | 27 ft 11 in |
| 28 ft | 7 ft 3 in | 28 ft 11 in | 32 ft |
| 32 ft | 8 ft 3 in | 33 ft 1 in | 36 ft 2 in |
Rounded to the nearest inch. Extension ladders lose length where the sections overlap. Compare with the maximum working length on the ladder's label, not its nominal size.
Before you climb
- Set the ladder on a stable, level surface, or secure it so it cannot move ((b)(6)-(7)).
- If the ladder cannot extend 3 ft above the landing, secure the top to a rigid support and provide a grab rail ((b)(1)).
- Do not load a ladder beyond its rated capacity ((b)(3)). For building stairs instead of ladders, use the stair calculator.
This calculator is a planning aid based on the federal OSHA construction standard. Follow your ladder's labels and any state plan or site rules.
Frequently asked questions
What is the 4-to-1 ladder rule?
For every 4 feet of ladder between the foot and the top support, the foot sits 1 foot out from the support. OSHA's construction standard says the horizontal distance should be about one-quarter of the working length (29 CFR 1926.1053(b)(5)(i)). That puts the ladder at about 75.5 degrees.
How far should a ladder be from the wall for a 12-foot roof?
About 3 ft 1 in. The working length is 12 ÷ 0.968 ≈ 12.4 ft, and a quarter of that is 3.1 ft. To step onto the roof, the rails must reach 3 ft above the roof edge, so you need about 15½ ft of ladder.
Is the quarter measured from the height or the ladder length?
From the working length: the distance along the ladder from the foot to the top support. Using a quarter of the height instead puts the foot slightly too close (3 ft instead of 3.1 ft for 12 ft).
How far above the roof must a ladder extend?
At least 3 feet above the upper landing surface when the ladder is used to get onto it. If the ladder is too short for that, secure the top to a rigid support and provide a grab rail (29 CFR 1926.1053(b)(1)).