Extension Ladder Calculator

Extension ladder calculator: length class for a given height

An extension ladder calculator picks the shortest ladder that reaches your contact point at a safe angle. It uses the 4 to 1 rule from OSHA, adds 3 ft above the roof edge, and adds the overlap between the sections. A gutter at 10 ft needs a working length of 10.3 ft. With the 3 ft extension and 3 ft of overlap, the ladder is 16.3 ft, so you need a 20 ft class ladder, with the base 2.6 ft from the wall.

The formula in plain words

Picture the ladder, the wall, and the ground as a right triangle. The ladder leans on the wall. The base sits a quarter of the ladder’s working length out from the wall. That sets the angle.

1. Working length = contact height ÷ √(1 − 0.25²), or height ÷ 0.968.
2. Base distance = working length ÷ 4.
3. Extended length = working length + extension (3 ft to reach a roof, 0 to work from the ladder).
4. Nominal length = extended length + overlap (3 ft, or 4 ft once the ladder passes 36 ft).
5. Ladder class = the next size up from the nominal length: 16, 20, 24, 28, 32, or 40 ft.

Worked examples

These match the calculator exactly.

Example 1: a 10 ft gutter, climbing onto the roof (3 ft extension).

• Working length: 10 ÷ 0.968 = 10.33 ft
• Base distance: 10.33 ÷ 4 = 2.58 ft
• Extended length: 10.33 + 3 = 13.33 ft
• Nominal length: 13.33 + 3 = 16.33 ft
• Ladder class: 20 ft

Example 2: an 18 ft eave, climbing onto the roof (3 ft extension).

• Working length: 18 ÷ 0.968 = 18.59 ft
• Base distance: 18.59 ÷ 4 = 4.65 ft
• Extended length: 18.59 + 3 = 21.59 ft
• Nominal length: 21.59 + 3 = 24.59 ft
• Ladder class: 28 ft

Ladder class and extended length

Ladder class	Maximum extended length	Overlap
16 ft	13 ft	3 ft
20 ft	17 ft	3 ft
24 ft	21 ft	3 ft
28 ft	25 ft	3 ft
32 ft	29 ft	3 ft
40 ft	36 ft	4 ft

The OCWR fact sheet gives the 24 ft example, where two 12 ft sections extend to 21 ft. Check the label on your own ladder, since the real extended length is printed on it.

Safety notes

• Set the base at one quarter of the working length. A ladder set too steep can tip back, and one set too flat can slide out.
• Extend the rails at least 3 ft above the landing you are stepping onto.
• Keep three points of contact while you climb.
• Do not climb above the highest standing level, about four rungs from the top (Ask The Builder).
• Set the feet on firm, level ground and check the locks first.
• Keep clear of power lines.

How to measure

• Measure from the ground to the gutter, the roof edge, or the point where the ladder will rest.
• Add the height of any porch or deck the base will stand on.
• Enter 3 ft of extension if you will step onto the roof, and 0 if you will work from the ladder.

Common mistakes

• Buying a ladder by its label length. The reach is shorter by the overlap and the extension.
• Placing the base too close to the wall to save space.
• Leaning the ladder on a gutter. Rest it against the wall or a stand-off.
• Forgetting the 3 ft above the roof when the job ends on the roof.

Sources

• OSHA 29 CFR 1926.1053(b)(1) and (b)(5)(i): 3 ft extension above the upper landing, and a base about one quarter of the working length from the support (ecfr.gov, 29 CFR 1926.1053).
• OSHA standard interpretation letter, March 16, 2009 (osha.gov/laws-regs/standardinterpretations/2009-03-16-3).
• Ask The Builder, extension ladder guidance (askthebuilder.com).
• OCWR, Fast Facts: Extension Ladders: overlap of 3 ft up to 36 ft ladders and 4 ft for 40 ft ladders, and the 24 ft ladder extending to 21 ft (ocwr.gov).

The maximum extended lengths in the table follow from the 3 ft overlap rule and our class sizes. Manufacturer labels govern. The 4 ft overlap case beyond 36 ft is simplified. This page does not replace the ladder label or your site safety rules.

Related calculators

For the slope of the roof you will be stepping onto, use the roof pitch calculator. A similar rise-and-run problem is in the stair calculator. Add up the lengths for a long job with the linear feet calculator.