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Inch to MM and Metric to Inch Fastener Converter
1 inch = 25.4 mm exactly. 1/4 in = 6.35 mm, 5/16 = 7.94 mm, 3/8 = 9.53 mm, 1/2 = 12.7 mm, and 3/16 in = 4.76 mm. Enter any fastener dimension below to convert between inches and millimeters.
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Standardizing Global Components
Metric (ISO) and inch (ANSI/ASME) hardware are routinely mixed in the same assembly. However, these two systems are fundamentally different. A common mistake is assuming that a “close enough” fit is acceptable, but mismatched threads or head sizes can lead to stripped components, loose joints, and safety risks.
Correctly converting between these standards is essential for sourcing replacement parts, designing new assemblies, and ensuring that your tooling (wrenches and sockets) matches the hardware perfectly.
System Benchmarks
Metric (ISO): Defined by diameter and thread pitch in millimeters (e.g., M8 x 1.25).
Inch (ANSI): Defined by diameter and threads per inch (TPI) (e.g., 5/16-18).
Interactive Fastener Conversion Tool
Use the utility below to compare Metric and Inch fasteners. Accurate identification ensures proper thread engagement and assembly integrity.
Metric to Inch Fastener Converter
Quickly toggle between Metric (mm) and Imperial (inch) sizing for fastener specs.
Final Calculated Result
Inches
Metric (mm)
0.00
Decimal Inch (in)
0.00
Closest Fraction (approx)
--
Nearest Metric Size
--
Scientific Value (Meters)
0.00e+0
*Conversions are based on standard industrial factors (1 inch = 25.4 mm).
Fractional results are rounded to the nearest 1/64th increment.
How to Use the Metric to Inch Converter
- Enter the diameter, length or other dimension.
- Choose the unit you have and the unit you need.
- Read the result in decimal, plus the nearest 1/64 inch fraction.
Conversion Formula, Examples and Nearest Sizes
Millimeters = inches x 25.4. Inches = millimeters / 25.4. For a fraction, divide first: 3/16 = 0.1875 in, x 25.4 = 4.7625 mm. For a metric size, M10 = 10 / 25.4 = 0.3937 in, just over 3/8 in (0.375 in).
| Inch bolt | Decimal in | Millimeters | Nearest metric bolt | Metric diameter (mm) |
|---|---|---|---|---|
| 1/4″ | 0.2500 | 6.350 | M6 | 6.0 |
| 5/16″ | 0.3125 | 7.938 | M8 | 8.0 |
| 3/8″ | 0.3750 | 9.525 | M10 | 10.0 |
| 7/16″ | 0.4375 | 11.113 | M11 (rare) / M12 | 12.0 |
| 1/2″ | 0.5000 | 12.700 | M12 | 12.0 |
| 5/8″ | 0.6250 | 15.875 | M16 | 16.0 |
| 3/4″ | 0.7500 | 19.050 | M20 | 20.0 |
| 7/8″ | 0.8750 | 22.225 | M22 | 22.0 |
| 1″ | 1.0000 | 25.400 | M24 | 24.0 |
Nearest by diameter only. Inch and metric threads never interchange: pitch, thread form and tolerances differ, so a 3/8-16 bolt will not go into an M10 x 1.5 hole.
Key Fastener Differences
| Specification | Metric Variation | Inch Variation |
|---|---|---|
| Thread Pitch | Distance between threads (mm). | Number of threads per inch (TPI). |
| Diameter | Based on whole millimeters. | Based on fractions of an inch. |
| Strength Grade | Property Class (e.g., 8.8, 10.9, 12.9). | Grade (e.g., Grade 2, 5, 8). |
| Tooling | Metric sockets/wrenches (mm). | SAE sockets/wrenches (inches). |
Fraction to MM Conversion Chart
Every 64th of an inch from 1/64 to 1 inch in decimal inches and millimetres. For example, 3/16 inch is 4.763 mm.
| Fraction (in) | Decimal (in) | Millimeters (mm) |
|---|---|---|
| 1/64" | 0.0156 | 0.397 |
| 1/32" | 0.0312 | 0.794 |
| 3/64" | 0.0469 | 1.191 |
| 1/16" | 0.0625 | 1.587 |
| 5/64" | 0.0781 | 1.984 |
| 3/32" | 0.0938 | 2.381 |
| 7/64" | 0.1094 | 2.778 |
| 1/8" | 0.1250 | 3.175 |
| 9/64" | 0.1406 | 3.572 |
| 5/32" | 0.1562 | 3.969 |
| 11/64" | 0.1719 | 4.366 |
| 3/16" | 0.1875 | 4.763 |
| 13/64" | 0.2031 | 5.159 |
| 7/32" | 0.2188 | 5.556 |
| 15/64" | 0.2344 | 5.953 |
| 1/4" | 0.2500 | 6.350 |
| 17/64" | 0.2656 | 6.747 |
| 9/32" | 0.2812 | 7.144 |
| 19/64" | 0.2969 | 7.541 |
| 5/16" | 0.3125 | 7.938 |
| 21/64" | 0.3281 | 8.334 |
| 11/32" | 0.3438 | 8.731 |
| 23/64" | 0.3594 | 9.128 |
| 3/8" | 0.3750 | 9.525 |
| 25/64" | 0.3906 | 9.922 |
| 13/32" | 0.4062 | 10.319 |
| 27/64" | 0.4219 | 10.716 |
| 7/16" | 0.4375 | 11.113 |
| 29/64" | 0.4531 | 11.509 |
| 15/32" | 0.4688 | 11.906 |
| 31/64" | 0.4844 | 12.303 |
| 1/2" | 0.5000 | 12.700 |
| 33/64" | 0.5156 | 13.097 |
| 17/32" | 0.5312 | 13.494 |
| 35/64" | 0.5469 | 13.891 |
| 9/16" | 0.5625 | 14.287 |
| 37/64" | 0.5781 | 14.684 |
| 19/32" | 0.5938 | 15.081 |
| 39/64" | 0.6094 | 15.478 |
| 5/8" | 0.6250 | 15.875 |
| 41/64" | 0.6406 | 16.272 |
| 21/32" | 0.6562 | 16.669 |
| 43/64" | 0.6719 | 17.066 |
| 11/16" | 0.6875 | 17.462 |
| 45/64" | 0.7031 | 17.859 |
| 23/32" | 0.7188 | 18.256 |
| 47/64" | 0.7344 | 18.653 |
| 3/4" | 0.7500 | 19.050 |
| 49/64" | 0.7656 | 19.447 |
| 25/32" | 0.7812 | 19.844 |
| 51/64" | 0.7969 | 20.241 |
| 13/16" | 0.8125 | 20.637 |
| 53/64" | 0.8281 | 21.034 |
| 27/32" | 0.8438 | 21.431 |
| 55/64" | 0.8594 | 21.828 |
| 7/8" | 0.8750 | 22.225 |
| 57/64" | 0.8906 | 22.622 |
| 29/32" | 0.9062 | 23.019 |
| 59/64" | 0.9219 | 23.416 |
| 15/16" | 0.9375 | 23.812 |
| 61/64" | 0.9531 | 24.209 |
| 31/32" | 0.9688 | 24.606 |
| 63/64" | 0.9844 | 25.003 |
| 1" | 1.0000 | 25.400 |
Computed exactly from 1 inch = 25.4 mm (the international inch), rounded to 3 decimal places.
Frequently Asked Questions
What is 3/16 inch in mm?
4.7625 mm (3/16 = 0.1875 in, x 25.4). The full 1/64 inch chart is further down this page.
What is the metric equivalent of a 3/8 inch bolt?
3/8 in is 9.525 mm, so the nearest metric size is M10 (10 mm). They are not interchangeable: the thread pitch and form differ.
How big is an M6 bolt in inches?
0.236 in, slightly smaller than a 1/4 inch bolt (0.250 in).
Is 1/2 inch the same as 12 mm?
No. 1/2 in is 12.7 mm, so an M12 bolt is 0.7 mm smaller in diameter and will be loose in a 1/2 inch hole.
Next Steps
- Bolt Size Chart: inch hex cap screw dimensions
- Thread pitch chart and calculator: inch and metric thread sizes
- Wrench Size Chart: socket sizes for inch and metric bolts
- Request a quote for the fasteners in your joint.
Related Calculators
More free tools for the same job:
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- Length Converter: Convert length between inches, millimeters, centimeters, meters and feet.
- Tap Drill Size Calculator: Find the tap drill size before tapping a thread.
- Bolt Size and Strength Calculator: Estimate the bolt size and strength class an application needs.
Navigating International Specifications?
Our engineering team specializes in standardizing hardware across global platforms. We can help you move between metric and inch requirements without redesign.