Excerpt from  American Ladder Institute

If you want to work safely on a ladder, it's important to purchase one that's the right length for the task at hand. While you may think you can look at a particular wall and have a sensible guess at the ladder height you need, you're never going to be 100% accurate. If you want to calculate the right ladder length, follow these steps.

Extension Ladder Length

In order to be used safely, extension ladders need to be leant up against a hard surface with a suitable pitch. If you're working on a platform like a roof, you need to make sure your ladder extends a few feet past the workspace. When calculating the right extension ladder length, it's important to take these extra few feet into account.

Here's an example to show you how to calculate the right ladder height for a 20ft wall.

To work safely, your extension ladder needs to be pitched at a ratio of 4:1. So for every 4 feet up, you need to move the ladder 1 foot out. This is known as the '1 in 4' rule.

So, if you want to work on a 20ft wall, the ladder needs to be moved 5ft out. Propping a ladder against a wall creates a right-angled triangle, with the ladder as the hypotenuse (longest side), so it's time to think back to math class: to calculate the length of the hypotenuse, we'll need to use Pythagoras's theorem.

In case you haven't brushed up on your trigonometry lately, Pythagoras's theorem states that the length of the hypotenuse squared = the triangle's height squared + the triangle's width squared. Let's apply this to the example above...

  • 20 squared is 400
  • 5 squared is 25
  • 400 + 25 = 425
  • The square root of 425 (to two decimal places) is 20.62

This means that your ladder needs to be 20.62 feet long (minimum) to allow for the ladder to be propped up against the wall at a safe angle.

Then, you need to add a few feet so that the extension ladder can extend past the working ledge. So, if you want to climb a 20ft wall, you need a ladder that's approximately 24ft long!