Buying the right kind of fasteners for whatever project you're working on can get confusing, especially when you're trying to match up the exact kind of bolt. Allen bolts, for instance, come in a variety of measurements with varying total length, shank length, threads per inch and diameter. They also are made to be manipulated by different sizes of Allen keys. Learn how to measure Allen bolts correctly, and you won't have to worry that you're buying bolts you'll have to take back later.
Line up your ruler's zero position with the head of your Allen bolt. Measure its total length. Write the number down, to the nearest 1/4 inch.
Keep the bolt head on the zero position and measure the portion of the bolt that doesn't have threads, called the shank. Write this down. This will vary according to the bolt's required tensile strength.
Line up the ruler's zero with the start of the Allen bolt's threads and see how many threads fit inside 1 inch. Write this down.
Measure the distance across the very top of the threaded portion of your Allen bolt. Write this diameter down. It will increase in increments of 1/16 inch.
Measure the Allen key opening at the top of the bolt, from one corner diagonally across to another. Write this down.
Using these measurements, a hardware store should be able match up the kind of Allen bolt you need. Bring in an old one, if you can, to ease the matching process.