There are six pieces of information that describe the measurement of a bolt. These are: diameter, thread pitch, shank length, grade, head and thread length. Bolt measurements can be calculated in millimetres or inches, but the manufacturer's specifications for bolts are international. This means that should you ask for a 1/2"x12x4" Grade 2 Hex Bolt Head with a 1" thread, the request is universally understood by bolt manufacturers.
Measure the diameter of the shank. The shank is the shaft-like part of the bolt. Use calibres to obtain an accurate measure of the shank's diameter. In the above example, 1/2 inch is the shank's diameter. It is the first measurement used to describe the bolt.
Determine the thread pitch by counting the threads along the shaft. Do this by measuring one inch of the shaft and counting how many threads (a rise and depth is one thread) are in that inch. In the example, there are 12 threads per inch, so the measure reads as 12.
Measure the length of the shank. The length of the shank is from the point where it meets the head to the opposite end. In the example, this is 4 inches.
Determine the grade of the bolt. The grade describes the grade of metal or the type of material used to manufacture the bolt. The grade of the bolt may be indicated by markings on the head. Manufacturers use a variety of ways to mark the grade on their bolts.
In the example, Grade 2 signifies a soft steel was used to make the bolt.
Look at the head of the bolt. The shape of the head indicates what type of bolt it is. If the head is square, it is a square head. A hex bolt head will have a six-sided head. A carriage will have a dome-shaped head with a square support between it and the shaft.
There are hundreds of different types of bolts, though the most common ones are included in the above listings. In the example, it is a hex bolt head.
Measure the thread length. This is the amount of shaft that is actually threaded. In the example, the thread length is 1 inch.
A bolt size, then, consists of these six indicators. Were you to contact a bolt manufacturer and say, "I need 20 1/2 inch x 12 x 4 inch Grade 2 Hex Head bolts with a 1 inch thread length," the manufacturer would know exactly what you mean.