What size bolt do I need for timber framing?

For typical timber framing with 50-100mm timber, M10 or M12 bolts are most common. The bolt diameter should be no more than 1/4 of the timber thickness. For heavy structural work, always consult a structural engineer.

Should I use washers with timber bolts?

Always use washers under both the bolt head and the nut when bolting timber. Large-diameter washers (at least 3x bolt diameter) spread the load and prevent the bolt head from pulling into the wood under load.

How tight should I torque timber bolts?

Tighten until the washer just begins to compress the wood surface, then give an additional quarter turn. Over-tightening crushes the wood fibres and reduces the joint strength. Re-check after a few weeks as wood shrinks.

WoodCalcs

Bolt Size Calculator — Timber Bolt Connections

Calculate bolt diameter, length, and washer requirements for timber connections and post-and-beam joinery.

Connection Type

First Piece Thickness

Second Piece Thickness

Number of Bolts

Price per Bolt Set (optional)

Bolt + nut + 2 washers

How We Calculate This

This calculator determines the correct bolt size for timber-to-timber and timber-to-steel connections based on timber thickness.

Bolt length calculation

Bolt length = Total timber thickness + (2 x washer thickness) + nut height + 5mm

The result is rounded up to the nearest standard metric bolt length.

Drill hole size

The drill hole should be 1mm larger than the bolt diameter to allow for easy insertion and slight timber movement.

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Always verify measurements and material quantities before cutting or purchasing.