Measure the diameter of the bolt shaft across the threads
Measure the length from under the head to the end of the bolt
Identify the thread pitch by counting threads per inch or measuring the distance between threads
Check the head type and drive style
Determine the bolt grade or strength marking
Verify whether the bolt uses metric or imperial sizing
Measure the shank length if the bolt has an unthreaded section
Compare the measurements to a bolt size chart
Use a caliper or thread gauge for accuracy