Metric Bolt Size Chart: Standard Sizes, Threads & Grades

A metric bolt size chart is a useful reference for identifying metric bolt diameters, thread pitches, head dimensions, and common bolt sizes. Metric bolts are widely used in automotive, mechanical, construction, manufacturing, machinery, furniture, and general hardware applications. Unlike imperial bolts, which are measured in inches, metric bolts are identified primarily by measurements in millimeters.

A typical metric bolt designation looks like M8 × 1.25 × 40. In this example, M8 represents the nominal bolt diameter, 1.25 represents the thread pitch in millimeters, and 40 represents the bolt length.

Metric Bolt Size Chart in mm

The following chart provides common metric bolt sizes and their standard coarse thread pitches. It is useful for quickly identifying the most frequently used metric fasteners.

Metric Bolt Size	Nominal Diameter	Coarse Thread Pitch	Approx. Diameter in Inches
M3	3 mm	0.50 mm	0.118 in
M4	4 mm	0.70 mm	0.157 in
M5	5 mm	0.80 mm	0.197 in
M6	6 mm	1.00 mm	0.236 in
M8	8 mm	1.25 mm	0.315 in
M10	10 mm	1.50 mm	0.394 in
M12	12 mm	1.75 mm	0.472 in
M14	14 mm	2.00 mm	0.551 in
M16	16 mm	2.00 mm	0.630 in
M18	18 mm	2.50 mm	0.709 in
M20	20 mm	2.50 mm	0.787 in
M22	22 mm	2.50 mm	0.866 in
M24	24 mm	3.00 mm	0.945 in
M27	27 mm	3.00 mm	1.063 in
M30	30 mm	3.50 mm	1.181 in
M33	33 mm	3.50 mm	1.299 in
M36	36 mm	4.00 mm	1.417 in
M39	39 mm	4.00 mm	1.535 in
M42	42 mm	4.50 mm	1.654 in
M45	45 mm	4.50 mm	1.772 in
M48	48 mm	5.00 mm	1.890 in

What Does M Mean on a Metric Bolt?

The letter M indicates that the bolt uses a metric thread system. The number following M represents the bolt’s nominal diameter in millimeters.

For example:

M6 = approximately 6 mm nominal diameter
M8 = approximately 8 mm nominal diameter
M10 = approximately 10 mm nominal diameter
M12 = approximately 12 mm nominal diameter
M16 = approximately 16 mm nominal diameter
M20 = approximately 20 mm nominal diameter

The number does not indicate the head size. An M10 bolt, for example, does not necessarily have a 10 mm hex head. The head dimension is specified separately.

How to Read Metric Bolt Sizes

Metric bolt specifications are generally easy to understand once the basic format is known.

Consider the example:

M10 × 1.5 × 50

This designation means:

M10 = 10 mm nominal thread diameter
1.5 = 1.5 mm thread pitch
50 = 50 mm bolt length

If a bolt is listed simply as M10 × 50, the thread pitch may be omitted because the standard coarse pitch is assumed in many general descriptions. However, the exact specification should always be confirmed when ordering replacement hardware.

Metric Bolt Diameter Chart

Bolt diameter is one of the most important dimensions when identifying a metric fastener. The nominal diameter refers to the approximate outside diameter of the threaded section.

Bolt Size	Nominal Diameter
M3	3 mm
M4	4 mm
M5	5 mm
M6	6 mm
M8	8 mm
M10	10 mm
M12	12 mm
M14	14 mm
M16	16 mm
M18	18 mm
M20	20 mm
M22	22 mm
M24	24 mm
M27	27 mm
M30	30 mm
M36	36 mm
M42	42 mm
M48	48 mm

Metric Bolt Thread Pitch Chart

Thread pitch is the distance between corresponding points on adjacent threads. It is measured in millimeters.

For example, an M8 bolt with a 1.25 mm pitch has 1.25 mm between adjacent thread crests.

Bolt Size	Coarse Pitch	Fine Pitch Example
M4	0.70 mm	0.50 mm
M5	0.80 mm	0.50 mm
M6	1.00 mm	0.75 mm
M8	1.25 mm	1.00 mm
M10	1.50 mm	1.25 mm
M12	1.75 mm	1.50 mm
M14	2.00 mm	1.50 mm
M16	2.00 mm	1.50 mm
M18	2.50 mm	1.50 mm
M20	2.50 mm	1.50 mm
M22	2.50 mm	1.50 mm
M24	3.00 mm	2.00 mm
M27	3.00 mm	2.00 mm
M30	3.50 mm	2.00 mm

Fine-thread availability varies by diameter and applicable standard, so the exact specification should be checked before purchasing.

Also Read: Seamless Pipe Size Chart: Your Complete Guide

Metric Bolt Length Chart

Bolt length is another important part of metric bolt identification. For most standard hex-head bolts, length is measured from the underside of the bolt head to the threaded end.

Bolt Length	Length in Inches
10 mm	0.394 in
15 mm	0.591 in
20 mm	0.787 in
25 mm	0.984 in
30 mm	1.181 in
35 mm	1.378 in
40 mm	1.575 in
50 mm	1.969 in
60 mm	2.362 in
70 mm	2.756 in
80 mm	3.150 in
90 mm	3.543 in
100 mm	3.937 in
120 mm	4.724 in
150 mm	5.906 in
200 mm	7.874 in

Bolt lengths are available in many additional increments depending on the bolt diameter, standard, manufacturer, and application.

Metric Hex Bolt Head Size Chart

The bolt head size determines the wrench or socket required to tighten or remove a hex-head bolt. Head dimensions are not necessarily equal to the nominal bolt diameter.

Metric Bolt	Common Head Width Across Flats
M4	7 mm
M5	8 mm
M6	10 mm
M8	13 mm
M10	16 mm
M12	18 mm
M14	21 mm
M16	24 mm
M18	27 mm
M20	30 mm
M22	34 mm
M24	36 mm
M27	41 mm
M30	46 mm
M36	55 mm

Exact head dimensions can vary according to the bolt standard and specification, so this table should be treated as a general reference.

Most Common Metric Bolt Sizes

Some metric bolt sizes are encountered much more frequently in general mechanical and hardware applications.

M6 bolts are commonly used for smaller mechanical components, equipment, brackets, and assemblies.

M8 bolts are widely used for machinery, automotive components, brackets, frames, and general hardware.

M10 bolts are common in machinery, automotive applications, equipment, structural components, and heavier assemblies.

M12 bolts are frequently used where a larger fastener is required, including machinery, automotive components, frames, and construction-related applications.

M16 and larger bolts are generally used for larger or higher-load applications, depending on the required engineering specification.

M6 Bolt Size Chart

M6 is one of the common smaller metric bolt sizes. Its nominal diameter is 6 mm and its standard coarse thread pitch is 1.0 mm.

M6 Specification	Dimension
Nominal Diameter	6 mm
Coarse Pitch	1.00 mm
Common Fine Pitch	0.75 mm
Common Head Width	10 mm
Common Lengths	10–100 mm+

An M6 × 30 bolt, for example, has a nominal diameter of 6 mm and a length of 30 mm.

M8 Bolt Size Chart

An M8 bolt has a nominal diameter of 8 mm. The standard coarse pitch is commonly 1.25 mm.

M8 Specification	Dimension
Nominal Diameter	8 mm
Coarse Pitch	1.25 mm
Common Fine Pitch	1.00 mm
Common Head Width	13 mm
Common Lengths	15–120 mm+

M8 fasteners are widely used for mechanical assemblies, equipment, brackets, automotive components, and general hardware.

M10 Bolt Size Chart

M10 bolts have a nominal diameter of 10 mm and commonly use a 1.5 mm coarse thread pitch.

M10 Specification	Dimension
Nominal Diameter	10 mm
Coarse Pitch	1.50 mm
Common Fine Pitch	1.25 mm
Common Head Width	16 mm
Common Lengths	20–150 mm+

Always confirm whether an M10 replacement requires coarse or fine threads before installation.

Also Read: Reamer Tool Size Chart: Complete Guide

M12 Bolt Size Chart

M12 bolts have a nominal diameter of 12 mm. The standard coarse pitch is commonly 1.75 mm.

M12 Specification	Dimension
Nominal Diameter	12 mm
Coarse Pitch	1.75 mm
Common Fine Pitch	1.50 mm
Common Head Width	18 mm
Common Lengths	25–200 mm+

M12 fasteners are frequently used in heavier mechanical assemblies, machinery, automotive components, frames, and equipment.

Metric Bolt Size vs Wrench Size

One common mistake is assuming that the metric bolt size and wrench size are identical. They are different measurements.

For example, a commonly encountered M8 hex-head bolt may require a 13 mm wrench, while an M10 bolt may require a 16 mm wrench under common modern metric hex-head dimensions.

The M number identifies the nominal thread diameter, while the wrench size refers to the width across the flats of the bolt head.

Therefore:

M8 ≠ 8 mm wrench

and

M10 ≠ 10 mm wrench

Always identify the correct head dimension before selecting a socket or wrench.

Metric Bolt Size Conversion to Inches

Metric bolts can be converted to approximate inch diameters for comparison purposes.

Metric Bolt	Diameter in mm	Approx. Diameter in inches
M6	6 mm	0.236 in
M8	8 mm	0.315 in
M10	10 mm	0.394 in
M12	12 mm	0.472 in
M14	14 mm	0.551 in
M16	16 mm	0.630 in
M18	18 mm	0.709 in
M20	20 mm	0.787 in
M22	22 mm	0.866 in
M24	24 mm	0.945 in
M27	27 mm	1.063 in
M30	30 mm	1.181 in
M36	36 mm	1.417 in

These conversions show diameter similarities but do not mean the metric and inch bolts are interchangeable. Thread pitch and thread form must also match.

How to Measure a Metric Bolt

If the bolt size is unknown, you can identify it by measuring several dimensions.

First, measure the outside diameter of the threaded section using a digital caliper. Next, use a thread pitch gauge to determine the distance between the threads. Measure the bolt length from the underside of the head to the end for a typical hex-head bolt.

Finally, measure the head width across the flats.

The basic process is:

Measure the threaded diameter.
Determine whether the bolt is metric.
Measure the thread pitch.
Measure bolt length.
Measure the head width.
Check the bolt head markings.
Compare the measurements with a metric bolt size chart.

Metric Bolt Grade and Strength

Metric bolt dimensions do not tell you everything about a fastener. The strength class is also important.

Common metric strength classes include 8.8, 10.9, and 12.9. These markings are associated with mechanical properties specified by the relevant fastener standard.

For example, two bolts can both be M10 × 50 but have different strength classes. Their dimensions may be similar while their mechanical properties differ.

When replacing a bolt, match the required diameter, length, thread pitch, material, and strength class rather than selecting a bolt based on size alone.

Also Read: Torque Wrench Size Chart: Complete Guide

How to Choose the Correct Metric Bolt Size

Choosing the correct metric bolt involves more than finding a bolt that physically fits through a hole. The bolt must have compatible threads, sufficient length, and the appropriate strength for the application.

Before purchasing a replacement, check:

Metric diameter
Thread pitch
Bolt length
Head style
Head dimensions
Thread length
Strength class
Material
Surface coating
Application requirements

For important mechanical or structural connections, always follow the manufacturer’s specifications or applicable engineering standards.

Metric Bolt Size Chart: Quick Reference

For quick identification, these are some of the most commonly encountered metric sizes:

Bolt Size	Diameter	Coarse Pitch	Common Head Width
M4	4 mm	0.70 mm	7 mm
M5	5 mm	0.80 mm	8 mm
M6	6 mm	1.00 mm	10 mm
M8	8 mm	1.25 mm	13 mm
M10	10 mm	1.50 mm	16 mm
M12	12 mm	1.75 mm	18 mm
M14	14 mm	2.00 mm	21 mm
M16	16 mm	2.00 mm	24 mm
M18	18 mm	2.50 mm	27 mm
M20	20 mm	2.50 mm	30 mm
M22	22 mm	2.50 mm	34 mm
M24	24 mm	3.00 mm	36 mm
M27	27 mm	3.00 mm	41 mm
M30	30 mm	3.50 mm	46 mm
M36	36 mm	4.00 mm	55 mm

Final Thoughts on Metric Bolt Sizes

A metric bolt size chart makes it easier to identify and compare common metric fasteners. The M designation identifies the nominal diameter, while the thread pitch identifies the spacing between threads. Bolt length and head dimensions are separate measurements that must also be considered.

For example, an M10 × 1.5 × 50 bolt has a 10 mm nominal diameter, a 1.5 mm thread pitch, and a 50 mm length. Understanding this format allows you to quickly identify thousands of metric fastener combinations.

When selecting a bolt, always verify the diameter, thread pitch, length, head dimensions, and strength class. For safety-critical applications, use the applicable manufacturer or engineering specification rather than relying solely on a general metric bolt size chart.

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