There are many different parameters that fully describe a bolt. The head type, the length, the shank length, and so on. But perhaps the most confusing to many people is the bolt thread.
Bolt threads are really very easy to define. They are described by two measurements--the thread diameter and the thread pitch. There are also two kinds of bolt measurements--imperial and metric.
To determine the thread type and size, you simply take the two measurements. You can then determine if the thread is imperial or metric.
Using a caliper, take the diameter of the bolt thread. Take the diameter at the plain shank portion of the bolt, if available. If there is no plain shank portion, make sure you are measuring across the outside tips (crests) of the thread.
Read the value from the caliper to determine the thread diameter. Measure using both imperial and metric units. The measurement that fits most closely to one particular unit, such as 1/2 inch or 12 mm, will usually indicate if the bolt is imperial or metric.
Measure the thread pitch. If you have a thread pitch gauge, mesh the serrated edge of each gauge arm in turn with the bolt thread. Note the value stamped on the arm that meshes tightly with the thread.
If you do not have a thread pitch gauge, use a ruler to measure the thread pitch. For a metric bolt, measure the distance between the tips of two adjacent threads. For an imperial bolt, count the number of thread tips per inch.
Write down the measurements in the correct form. For imperial threads, the form is thread diameter followed by threads per inch (TPI). For example 1/2-32. Imperial bolts less than 1/4 inch diameter use a number rather than a measurement for the diameter. For example 10-32. You will need to look up these values on a thread table. Refer to InStockFasteners.com in the Resources links.
For metric threads, the form is thread diameter followed by thread pitch. For example, 12 x 1.25.