SettingsSettings for this calculationUS
The height of the support point (eave, wall top, or landing) you need to reach.
Measure from the ground to the point the ladder will rest against.
Minimum ladder length
15.4 ft (minimum ladder length)
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
- 12 ft
They open the calculator with your figures already in it
Extension Ladder Size Calculator: 15.39 ft (minimum ladder length) — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- OSHA 29 CFR 1926.1053(b)(5)(i): "Non-self-supporting ladders shall be used at an angle such that the horizontal distance from the top support to the foot of the ladder is approximately one-quarter of the working length of the ladder (the distance along the ladder between the foot and the top support)" — about 75.5° from the ground; 1926.1053(b)(1): when portable ladders are used for access to an upper landing surface, the side rails shall extend at least 3 feet (0.9 m) above it
Inputs used
- Height to Reach
- 12 ft
Intermediate steps
- Height to reach
- 12 ft
Confidence note: 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.
What this calculation does not cover
- The setup angle is fixed at OSHA's quarter-of-the-working-length rule, a single multiplier of 1.0328 on the height, so a lean forced shallower or steeper by a fence, a flowerbed or a driveway you cannot plant the feet on changes the rail length you actually need and nothing in the arithmetic responds.
- Three feet of extension above the support point is added on every run, including the case where you will stand on the rungs and work rather than step off at the top, and that allowance is not offered as an input you can switch off.
- The support point is taken to sit against the wall face the 4:1 setback is measured to; a projecting eave, gutter, windowsill or standoff bracket pushes the base further out and adds rail length the formula carries no term for.
- The answer comes back as a raw decimal — about 15.2 ft at the 3.6 m default — rather than a size ladders are sold in, so round up to the next stocked length yourself.
- Ground condition sits outside the calculation entirely: sloping, soft or stepped footing, levelers under the rails and tying off the top all decide whether the climb is safe, and none of them is asked for.
- Reach is capped by the input itself, which refuses anything above 15 m, so the tool covers house-height access rather than the taller sectional and pole ladders used on commercial frontages.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-10-05 · v1.1.0
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