How to Measure the Size of a Bolt
A bolt’s size is classified based on a number of dimensions, namely the bolt’s diameter, length of shank, thread pitch, grade, thread length and head size. Bolts can me specified in metric using millimeters, but the standard, internationally-recognized descriptions of bolt sizes are in inches.
If you’re looking to figure out the size of a bolt needed in an assembly and have a bolt that has worked on hand, follow the steps below to identify the dimensions of the bolt:
Step 1: Measure the shank’s diameter
The shaft of the bolt is called the shank, and its diameter is the first dimension used to describe a bolt size. This can be done using calipers or a bolt gauge.
Step 2: Determine the thread pitch
Thread pitch is a designation related to the number of threads per inch on the bolt’s shank. You can complete this measurement by simply counting the number of threads in an inch worth of shank. If the shank is less than one inch, you’ll need to multiply the number of threads to reach a full inch worth of threading.
Step 3: Measure the length of the shank
This is the measurement from where the shank meets the head to the very tip of the fastener.
Step 4: Determine the bolt’s grade
The grade of a bolt is determined by the type of metal used in the manufacturing of the bolt, as indicated by the bolt’s head markings. No one expects you to know all of the head markings by heart, just use our handy bolt head marking chart to make the determination.
Step 5: Determine the shape of the bolt’s head
This should be a simple one. While the most common bolt head is hex (six-sided), some applications require a four-sided square head or the dome-shaped head of a carriage bolt.
Step 6: Measure the length of the threaded portion of the bolt
As opposed to the shank length measured in Step 3, this measurement should specify only the length of the threaded portion of the bolt. One instance in which this would be useful to know is when determining whether you need a standard hex bolt or rather a fully threaded tap bolt.
