How to measure bolts and set screws

1 min read

In a blog post, Thomas Smith Fasteners examines the most common methods for measuring bolts and screws.

(Image credit: Thomas Smith Fasteners)

Measuring bolts and set screws is an important task that both DIY enthusiasts and professional engineers should be familiar with. Accurate measuring ensures that you use the right tool for the job and avoid any potential problems that could arise from using the wrong size.


The length of a hexagon bolt is determined by measuring from underneath the head to the end of the bolt.

If the bolt has a countersunk head, the full length of the bolt, including the head, is measured.


To determine bolt diameter, a caliper, micrometre or bolt gauge can be used. Place the measuring tool around the threaded portion of the bolt and take a reading.

For metric bolts, the prefix 'M' is added to denote Metric. For example, a 4mm diameter bolt is commonly known as an 'M4' bolt. Imperial bolts are measured in inches.


Metric bolts are identified by measuring the distance between the threads - a 1mm pitch bolt has 1mm between the threads.

The thread pitch of an imperial bolt is denoted by the number of threads per inch.

The Thread Detective is a quick and easy solution to the problem of identifying bolt and nut threadforms. It allows quick and precise identification of Metric, UNC, and UNF bolts and nuts.


Measuring head size determines the size of the socket or wrench necessary. To measure the head size, use a ruler or a socket measuring tool to determine the distance across the flats of the bolt head.

Following these steps can ensure the right tool is used for the job while also avoiding any potential problems that could arise from using the wrong size.