Unlike fixed ladders that are one certain length, extension ladders are adjustable. They have at least two sections that slide out when they are set up. Extension ladders come in sizes that range from 6 feet to 60 feet. To choose the rightsized ladder for a task, you must know how long a ladder is. There are three ways to find this out.
Things You'll Need
 Measuring tape

Check the outside rail on an extension ladder to find the load capacity sticker. This tells you how many pounds the ladder can safely hold. The load capacity sticker also tells you what size the ladder is in feet.

Count the rungs (also called pins) on an extension ladder. Each rung is approximately 1 foot apart. If a ladder has 24 rungs, for example, that means you can roughly determine its size as being 24 feet long.

Use a measuring tape to find out the length of one section of the extension ladder. Multiply that number by the number of sections the ladder has, and you'll be able to determine the size of an extension ladder.
Tips & Warnings
 Be careful to choose the rightsized extension ladder for the job. If a ladder has two sections and each is 12 feet long, the ladder measures 24 feet long. However, they're are not all usable feet. The sections on extension ladders overlap each other by at least 1 1/2 feet. You'll need to subtract the overlap from the total number of feet.