Metric Bolt Size Chart – Complete Reference (M3–M36)
12th Mar 2026
Metric Bolt Size Chart – Complete Reference (M3–M36)
What is a Metric Bolt Size?
Metric bolts are identified using the “M” designation, followed by the nominal diameter of the bolt in millimetres.
For example:
Metric fasteners follow international standards such as ISO 261 and ISO 965, which ensure consistent thread sizes and dimensions across industrial and engineering applications.
Metric bolts are widely used in:
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industrial machinery
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structural construction
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automotive manufacturing
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fabrication and engineering assemblies
Understanding metric bolt sizes helps engineers select the correct fastener for load capacity, clearance, and spanner compatibility.
Metric Bolt Size Chart
The table below provides a reference for common metric bolt sizes including thread pitch, spanner size and clearance hole diameter.
| Bolt Size | Thread Pitch | Spanner Size | Clearance Hole |
|---|---|---|---|
| M3 | 0.5 mm | 5.5 mm | 3.2 mm |
| M4 | 0.7 mm | 7 mm | 4.2 mm |
| M5 | 0.8 mm | 8 mm | 5.2 mm |
| M6 | 1.0 mm | 10 mm | 6.2 mm |
| M8 | 1.25 mm | 13 mm | 8.2 mm |
| M10 | 1.5 mm | 17 mm | 10.5 mm |
| M12 | 1.75 mm | 19 mm | 13 mm |
| M14 | 2.0 mm | 22 mm | 15 mm |
| M16 | 2.0 mm | 24 mm | 17 mm |
| M20 | 2.5 mm | 30 mm | 21 mm |
| M24 | 3.0 mm | 36 mm | 25 mm |
| M30 | 3.5 mm | 46 mm | 31 mm |
| M36 | 4.0 mm | 55 mm | 37 mm |
This chart is commonly used by engineers and installers to determine the correct bolt diameter, wrench size and hole clearance.
Metric Bolt Thread Pitch Explained
Thread pitch refers to the distance between threads, measured in millimetres.
Metric bolts typically use coarse thread pitch, which is the standard for most industrial fasteners.
Coarse Thread
Advantages include:
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faster installation
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stronger threads in softer materials
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better resistance to thread stripping
Fine Thread
Fine threads provide:
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increased clamping force
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better vibration resistance
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more precise adjustment
However, fine threads are typically used only in specialised engineering applications.
Metric Bolt Head and Spanner Sizes
Metric hex bolts are designed with standard head sizes to match common spanners and sockets.
Examples include:
Using the correct spanner size ensures proper torque application and prevents fastener damage.
Metric Bolt Clearance Hole Sizes
Clearance holes allow bolts to pass freely through components without engaging the threads.
These holes are slightly larger than the bolt diameter to allow for alignment during assembly.
Examples:
Correct clearance hole sizing helps ensure accurate installation and reliable joint performance.
Common Metric Bolt Sizes Used in Industry
Although metric bolts range from M1.6 to M64, several sizes are particularly common in industrial applications.
M6 Bolts
Commonly used for machinery covers, brackets and light assemblies.
M8 Bolts
Widely used in automotive and fabrication applications.
M10 Bolts
Frequently used in structural assemblies and machine frames.
M12 Bolts
Used in heavy equipment and structural engineering applications.
Metric Bolt Strength Grades
Metric bolts are manufactured in different strength grades, which determine their load capacity.
Common grades include:
| Bolt Grade | Typical Applications |
|---|---|
| 8.8 | General engineering |
| 10.9 | Heavy machinery |
| 12.9 | High-strength mechanical assemblies |
Higher strength grades provide greater tensile strength and improved fatigue resistance.
Learn more in our Bolt Grades Explained guide, or download our Metric Bolt Grades Chart
Metric Bolt Torque Considerations
Correct tightening torque is critical to ensure bolts achieve the correct clamping force.
Under-tightening can cause:
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joint loosening
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vibration failure
Over-tightening may cause:
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bolt stretching
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thread damage
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fastener failure
Refer to our Metric Bolt Torque Chart to determine recommended tightening values.
How to Identify Metric Bolt Size
To determine the size of a metric bolt:
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Measure the outer thread diameter using calipers.
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Identify the thread pitch using a thread gauge.
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Refer to a metric bolt size chart to determine the correct spanner size and clearance hole.
This process ensures the correct fastener is selected for engineering and construction applications.
You can download our Metric Bolt Size Chart, perfect for engineers.
Frequently Asked Questions
What does M8 mean on a bolt?
The “M” indicates a metric thread, while the number represents the nominal thread diameter in millimetres.
What is the most common metric bolt size?
M6, M8 and M10 are among the most widely used metric bolt sizes in industrial and engineering applications.
What spanner fits an M10 bolt?
An M10 hex bolt typically requires a 17 mm spanner or socket.
Shop Metric Bolts and Fasteners
Konnect Fastening Systems supplies a comprehensive range of metric fasteners and industrial bolts, including:
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socket head cap screws
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countersunk socket screws
All products are sourced from trusted manufacturers and meet relevant engineering standards.
Related Fastener Guides
You may also find these resources helpful:
These guides provide additional technical information for selecting and installing metric fasteners.