Ladder angle & length calculator
Get the ladder length and the base distance for a height, or check a ladder's angle.
The roof edge or wall point where the ladder rests, measured straight up from the ground.
The length the ladder is extended to, along its rails.
What angle should a ladder be set at?
OSHA requires a straight or extension ladder to be used at an angle where the horizontal distance from the top support to the foot is approximately one-quarter of the working length — the distance along the ladder from the foot to the top support (29 CFR 1926.1053(b)(5)(i)). That is the 4-to-1 rule, an angle of about 75.5° from the ground. A ladder with a 20 ft working length stands 5 ft from the wall.
How long a ladder do I need to reach a roof?
Divide the height of the roof edge by 0.968 to get the working length, then add 3 feet, because OSHA says the side rails must extend at least 3 feet (0.9 m) above the landing (1926.1053(b)(1)). For a 12 ft eave the working length is 12.4 ft and the ladder must be at least 15.4 ft long, with the foot 3 ft 1 in from the wall. The calculator measures the 3 feet along the rails; the rule does not say whether along the rails or straight up. An extension ladder reaches less than its labeled length because its sections overlap, so check its label for the length it extends to.
How far from the wall should the ladder base be?
One-quarter of the working length: about 1 foot out for every 4 feet of ladder. A ladder with 21 ft between the foot and the top support gets a foot distance of 5 ft 3 in, and its top support is 20.3 ft up. Too close to the wall and the ladder is steeper than the rule and can tip backward; too far out and it is shallower and can slide.
How do I check the angle of a ladder I already set up?
Measure how far the foot is from the wall and how high the ladder touches it, and pick Check my setup. The calculator works out the angle and the working length, and tells you whether the foot should move in or out. For example, a foot 4 ft from the wall against a point 20 ft up is at 78.7°, steeper than 75.5°: the foot should be 5 ft 1 in out. OSHA's text says “approximately” one-quarter and gives no tolerance.
Do job-made ladders follow the same rule?
Not all of them. For wood job-made ladders with spliced side rails OSHA sets the distance at one-eighth of the working length (1926.1053(b)(5)(ii)). The one-quarter rule is for non-self-supporting ladders such as straight and extension ladders; a step ladder stands on its own and is opened fully instead.
Sources: OSHA, 29 CFR 1926.1053(b)(1) and (b)(5), Ladders (eCFR text); OSHA QuickCard “Portable Ladder Safety” and fact sheet 3660, “Safe Use of Extension Ladders”. The 75.5° angle is arccos(¼). Any other right triangle: right triangle calculator. How we check our calculators.