About the Ladder Angle Calculator
This ladder angle calculator answers two everyday questions: how far should the foot of the ladder be from the wall, and what size ladder do I need? Enter the height where the ladder rests against the wall or gutter and it applies the 4-to-1 rule — one foot out for every four feet up — to give the base distance, the ladder length along the rails and the resulting angle of about 76°. (OSHA words the rule as one quarter of the ladder’s working length, which gives 75.5° — the difference is only a few inches at the foot.)
If you are climbing onto a roof or landing, switch on the extension so the rails reach 3 feet above the edge, as OSHA requires for access ladders. The calculator then suggests the smallest standard extension ladder that still reaches after accounting for the overlap between its sections.
You can also enter the base distance you actually have to check the angle of an existing setup. Angles far above 76° risk the ladder tipping backwards; much lower angles risk the foot sliding out.
With the default inputs, the ladder length needed is 23.6 ft. Change any value above to recalculate instantly.
How to use the ladder angle calculator
- 1Measure the height where the top of the ladder will rest.
- 2Turn on the +3 ft extension if you will step off onto a roof or landing.
- 3Read the ladder length needed and the extension ladder size to buy.
- 4Place the feet at the base distance shown — about 1 ft out per 4 ft up.
- 5Optionally enter your real base distance to check the angle.
Formula and method
The 4-to-1 rule places the base one quarter of the working length away from the wall: for a support point H feet up, the base sits H ÷ 4 feet out. The ladder, wall and ground form a right triangle, so the rail length is the hypotenuse √(H² + (H/4)²) ≈ 1.03 × H, and the angle to the ground is arctan(4) ≈ 75.96°.
When the ladder is used to reach a roof or landing, its side rails must extend at least 3 feet above the upper surface, so 3 feet is added. Extension ladders are sold by total section length, but the sections overlap (3 ft for ladders up to 36 ft, 4 ft above), so the suggested size is the smallest whose usable length still covers the total.
- H
- Height of the support point (ft)
- Base
- Horizontal distance from wall to ladder feet (ft)
- Angle
- Angle between ladder and ground (degrees)
Worked examples
Getting onto a roof with a 20 ft eave
The base goes 20 ÷ 4 = 5 ft out. The rails reach the eave after √(20² + 5²) = 20.62 ft, plus 3 ft above the edge = 23.6 ft. A 24 ft ladder only reaches 21 ft with overlap, so buy a 28 ft extension ladder (25 ft usable).
Cleaning gutters at 12 ft (no roof access)
For a 12 ft support point the foot sits 3 ft from the wall and the ladder must be 12.37 ft long. A 16 ft extension ladder gives about 13 ft of usable length.
Checking a setup with the base 6 ft out at 16 ft
With the feet 6 ft out the angle is arctan(16/6) = 69.4°, flatter than the 4:1 target, so the foot could slide. Moving the base to 4 ft restores roughly 76°.
Frequently asked questions
What is the 4 to 1 rule for ladders?+
For every 4 feet of height to the point where the ladder rests, place the base 1 foot out from the wall. This gives an angle of about 75–76°, which OSHA references for non-self-supporting ladders.
What size ladder do I need for a 2-story house?+
A typical two-story eave is 18–22 ft high. With the 4:1 angle and 3 ft of extension above the roof edge you need roughly 22–26 ft of usable length, which usually means a 28 or 32 ft extension ladder.
How far should a ladder extend above the roof?+
OSHA requires the side rails of a portable ladder used to access an upper surface to extend at least 3 feet above that surface, so you have something to hold while stepping on and off.
Why is an extension ladder shorter than its size?+
The sections overlap to lock together. Ladders up to 36 ft typically lose about 3 ft to overlap and longer ones about 4 ft, so a 28 ft ladder gives roughly 25 ft of working length.
What is the correct angle for a ladder?+
About 75–76° to the ground — the 4:1 ratio (76° measured by height, 75.5° when the base is a quarter of the ladder length). A much steeper angle risks tipping backwards, and a shallower one risks the base kicking out, especially on smooth surfaces.
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