Pitch converts to about 20.32 threads per inch.
Metric Bolt Size Converter
Convert metric bolt sizes such as M6, M8, M10, and M12 into inch diameter, pitch in TPI, closest inch bolt references, wrench size, clearance drill, and tap drill estimates.
Convert a metric bolt
Choose a common metric bolt or enter a custom diameter and pitch. Results update instantly with practical workshop references.
Diameter8 mm / 0.3150 in
Pitch1.25 mm / 20.32 TPI
Tap drill estimate6.75 mm / 0.2657 in
Clearance drill guide8.4 mm close / 9.0 mm normal
Nearest SAE wrench1/2 in
Thread typeCoarse metric pitch
M8 × 1.25 is close to a 5/16 inch bolt diameter, but it is not the same thread standard.
How to use the metric bolt converter
- Select a common metric bolt size or type your own diameter and pitch.
- Read the inch diameter and pitch/TPI equivalents for measuring or comparing parts.
- Use the wrench and drill references as quick shop guidance, then verify against the fastener standard for critical work.
Metric bolt size meaning
A metric bolt label starts with the outside diameter in millimeters. M6 is about 6 mm across the threads, M8 is about 8 mm, and M10 is about 10 mm.
The pitch value tells you the distance from one thread crest to the next. M10 × 1.5 is the common coarse thread; M10 × 1.25 is a finer thread.
Common metric bolt size chart
Use this chart for quick identification. Exact wrench sizes and clearance holes can vary by standard and application.
| Metric bolt | Diameter in inches | Coarse pitch | Approx TPI | Common wrench | Close inch diameter |
|---|
Metric bolt conversion FAQ
How do I convert a metric bolt size to inches?
Divide the metric diameter by 25.4. An M8 bolt is 8 ÷ 25.4 = 0.315 inches, which is close to 5/16 inch by diameter.
Can I replace a metric bolt with the closest inch bolt?
Usually not without checking the application. The diameter may be close, but thread pitch and fit are different. Use the closest inch size as a measuring clue, not an automatic substitute.
How is metric pitch converted to TPI?
Divide 25.4 by the pitch in millimeters. A 1.25 mm pitch is about 20.32 TPI, and a 1.5 mm pitch is about 16.93 TPI.
What tap drill should I use for a metric bolt?
A common estimate is major diameter minus pitch. For example, M8 × 1.25 estimates to 6.75 mm. Always check a tap drill chart for the exact thread percentage and material.