How the 4 to 1 ladder rule works
The ladder, the wall and the ground make a right triangle. The ladder is the long side, so its working length is the hypotenuse. The working length runs along the ladder from the foot to the top support. The 4 to 1 rule puts the foot one unit from the wall for every four units of working length. That is the base-distance rule in 1926.1053(b)(5)(i), which says approximately one-quarter.
- Base distance = working length ÷ 4. A 20 ft working length sets the foot 5 ft from the wall.
- Height reached = working length × √15 ÷ 4, about 0.968 times the working length. This is the Pythagorean theorem with the base at one-quarter of the length.
- Angle = arccos(1 ÷ 4) = 75.52 degrees from the ground, about 75.5. Every ladder set at 4 to 1 has this angle, whatever its length.
- Working length for a landing height = 4 × height ÷ √15, about 1.033 times the height.
Too far out and the ladder is flat and can slide at the foot. Too close and it is steep and can tip back. The angle is only half the setup. The second half is the extension.
How far the ladder must extend above the landing
When a portable ladder gives access to an upper landing, 1926.1053(b)(1) requires the side rails to extend at least 3 feet (0.9 m) above the landing surface. The rails give you something to hold while you step on or off. If the ladder is too short for that, the same paragraph has a different route: secure the top to a rigid support and provide a grasping device. For the length to buy, add the 3 feet to the working length. The result is the extended length, foot to top of the rails.
Extension ladders are sold by the combined length of their sections, and the sections overlap when extended, so the extended length is less than the sum. The calculator has an optional overlap field. Read the overlap and the maximum extended length from the ladder label, and pick a ladder whose labeled extended length is at least the number shown. For the full list of ladder rules, read OSHA ladders and stairways (Subpart X) for the CHST.
Worked example 1: the ladder to buy for a 15 ft landing
The landing is 15 ft above the ground. These are invented study numbers.
- Working length = 4 × 15 ÷ √15 = 60 ÷ 3.873 = 15.49 ft, which is 15 ft 6 in.
- Base distance = 15.49 ÷ 4 = 3.87 ft, which is 3 ft 10 in.
- Check the angle: cos⁻¹(3.873 ÷ 15.492) = 75.5 degrees.
- Add the 3 ft extension: 15.49 + 3 = 18.49 ft from foot to top of rails when extended.
- If the label shows a 3 ft overlap, the sections add up to 21.49 ft, so the next ladder size up from that is the one to look for.
Worked example 2: how high does a 20 ft working length reach?
- Base distance = 20 ÷ 4 = 5 ft.
- Height = √(20² − 5²) = √375 = 19.36 ft, which is 19 ft 4 in.
- The angle is 75.5 degrees, the same as every 4 to 1 setup.
A 20 ft working length reaches a landing 19.36 ft high, but it leaves no extension. Under (b)(1) a landing that high needs a longer ladder, since the rails must stand 3 ft above it. These are original arithmetic examples, not exam questions.
Ladder angle questions
Is 4 to 1 the same as 75 degrees?
It is the same setup. A ladder with its foot one-quarter of its working length from the wall sits at 75.5 degrees from the ground. You will see 75 degrees quoted as the rounded figure.
Do I measure 4 to 1 along the wall or along the ladder?
Along the ladder. The 4 is the working length, foot to top support, and the 1 is the horizontal distance from the foot to the wall. It is not the height. A common slip is dividing the landing height by 4, which sets the base about 3% too close.
Does the 3 foot extension count inside the working length?
No. The working length stops at the top support. The 3 feet is the part of the rails sticking up above it, so you add it when you work out how long a ladder to get.
The ladder rules are on the CHST, in Domain 1 under working at heights. The calculator covers the arithmetic, and the guide covers the rest of Subpart X.