Ladder Height Calculator

Ladder safety depends on both height selection and proper angle. Using the wrong ladder height, or leaning it at the wrong angle, is one of the most common causes of falls at home and on job sites.

Safe working angle (4:1 Rule): Base Distance = Ladder Length / 4

The base of the ladder should sit 1 foot out from the wall for every 4 feet of ladder, which puts it at about 75 degrees. Note that the divisor applies to the ladder’s own length, not to the height you are trying to reach. Those are different numbers on a leaning ladder, because it also has to poke 3 feet above the roof edge and it stands at an angle rather than straight up. Getting this backwards sets the base too close and is exactly how a ladder slides out from under someone.

Step ladder: how tall a one do you need? Step Ladder Height = Reach Height − 4 ft (Your standing reach from a step ladder is roughly 4 feet above its top step, and you should never stand on the top two.)

Extension ladder: a different sum entirely An extension ladder leans, so only about 97% of its length becomes height, and OSHA wants 3 feet of it standing proud above the landing point: Ladder Length = (Working Height + 3 ft) ÷ 0.97

Worked example, gutters 16 feet up:

• As an extension ladder: (16 + 3) ÷ 0.97 = 19.6, so a 20-foot ladder
• Safe base distance: 20 ÷ 4 = 5 feet from the wall
• The same job with a step ladder would want 16 − 4 = a 12-foot step ladder, though a 12-foot step ladder is an awkward, top-heavy thing to work off outdoors and most people reach for the extension

Ladder type selection guide:

Weight rating classes:

• Type III (Light Duty): 200 lbs, household use only
• Type II (Medium Duty): 225 lbs, light trade use
• Type I (Heavy Duty): 250 lbs, industrial use
• Type IA (Extra Heavy): 300 lbs
• Type IAA (Special): 375 lbs

Always include your body weight plus the weight of tools and materials.