Ladder Safety Calculator (4:1 Rule)
Proper extension ladder setback, working height, and reach per OSHA 1926.1053. Get the angle right or get hurt.
Where the ladder leans against the upper structure (eave, beam, edge).
Manufacturer's labeled length (extension ladders are measured at full extension).
Typical 4 ft from feet on rung to fingertip overhead.
Ladders kill roughly 130 construction workers a year. Wrong angle + missing tie-off accounts for most. The 4:1 rule is non-negotiable.
Required setback (base from wall)
Set ladder base 3.00 ft from wall
4:1 rule — height ÷ 4
LADDER LENGTH OK
Your 16.00 ft ladder meets the 15.37 ft requirement (with 3 ft extension above landing).
| Required setback (4:1) | 3.00 ft |
| Required ladder length | 15.37 ft |
| Your ladder length | 16.00 ft |
| Actual angle | 76.0° |
| Target angle | 75.5° |
| Angle delta from target | 0.5° |
| Working height (no top 3 rungs) | 13.00 ft |
| Total safe reach | 17.00 ft |
OSHA 1926.1053 key rules
- 4:1 rule — base is 1 ft out for every 4 ft of working height (1926.1053(b)(5)(i)).
- 3-foot extension above landing — side rails must extend at least 3 ft above the upper landing surface (1926.1053(b)(1)).
- Secure the top — tie off, block, or otherwise secure the top of the ladder to prevent displacement.
- Three points of contact — always maintain two hands + one foot, or two feet + one hand, while climbing.
- Do not stand on top 3 rungs of an extension ladder, or top 2 steps of a step ladder.
- Inspect before use — qualified-person inspection per 1926.1053(b)(15) for visible defects (cracked rails, missing rungs, damaged feet).
Methodology: Setback = height ÷ 4 (OSHA 4:1 rule). Required ladder length = √(height² + setback²) + 3 ft extension above landing. Actual angle = arctan(height / setback). Target is 75.5° per ANSI ladder safety guidance. Working height excludes the top 3 rungs.
Frequently Asked Questions
What is the 4:1 rule?
The 4:1 rule (OSHA 1926.1053(b)(5)(i)) says that for every 4 feet of vertical height to where the ladder rests against the upper support, the base of the ladder must be set 1 foot away from the wall. A ladder against a 16 ft eave needs the base 4 ft out. This produces a ladder angle of approximately 75.5° — steep enough to be stable on its feet, shallow enough that it will not tip backward. Too steep and the ladder kicks back; too shallow and the base slides out.
How much should the ladder extend above the landing?
OSHA 1926.1053(b)(1) requires side rails of an extension ladder to extend at least 3 feet (about 1 meter) above the upper landing surface — or the ladder must be secured at its top to a rigid support, and a grasping device (such as a grab rail) must be provided to assist the worker in mounting and dismounting. The 3 ft extension gives the worker something solid to hold onto when stepping off onto the landing, which is when most ladder falls occur.
Why can't I stand on the top 3 rungs?
Standing on the top 3 rungs of an extension ladder (or the top 2 steps of a step ladder) puts your center of gravity above the ladder's upper support point. The ladder loses lateral stability, and any small movement — leaning to one side, reaching, getting startled — can pitch you over. ANSI A14 and OSHA 1926.1053(b)(13) prohibit it. If you need more height, get a taller ladder or use a different access method (scaffold, lift, scissor lift).
Step ladder vs extension ladder — what's the difference?
A step ladder is self-supporting — it stands on its own four feet in an A-frame configuration. Max safe step is the second-from-top. Never use a step ladder leaned against a wall (folded closed) — the feet are designed for the A-frame stance, and the ladder will slide. An extension ladder is non-self-supporting — it has two sections that slide to extend length, and it relies on leaning against an upper support. The 4:1 rule, 3 ft extension, and top-3-rungs-off-limits apply to extension ladders. Different tool, different rules — pick the right one for the job.
Estimates only. Not a substitute for OSHA-compliant fall protection plan, PE-stamped scaffolding design, or qualified-person ladder inspection. Verify with a Competent Person.