How to Measure a Bolt

1. Measure the thread diameter across the outside of the threads (the major diameter) — this is the first number, e.g. 1/4 in or M6.
2. Count the thread pitch: threads per inch for imperial (e.g. 20 = 1/4-20), or millimeters per thread for metric (e.g. 1.0 = M6 x 1.0).
3. Measure length from under the head to the tip for most bolts — not including the head.
4. For flat and countersunk heads, measure the overall length including the head.
5. Identify the grade from the head markings (lines or numbers).
6. Match these to the spec table to order a replacement.

Related Fasteners

• Hex Bolt
• Machine Screw
• Carriage Bolt
• Threaded Rod