Ladder Angle & Length Calculator (4-to-1 Rule)

How it works

Length along the ladder = required top height ÷ sin 75.5°; base distance = contact height ÷ 4; usable length = nominal − section overlap (3 ft ≤ 32 ft, 4 ft to 36, 5 ft to 48); roof access adds 3 ft above the landing

The 4-to-1 rule — one foot out from the wall for every four feet of height to the contact point — sets a ladder at about 75°, the angle at which the base is least likely to kick out and the ladder least likely to tip backward. Because the ladder leans, the length along the rails is longer than the height it reaches (about 3% at 75°), extension ladders lose several feet to the overlap between sections, and a ladder used to climb onto a roof must stick up three feet past the edge for a handhold, so the nominal ladder size needed is always well above the height in question: an 18 ft eave takes a 24 ft ladder. Working from a ladder is more restrictive still, since the top three rungs are off limits and reach is only about four feet above the standing rung.

Worked example

To climb onto an 18 ft roof edge the ladder must reach 21 ft, which at 75° is 21.7 ft along the rails; with a 3 ft section overlap that means a 28 ft extension ladder (25 ft usable) with its feet 4.5 ft from the wall. A 24 ft ladder (21 ft usable, reaching 20.3 ft) is 0.7 ft short — it suits eaves up to about 17 ft.

Frequently asked questions