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Extension Ladder Size Calculator

Find the minimum extension ladder length needed to safely reach a given height, per OSHA's 4:1 angle rule.

Computed in your browser — nothing you enter is uploaded. Figures are presented for United States against IRC 2024, and every formula is cited under regulatory standards below.

Last verified 2026-08-26 · v1.0.0

Market
Imperial · sales tax
11.81 ft
Schematic, drawn to the proportions you entered — not to scale on screen.

Minimum ladder length

15.17 ft (minimum ladder length)

Check your inputs

Extension ladders have overlap between their sections, so a ladder's labeled length is longer than its maximum safe extended reach — check the manufacturer's duty rating and maximum working length, not just the labeled overall length, before buying.

Height to reach
11.81 ft

For the dimensions entered, expect a minimum ladder length of 15.2 ft (minimum ladder length). Moderate confidence — sound arithmetic, but allow for the variation any real site introduces. Set for United States against IRC 2024. The market selector changes both the units and the code cited.

Estimated cost — your price

This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — confirmed fixes become pinned regression tests.

[Schema Verified] Computed in alignment with American Concrete Institute (ACI 318-19) formulas and International Residential Code (IRC 2024) spatial boundaries.

Regulatory standards & verification citations

  • OSHA recommends setting an extension ladder's base 1 ft away from the wall for every 4 ft of height (a 4:1 ratio), and extending the ladder at least 3 ft above the support point when used for access

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Frequently asked questions

Why does OSHA recommend a 4:1 base-to-height ratio?
This angle (about 75.5° from horizontal) balances stability against tipping backward with enough grip to prevent the base from sliding out — too steep or too shallow an angle both increase fall risk.
Why extend 3 ft above the landing point?
The extra length gives something stable to hold onto while stepping on and off the ladder at the top, rather than having to reach past the top rung to grab the roof edge or wall.
Does a ladder's labeled length equal its usable reach?
No — extension ladder sections overlap when extended, so a '24 ft' ladder's maximum working length is shorter than 24 ft. Check the manufacturer's specific maximum extension and duty rating (weight capacity) before buying.