When working on mechanical projects, construction, automotive repair, or even simple DIY fixes, bolts are one of the most essential fasteners you will encounter. Choosing the right bolt size is critical for safety, accuracy, and efficiency. That’s where a metric bolt size chart comes in handy.
In this guide, we’ll break down everything you need to know about metric bolt sizes: what they mean, how to read them, the standard sizes available, and how to use charts to make the right selection every time.
What is a Metric Bolt?
A metric bolt is a type of fastener that uses the metric system for measurement rather than the imperial system (inches). Metric bolts are measured in millimeters (mm), and their sizing follows a consistent international standard, making them widely used across the world.
The size of a bolt is generally defined by:
Diameter (M) – The thickness of the bolt’s shank in millimeters.
Pitch – The distance between the threads (also in millimeters).
Length – The measurement from the head of the bolt to the tip.
For example: M10 × 1.5 × 40 means:
Bolt diameter = 10 mm
Thread pitch = 1.5 mm
Length = 40 mm
Why Use a Metric Bolt Size Chart?
Even experienced engineers or mechanics can get confused when selecting bolts. A metric bolt size chart helps you:
Quickly identify the right diameter, pitch, and length.
Match bolts with the correct nut and washer sizes.
Prevent errors in assembly that could cause damage or safety issues.
Save time by avoiding trial-and-error selections.
Think of it as a reference sheet that ensures accuracy in every project.
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Key Terms You Need to Know
Before reading the chart, let’s define a few important terms:
Major Diameter (M): The outer diameter of the bolt.
Thread Pitch: The distance between two thread peaks. Coarse pitches have wider spacing, while fine pitches are closer.
Head Size: The width across the flats of the bolt’s head, usually where a wrench grips.
Tensile Strength: The maximum load the bolt can handle before breaking.
Standard Metric Bolt Sizes
Metric bolts typically follow the ISO (International Organization for Standardization) standards. The most common sizes range from M1 (tiny screws) up to M100 (huge bolts for heavy machinery).
Here’s a simplified chart with some of the most frequently used sizes:
Metric Bolt Size Chart (Diameter, Pitch & Head Size)
| Bolt Size | Thread Pitch (coarse) | Hex Head Width (mm) | Typical Applications |
|---|---|---|---|
| M3 | 0.5 mm | 5.5 mm | Electronics, small appliances |
| M4 | 0.7 mm | 7 mm | Household devices |
| M5 | 0.8 mm | 8 mm | Furniture assembly |
| M6 | 1.0 mm | 10 mm | Bicycles, light machinery |
| M8 | 1.25 mm | 13 mm | Automotive, construction |
| M10 | 1.5 mm | 17 mm | Machinery, engines |
| M12 | 1.75 mm | 19 mm | Structural steel |
| M14 | 2.0 mm | 22 mm | Heavy equipment |
| M16 | 2.0 mm | 24 mm | Automotive suspension |
| M20 | 2.5 mm | 30 mm | Industrial equipment |
| M24 | 3.0 mm | 36 mm | Large-scale construction |
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Coarse vs. Fine Thread Bolts
Metric bolts come in coarse and fine thread varieties:
Coarse Threads (standard):
Easier to assemble.
Better for soft materials.
More resistant to damage and wear.
Fine Threads:
Provide a stronger hold.
Allow more precise tensioning.
Common in automotive and aerospace applications.
A metric bolt size chart usually lists both coarse and fine thread pitches. For example, an M10 bolt might have:
Coarse thread pitch: 1.5 mm
Fine thread pitch: 1.25 mm or 1.0 mm
Metric Bolt Strength Grades
Besides size, bolts are also categorized by strength grades. This ensures the bolt can withstand the intended load. Common metric grades include:
8.8: Standard strength for general use.
10.9: High-strength bolts, often used in automotive.
12.9: Extra-high strength, used in critical machinery and aerospace.
On a bolt head, you’ll often see these numbers stamped (e.g., “10.9”).
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How to Read a Metric Bolt Size
Let’s break down an example: M12 × 1.75 × 80 – 10.9
M12 = Diameter 12 mm
1.75 = Thread pitch (distance between threads)
80 = Length of bolt (in mm)
10.9 = Strength grade
So, this bolt is 12 mm wide, 80 mm long, with a medium-fine thread, and is strong enough for heavy-duty applications.
Using Metric Bolts with Nuts and Washers
Every bolt typically needs a matching nut and often a washer:
Nut Size: Usually matches the bolt’s diameter (M8 bolt fits an M8 nut).
Washer Size: Slightly larger hole to fit the bolt shaft.
The bolt size chart often includes the corresponding nut and washer dimensions, which makes it easier to ensure compatibility.
Applications of Common Metric Bolt Sizes
M3–M6: Electronics, furniture, bicycles, small equipment.
M8–M12: Cars, motorcycles, home construction, light machinery.
M14–M20: Heavy machinery, trucks, structural frameworks.
M24+: Industrial machinery, bridges, and large-scale structures.
Tips for Choosing the Right Bolt
Check the load requirement: Use stronger grades (10.9 or 12.9) for heavy-duty applications.
Measure carefully: Use a caliper or thread gauge for accurate sizing.
Don’t mix metric and imperial: An M8 bolt won’t fit properly in a 5/16″ nut.
Use washers where needed: They distribute the load and protect surfaces.
Consult the chart every time: Even small differences can cause big problems in performance.
Printable Metric Bolt Size Chart
It’s always helpful to keep a printable bolt size chart in your workshop, garage, or toolbox. Many mechanics and engineers laminate one for quick reference. This saves time and avoids costly mistakes when selecting fasteners.
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Final Thoughts
A metric bolt size chart is more than just a reference – it’s an essential tool for anyone working with fasteners. From DIY projects to industrial applications, knowing the right bolt size ensures safety, durability, and efficiency.
Whenever you’re unsure about bolt sizes, thread pitches, or compatible nuts and washers, always check the chart before making a decision.
By understanding how to read and use these charts, you’ll make better choices and avoid the frustration of mismatched or weak fasteners.