Step ladders can be used to help you reach high shelves, clean unreachable areas, paint rooms and install new fixtures. One of the problems homeowners struggle with when they are looking to purchase a new step ladder is not only what size they need, but how to measure a ladder in the first place. The advertised height of a step ladder isn't necessarily the same as the length you'll be able to use. When measuring a step ladder, take into account safety precautions as well as usable space. There are two different measurements you should take when measuring a step ladder.
Close the step ladder and stand it straight up. Have a family member or friend hold the step ladder level or lean it up against a wall as straight as possible. It's more important that you measure the actual structure of the ladder, so don't worry about getting the ladder perfectly level.
Measure the height from the bottom to the top of the ladder. Measure one of the supporting beams instead of the ladder rungs, since most rungs start off the ground. Make a note of the measurement. This will be the measurement that's used commercially to note the height of a step ladder that is being sold. Realize that this won't be the safe standing height on the ladder, but the height of the structure itself.
Measure the height from the bottom to the third-to-last rung of the ladder. For safety reasons, you should never stand on the top two rungs of a step ladder. Since rungs are usually spaced a foot apart, this will take away at least two feet from the usable height of the ladder. Note this measurement as the usable height of the ladder.