How to use the ladder length calculator
- Choose Ladder length to find what you need, or Reach height to find how high a ladder you already have will reach.
- Enter the height of the support point, where the top of the ladder touches the wall or roof edge. For the reach option, enter the ladder's length instead.
- Enter how far the ladder extends past the support point, if it does. Then set the lean either as an angle or as a ratio of ladder length to base distance.
- Read the ladder length, the length up to the support, the base distance and the angle.
Formula
Length up to the support = support height / sin(angle)
Overall length = length up to the support + extension
Base distance from the wall = length up to the support x cos(angle)
Ratio setup: cos(angle) = 1 / ratio
The angle is measured from the ground. A ladder length to base ratio of 4 means the base sits one quarter of the working length from the wall, so cos(angle) = 1/4.
Reaching a point 12 feet up
At an angle of 75 degrees, sin(75) = 0.9659, so the length up to the support is 12 / 0.9659 = 12.42 feet. The base sits 12.42 x cos(75) = 12.42 x 0.2588 = 3.22 feet, or 38.6 inches, from the wall. If the ladder must extend 3 feet past the support, the overall length is 15.42 feet.
With a ratio of 4 to 1 the angle is the angle whose cosine is 0.25, which is 75.52 degrees. The ladder is then 12 / sin(75.52) = 12.39 feet long with the base 3.10 feet out. Going the other way, a 16 foot ladder with 3 feet extending past the support, at 75 degrees, reaches a support point 13 x 0.9659 = 12.56 feet high with the base 3.36 feet out.
This is geometry, not a safety rating
The tool turns a height and an angle into a length using right-triangle geometry. The angle and the ratio are example values. The correct setup for a particular ladder is the one in its label and the manufacturer's instructions, along with its duty rating, so follow those. The number shown is the minimum length for the geometry, and a ladder that is too short for it is not safe to use at that height.
Ladders must also sit on firm, level footing, and the top must rest on something that will carry the load. The calculation does not check any of that.
Choosing a ladder
Add the extension to the length if you need to step onto a roof or platform, and then pick the next standard size up from your result. A stepladder is freestanding and is not leaned against a wall, so it does not use this calculation.
For the geometry behind it, see the Pythagorean theorem calculator and the trigonometry calculator. Roof work may also involve the roof pitch calculator and the rafter length calculator.
Assumptions and limits
- The ground is level and the wall is vertical.
- The angle of 75 degrees and the ratio of 4 are example values you can change. They are not a safety standard.
- The extension is measured along the ladder past the support point.
- Uneven or soft footing, ladder duty ratings and loads are not modeled.
Frequently asked questions
How do I find the ladder length I need?
Divide the height of the support point by the sine of the angle the ladder leans at, then add any length that should extend past the support. At 75 degrees and 12 feet the ladder must be at least 12.42 feet long before any extension.
How far from the wall should the base of the ladder be?
It is the length up to the support times the cosine of the angle. At 75 degrees that is about 26 percent of the length up to the support. Use the angle or ratio on your ladder's label.
What is the 4 to 1 rule for ladders?
It is a commonly cited setup in which the base sits one quarter of the working length from the wall, an angle of about 75.5 degrees. Here it is only an editable example, so follow the instructions and labels for your own ladder.
Can I use this for a stepladder?
No. A stepladder stands on its own and has its own height and use limits. This tool is for a straight or extension ladder leaned against a support.