What size bolt do I need for timber framing?

For typical timber framing with 50-100mm timber, M10 or M12 bolts are most common. As a rule of thumb keep the bolt diameter to no more than 1/4 of the timber thickness, so thinner members (around 24-32mm) are better suited to M6 and 32-40mm to M8. This calculator follows that guideline when recommending a size. 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.uk

Bolt Size Calculator — Timber Bolt Connections

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

Connection TypeTimber-to-timber and post-and-beam use a washer both ends; a steel bracket bears on the plate, so only one timber-side washer is added to the bolt length.

First Piece Thickness

Thickness of the main (first) timber member in mm

Second Piece Thickness

Thickness of the second timber (or the steel bracket) being joined, in mm

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 thickness + (washers x washer thickness) + nut height + protrusion

Timber-to-timber and post-and-beam joints add a washer under both the head and the nut (two washers); a timber-to-steel bracket bears on the steel plate so only one washer is added. Washer thickness and nut height follow the standard DIN 125-A washer and DIN 934 nut sizes, and 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 (BS EN 1995-1-1 limits the bolt hole to no more than 1mm over the bolt diameter).

Nut spanner size

The spanner (across-flats) size is the standard DIN 933/934 metric hex size: M6 = 10mm, M8 = 13mm, M10 = 17mm, M12 = 19mm, M16 = 24mm.

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.