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Identifying the size of my Bolt or Screw
Bolt Identifier Tool
Not sure what thread your bolt or screw is? Measure the diameter, count the threads, and we’ll match it for you.
1Measure the outside diameter
Measure across the outside of the threads (the major diameter).
Close the calipers across the outside of the threads (the major diameter) and read the measurement.
Enter your measurement, or select a range from the dropdown.
2Work out the thread spacing
Lay the thread along a ruler and count the thread crests across one of the lengths below — we’ll work out the pitch in millimetres for you. A longer length is more accurate, so use the longest your screw allows.
Listed best first — a longer length gives a more accurate reading, so use the longest your thread allows.
Tip: count over a true inch mark (not the rounded metric value), and over the longest length your screw allows, for the most accurate result.
Quickest way to be sure, especially metric vs imperial. Start with how you turn the fastener and we’ll show the size to check against yours.
Pop a spanner on the head. The size is the distance across the flats (A/F) — between two opposite faces of the hex, not corner to corner. Check it against the size below.
Pick the head type and we’ll show the key size to check against yours.
Key sizes from the BNA socket-screw chart.
Match the driver tip to the recess. Screws like this (machine screws and self-tappers) don’t carry grade markings, so go by the diameter and thread spacing in your result.
Your result
Enter a diameter and the thread spacing above to see the matching thread.
© 2026 Bolt & Nut Australia Pty Ltd. All rights reserved. This bolt identifier tool, including its design, content and data, is protected by copyright and may not be copied, reproduced or republished without written permission.
