Screws
Nails
Fasteners & Fixings
Adhesives & Building Chemicals
Powertool Accessories
Fence & Gate Hardware
Building Supplies
Security & Ironmongery
Site and Access
Hand Tools
Painting & Decorating
Workwear, PPE & Safety
Understanding Metric Bolt Sizes: M6, M8, M10 & M12 Explained
Understanding Metric Bolt Sizes: M6, M8, M10 & M12 Explained
When selecting bolts for construction, landscaping or general fixing work, choosing the correct size is essential for strength, safety and long-term performance. Metric bolt sizes such as M6, M8, M10 and M12 are widely used across UK building projects, but it’s not always clear which size is best for each application.
This guide explains what metric bolt sizes mean, how M6, M8, M10 and M12 compare, and how to choose the right size and finish for your job.
What Do Metric Bolt Sizes Mean?
Metric bolt sizes refer to the diameter of the threaded shank, measured in millimetres:
- M6 = 6mm diameter
- M8 = 8mm diameter
- M10 = 10mm diameter
- M12 = 12mm diameter
As the diameter increases, the bolt’s strength and load capacity also increase. Bolt length is specified separately and should be selected based on the thickness of the materials being fixed together.
Why Bolt Size Selection Matters
Choosing the correct bolt size helps ensure:
- Adequate strength for the load being applied
- Secure clamping without movement or flex
- Reduced risk of timber crushing or fixing failure
- Correct hole sizing and compatibility with washers and nuts
Using a bolt that’s undersized can compromise safety, while oversizing may cause unnecessary damage to the surrounding material.
Common Uses by Bolt Size
Common Uses by Bolt Size
M6 Bolts
M6 bolts are generally used for light-duty fixing, including:
- Brackets and fittings
- Light timber assemblies
- Small framing and fix-out work
- Situations where space is limited
They’re best suited where high structural strength is not required.
M8 Bolts
M8 bolts are a widely used general-purpose size, suitable for:
- Timber fencing and gates
- General construction and landscaping
- Timber-to-timber connections
- Medium load-bearing applications
They offer a good balance between strength and ease of installation.
M10 Bolts
M10 bolts are used for heavier-duty applications, such as:
- Decking subframes
- Structural timber connections
- Posts, beams and sleepers
- Timber-to-steel fixing
This size is commonly chosen where increased rigidity and load resistance are required.
M12 Bolts
M12 bolts are intended for structural and load-bearing applications, including:
- Large timber structures
- Pergolas and carports
- Heavy fencing and landscaping work
- Structural posts and beams
They provide high strength for demanding installations.
Choosing the Right Bolt Size for Your Job
| Bolt Size | Typical Uses | Load Level |
|---|---|---|
| M6 | Brackets and fittings Light timber assemblies Small framing and fix-out work Limited-space applications | Light duty |
| M8 | Timber fencing and gates General construction and landscaping Timber-to-timber connections | Medium duty |
| M10 | Decking subframes Structural timber connections Posts, beams and sleepers Timber-to-steel fixing | Heavy duty |
| M12 | Structural timber construction Pergolas and carports Heavy fencing and landscaping Structural posts and beams | Structural / load-bearing |
When to Choose Stainless Steel Bolts
Stainless steel bolts are used where corrosion resistance is critical, including:
- External applications
- Damp or exposed environments
- Treated or green timber
- Coastal or high-humidity locations
Stainless steel helps prevent rust staining and offers long-term durability where plated or coated fixings may degrade over time.
Stainless Steel Bolt & Nut Sets (DIN 934)
Where stainless steel carriage bolts are supplied with nuts, the nuts conform to DIN 934, the standard for hexagon full nuts.
- DIN 934 specifies:
- Thread compatibility
- Nut dimensions
- Consistent strength and fit
This ensures reliable clamping performance when stainless steel carriage bolts are supplied as complete bolt and nut sets.
Stainless steel carriage bolts may also be supplied without nuts, depending on the application.
Why Carriage Bolts Are Common for Timber Fixing
Carriage bolts are widely used in timber construction because they feature:
- A domed head for a neat, low-profile finish
- A square shoulder beneath the head to prevent rotation
- Secure clamping when used with washers and nuts
They’re commonly used for fencing, decking, gates, landscaping structures and general timber construction.
Bolt Finishes and Supply Options
Metric carriage bolts are available in a range of finishes to suit different environments and applications, including:
- Zinc plated carriage bolts supplied with nuts
- Exterior green coated carriage bolts supplied with washers and nuts
- Exterior silver coated carriage bolts supplied with washers and nuts
- Hot dipped galvanised carriage bolts supplied with nuts
- Stainless steel carriage bolts, supplied as:
- Bolt only
- Or bolt and nut sets (with DIN 934 nuts)
Each finish provides a different level of corrosion resistance depending on exposure and environment.
Exterior Green Carriage Bolt & Nut
Zinc Carriage Bolt & Nut
Choosing the Right Bolt Size and Finish
| Consideration | What to Look For |
|---|---|
| Load requirement | Choose a bolt size that safely supports the weight and forces involved. Structural applications will typically require larger diameters such as M10 or M12. |
| Materials being joined | Timber-to-timber, timber-to-steel or metal-to-metal connections may influence both bolt size and the need for washers to spread the load. |
| Environment | Use zinc-plated bolts for internal use, exterior-coated or galvanised bolts for outdoor applications, and stainless steel for damp, coastal or corrosive environments. |
| Nut and washer | A nut and washer provide secure clamping, spread the load and help prevent loosening over time, especially on timber connections. |
| Corrosion resistance | Upgrade the finish where moisture, weather exposure or treated timber is present to ensure long-term performance. |
How Fast Fixings Online Can Help
At Fast Fixings Online, we supply a practical range of carriage bolts and fixings in the most commonly used sizes and finishes, selected for strength, reliability and suitability for trade use. Our range covers M6, M8, M10 and M12 carriage bolts, with options for internal, external and corrosion-resistant applications.
We offer:
- Clear size and finish options to help you choose the right bolt for your load and environment
- Trade pricing with next working-day delivery when you order before 3pm
- A focused range of finishes including zinc plated, exterior coated, hot dipped galvanised and stainless steel
Whether you’re fixing fencing, gates, decking frames or structural timber, choosing the correct bolt size and finish is essential for strength, durability and long-term performance. Browse our carriage bolt range online or contact Fast Fixings Online if you need help selecting the right option for your project.
