How to Calculate Thread Length on a Bolt

At Arun Fasteners we are often asked about the thread length on differing size Hex Head Bolts.  Below is a helpful guide.

NOTE: It is measured from UNDER the head

Metric Bolt Thread Length

0 to 125mm2 x Diameter + 6mm
125 to 200mm2 x Diameter + 12mm
200+mm2 x Diameter + 25mm

Imperial Bolt Thread Length

Thread length = 2 x diameter + 1/4″ (up to 6″), 2 x diameter & 1/2″ (over 6″)

NOTE: It is measured from UNDER the head

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