How to Measure Bolt Size
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 millimeters 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. Does this Spark an idea?
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Instructions
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How to Measure Bolt Size
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1
Measure the diameter of the shank. The shank is the shaft-like part of the bolt. Use calibers 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.
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2
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.
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3
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.
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4
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.
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5
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.
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6
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.
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Comments
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can u show me the example of calculation.
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can show me the example of calculation.
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Anonymous
Jul 12, 2011
While it may be short, great post on an intro to measuring bolts. Of course there is much more to be written on and expanded upon- but great post for people just starting to figure out what all the bolt terms mean! -
robnet
Jul 28, 2010
Dear Ms McRae: We felt a need to respond to your bolt-measuring article because we believe it is incorrect in some ways and misleading in others. Firstly, your choice of a ½ diameter bolt with 12 threads per inch is a flawed example since a ½-inch bolt has 13 threads per inch. Within the industry, this typically would be written ½-13 x 4 rather than 1/2x13x4 as you have offered. Your method of measuring threads per inch may work for some medium and larger diameter bolts, but would not work for others. In a smaller diameter bolt with fine threads, for instance, it would be very difficult to measure the threads with the method you describe. One would need a thread pitch gage to be sure. Perhaps this is the very reason you came up with 12 threads on the ½ bolt in your example, in effect proving just how imprecise this method can be! Moreover, there has been a significant...
