Bolt Diameters


In practice, the vast majority of bolts used for load bearing connections will be between M16 (5/8”) and M24 (1”). Anything less than M16 is unlikely to be certified and should be avoided. Greater than M24 would be used only in the most exceptional circumstances.

The table below will provide some idea of how the various sizes may be applied.

Metric Bolt Dia. Inch Series Bolt Dia, Application
M 12 1/2” May be used for non-critical connections for bolting handrail, stair treads etc. they may also be applied to 3rd party ancillary equipment where specified by the supplier.
M 16 5/8” Again, would usually apply to non-critical connections as noted above
M 20 3/4” The de-facto standard bolt for structural connections
M 22 7/8” This is a non-preferred bolt dia. for metric bolts – more commonly used for inch bolts
M 24 1” Usually limited to heavy duty – Moment resisting connections

A general rule-of-thumb when selecting bolts diameters is to used Class 4.6 (ASTM A307) for bolt diameters of M12 and M16 (1/2″ and 5/8″) and class 8.8 (ASTM A325) for M20 bolts – these are high-strength bolts and would be applied to 90% of structural connections.
Class 10.9 and 12.9 (ASTM A490) would generally apply to moment resisting connections – Note! that this should be specified by the responsible engineer.

Bolt Hole Diameters

The standard clearance hole for bolted joints should be 2 mm larger than the bolt diameter, (or for inch bolts 1/16”) – there will be instances where an oversized or short slotted hole may be required to allow additional erection tolerances, while a long slotted hole is often used where movement of the joint may be desirable to accommodate thermal expansion. Generally, for oversized or slotted holes, washers will be required (See washers)

Anchor bolt-holes are always oversized because the tolerances for field placement of anchor bolts usually exceed those that normally apply to fabricated steelwork. In practice, even these relaxed tolerances are exceeded.

! Note – Anchor bolt holes will usually require fabricated plate washers. This should be approved by the responsible engineer.

Metric Series

Bolt Dia. Standard Dia. Oversize Dia. Short Slot (W x L) Long Slot (W x L) Anchor Bolt Dia.
M 12 14 16 14 x 18 14 x 30
M 16 18 22 18 x 22 18 x 40
M 20 22 26 22 x 24 20 x 45 26
M 22 * 24 28 24 x 28 24 x 55 30
M 24 26 30 28 x 32 26 x 65 36

Inch Series

Bolt Dia. Standard Dia. Oversize Dia. Short Slot (W x L) Long Slot (W x L) Anchor Bolt Dia
½” 9/16” 5/8” 9/16 x 11/16 9/16 x 1-1//16
5/8” 11/16” 13/16” 11/16 x 7/8 11/16 x 1-9/16
¾” 13/16” 15/16” 13/16 x 1 13/16 x  1-7/8 1-5/16

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