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M3 Bolt Specifications

Reviewed by MLC Editorial Team | 31 May 2026

Verified by Rachel Mayfield, Supply Chain Analyst - May 2026

Close-up image of three steel bolts standing upright against a contrasting yellow background. — Photo by Győző Mórocz on Pexels

Every dimension, torque value, and thread specification for the M3 metric bolt in one reference card. M3 Bolt Specifications M3 is one of the most common metric fastener sizes in UK engineering, used across small panel mounting and instrumentation.

Property	Value	Standard
Nominal diameter	3.0 mm	ISO 261
Coarse pitch	0.5 mm	ISO 261
Fine pitch	0.35 mm	ISO 261
Tap drill (coarse, 75% engagement)	2.5 mm	ISO 2306
Tap drill (fine)	2.65 mm	ISO 2306
Clearance hole - close	3.2 mm	ISO 273
Clearance hole - medium	3.4 mm	ISO 273
Clearance hole - coarse	3.6 mm	ISO 273
Hex head A/F (spanner size)	5.5 mm	ISO 4014
Hex head height	2.0 mm	ISO 4014
Socket head cap screw head dia	5.5 mm	ISO 4762
Socket head hex key size	2.5 mm	ISO 4762
Tensile stress area	5.03 mm^2	ISO 898-1
Minimum tensile strength 8.8	800 MPa	ISO 898-1
Minimum tensile strength 10.9	1040 MPa	ISO 898-1
Proof load 8.8	2.9 kN	ISO 898-1
Proof load 10.9	4.2 kN	ISO 898-1
Torque dry, class 8.8	1.3 Nm	VDI 2230
Torque dry, class 10.9	1.8 Nm	VDI 2230
Torque dry, class 12.9	2.1 Nm	VDI 2230

M3 thread pitch

Standard M3 bolts use a 0.5 mm coarse pitch. Fine pitch M3 x 0.35 is used where vibration resistance matters or in thin-wall components. Coarse pitch is the default - if a drawing says M3 with no pitch specified, it means M3 x 0.5 coarse.

M3 tap drill size

For M3 x 0.5 coarse pitch with 75% thread engagement, drill 2.5 mm. For fine pitch, drill 2.65 mm. Smaller drills give more engagement but increase tap breakage risk without meaningful strength gain.

M3 clearance hole

For a clearance hole where the bolt passes through, use 3.2 mm (close fit), 3.4 mm (medium), or 3.6 mm (coarse) per ISO 273.

M3 torque values

Dry M3 property class 8.8: torque to 1.3 Nm. Class 10.9: 1.8 Nm. Class 12.9: 2.1 Nm. These values target 90% proof load with K=0.20 dry thread friction. If threads are oiled or coated, reduce by 20-25%. For critical joints, always verify against the fastener manufacturer's datasheet.

M3 spanner and hex key

An M3 hex head bolt (ISO 4014) has 5.5 mm across flats, so you need a 5.5 mm spanner or socket. M3 socket head cap screws (ISO 4762) have 5.5 mm head diameter and need a 2.5 mm hex key (Allen key).

Related standards

ISO 261 - ISO general-purpose metric screw threads
ISO 273 - Fasteners: clearance holes for bolts and screws
ISO 898-1 - Mechanical properties of carbon steel fasteners
ISO 4014 - Hexagon head bolts, product grades A and B
ISO 4762 - Hexagon socket head cap screws
VDI 2230 - Systematic calculation of bolted joints

References

ISO 261:1998 - ISO metric screw threads
ISO 898-1:2013 - Mechanical properties of fasteners
VDI 2230:2015 - Calculation of bolted joints

M3 bolts and tools on Amazon

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Frequently Asked Questions

What is the thread pitch of an M3 bolt?

A standard M3 bolt has a coarse thread pitch of 0.5 mm per ISO 261. The fine pitch version is M3 x 0.35. If a drawing says M3 with no pitch specified, it means M3 x 0.5 coarse.

What is the tap drill size for M3?

For M3 x 0.5 coarse pitch with 75% thread engagement, drill 2.5 mm. For fine pitch, drill 2.65 mm. These values give a good balance of thread strength and tapping ease.

What torque is an M3 bolt rated for?

For a dry M3 property class 8.8 bolt, the standard torque is 1.3 Nm. Class 10.9 takes 1.8 Nm, class 12.9 takes 2.1 Nm. Values are per VDI 2230 at 90% proof load with nut factor K=0.20. Reduce by 20-25% if threads are lubricated.

What spanner size fits an M3 bolt?

An M3 hex head bolt (ISO 4014) has a 5.5 mm across-flats (A/F) dimension, so you need a 5.5 mm spanner or socket. An M3 socket head cap screw uses a 2.5 mm hex key (Allen key).

References

Independent UK engineering trade publication. Company profiles cross-referenced with Companies House.