Ladder Height Calculator
Find the safe ladder length needed for a given working height.
About Ladder Height Calculator
A ladder is the hypotenuse of a right triangle, so its length is set by two things: how high you need to reach and how far the base sits from the wall. The 4:1 rule fixes the second - one foot out for every four feet of height - which puts the ladder at roughly 76 degrees, steep enough not to slide out at the base and shallow enough not to tip backwards. That makes the minimum length sqrt(reach^2 + base^2), always about 3% more than the reach itself. If you are stepping off onto a roof rather than working from the ladder, the rails have to carry on three feet past that landing, so the reach figure grows before the triangle is solved. Buy the next size up: the result is a minimum, extension ladders lose a foot or two to section overlap, and you should never stand on the top three rungs. Setting a ladder too upright makes it tip backwards and too shallow makes the feet slide out, and both failures happen at the moment you lean sideways rather than while you are climbing.
Formula
Ladder Height
Base = reach / 4 (the 4:1 rule); ladder length = sqrt(reach^2 + base^2); add 3 ft of reach when stepping onto a roof
- 4:1 rule
- OSHA and ANSI put the base one quarter of the working height out from the wall, which sets the ladder at about 76 degrees
- 3 ft
- Side rails have to extend at least three feet above any landing you step off onto (29 CFR 1926.1053)
Worked example
With the default values below, Ladder Height Calculator returns Minimum Ladder Length of 20.6. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Height of the support pointaccepts 3–60 ft
- 20 ft
- Stepping off onto a roof or landing
- No
Results
- Minimum Ladder Length
- 20.6 ft
- Base Distance from Wall
- 5.0 ft
- Height the Ladder Must Reach
- 20.0 ft
- Set-up AngleThe 4:1 rule always lands near 76 degrees.
- 76.0 deg
Step by step
- 1Values used
- 2Ladder Height
- 3Minimum Ladder Length= 20.6 ft
- 4Base Distance from Wall= 5.0 ft
- 5Height the Ladder Must Reach= 20.0 ft
- 6Set-up Angle= 76.0 deg
Assumptions and limits
- The result is derived only from the 2 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 1 decimal place, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
How is Ladder Height calculated?
Base = reach / 4 (the 4:1 rule); ladder length = sqrt(reach^2 + base^2); add 3 ft of reach when stepping onto a roof A ladder is the hypotenuse of a right triangle, so its length is set by two things: how high you need to reach and how far the base sits from the wall. The 4:1 rule fixes the second - one foot out for every four feet of height - which puts the ladder at roughly 76 degrees, steep enough not to slide out at the base and shallow enough not to tip backwards. That makes the minimum length sqrt(reach^2 + base^2), always about 3% more than the reach itself. If you are stepping off onto a roof rather than working from the ladder, the rails have to carry on three feet past that landing, so the reach figure grows before the triangle is solved. Buy the next size up: the result is a minimum, extension ladders lose a foot or two to section overlap, and you should never stand on the top three rungs.
Why does Ladder Height matter?
Setting a ladder too upright makes it tip backwards and too shallow makes the feet slide out, and both failures happen at the moment you lean sideways rather than while you are climbing.
What values do I need to enter?
This calculator takes 2 inputs: Height of the support point, Stepping off onto a roof or landing. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.