Coach Bolt Size Guide: M6 to M20 Sizes and Uses

Choosing the right coach bolt size is important for strength, fit and installation. Using the wrong diameter or length can lead to weak connections or unnecessary rework on site.

This guide explains coach bolt sizes, how to measure them, and how to select the correct size for timber fixing and construction work.

How coach bolt sizes are measured

Coach bolts are measured by two key dimensions:

Diameter (M size) – the thickness of the bolt shaft, and the nominal thread diameter
Length – measured from under the head to the end of the thread

For example, an M10 x 120mm coach bolt has a 10mm diameter shank with M10 thread, and is 120mm long from underneath the head.

Common coach bolt sizes

Size	Typical Uses
M6	Light duty fixing, small brackets
M8	General timber fixing, fencing, light structural work
M10	Common construction use, gates, decking frames
M12	Heavy timber connections, structural joins
M16	High load applications, large timber sections
M20	Specialist structural applications

Choosing the right length

The bolt needs to pass through all materials and allow enough thread for a washer and nut.

Add the thickness of all materials being fixed
Allow extra length for washer and nut
Avoid excessive overhang where possible

Important: Thread length is not standard under DIN 603 and can vary between products.

Coach bolts for timber applications

For most timber work:

M8 and M10 are commonly used for general fixing
M12 and above are used where higher strength is required

For comparison with alternative fixings, see our coach bolts vs coach screws guide.

Material selection

Material choice should match the environment:

Zinc plated – internal use
Stainless steel – external or damp environments
High tensile – higher load requirements

View available ranges:

Common mistakes to avoid

Using a bolt that is too short
Choosing diameter based on availability rather than load
Not allowing for washer and nut space
Using zinc plated bolts in external environments

Summary

Selecting the correct coach bolt size comes down to diameter, length and application. Getting this right improves fixing strength, installation efficiency and long-term performance.