Fastener Guide

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.

    Save Step 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).

    Save Step 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.

    Save Step 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.

    Save Step 4: For flat and countersunk heads, measure the overall length including the head.

  5. Identify the grade from the head markings (lines or numbers).

    Save Step 5: Identify the grade from the head markings (lines or numbers).

  6. Match these to the spec table to order a replacement.

    Save Step 6: Match these to the spec table to order a replacement.

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