Ladder Angle Calculator (4 to 1 Rule)
Set an extension ladder safely with the 4-to-1 rule.
Enter your working height to get the base distance, lean angle, and minimum ladder length.
An extension ladder is only safe at the right angle, and the trick the trades use is the 4-to-1 rule: for every four feet of height up to where the ladder touches the wall, the feet sit one foot out from the base. That works out to a lean of about 75 degrees, steep enough that you are not pulling the ladder off the wall as you climb, but not so steep that it tips backward.
To use it, measure the height from the ground to the point where the ladder will rest, divide by four, and that is how far to pull the feet out. A ladder touching a gutter 16 feet up should have its feet 4 feet from the wall. Get this wrong in either direction and you set up one of the two classic ladder accidents. Too shallow and the base kicks out from under you. Too vertical and the whole ladder peels backward off the wall as you near the top.
If you are climbing onto a roof rather than just working against the wall, the ladder has to stick up at least 3 feet above the edge so you have something to hold while you step off. This calculator adds that automatically when you pick roof access. One more habit worth keeping: never stand on the top three rungs, and keep three points of contact, two feet and a hand or two hands and a foot, whenever you move. On soft or uneven ground, tie the top off or have someone foot the base.