Quick Metric ↔ Inch Conversion
This chart covers the conversions you’ll use most often — common metric sizes and their nearest fractional inch equivalents. Perfect for when you’re working with mixed-unit drawings or trying to find the closest standard tool size.
Millimeters to Inches — Common Sizes
| mm | Decimal (in) | Nearest Fraction | Notes |
|---|---|---|---|
| 1 | 0.0394 | — | |
| 2 | 0.0787 | 5/64 (0.0781) | |
| 3 | 0.1181 | 1/8 (0.1250) | Close but not exact |
| 4 | 0.1575 | 5/32 (0.1562) | |
| 5 | 0.1969 | 13/64 (0.2031) | |
| 6 | 0.2362 | 15/64 (0.2344) | Close to 1/4 (0.2500) |
| 8 | 0.3150 | 5/16 (0.3125) | Very close match |
| 10 | 0.3937 | 25/64 (0.3906) | Close to 3/8 (0.3750) |
| 12 | 0.4724 | 15/32 (0.4688) | Close to 1/2 (0.5000) |
| 14 | 0.5512 | 35/64 (0.5469) | |
| 16 | 0.6299 | 5/8 (0.6250) | Very close match |
| 18 | 0.7087 | 45/64 (0.7031) | |
| 20 | 0.7874 | 25/32 (0.7812) | Close to 3/4 (0.7500) |
| 22 | 0.8661 | 55/64 (0.8594) | Close to 7/8 (0.8750) |
| 24 | 0.9449 | 15/16 (0.9375) | |
| 25 | 0.9843 | 63/64 (0.9844) | Almost exactly 1 inch |
| 25.4 | 1.0000 | 1″ | Exact conversion factor |
| 30 | 1.1811 | 1-3/16 (1.1875) | |
| 40 | 1.5748 | 1-37/64 (1.5781) | |
| 50 | 1.9685 | 1-31/32 (1.9688) | Almost 2 inches |
The One Number to Remember
25.4 mm = 1 inch (exactly)
- mm to inches: divide by 25.4
- inches to mm: multiply by 25.4
“Close Enough” Matches — Watch Out!
Some metric and inch sizes are dangerously close but not interchangeable:
| Metric | Inch | Difference | Danger Zone? |
|---|---|---|---|
| M6 bolt (6mm) | 1/4″ bolt (6.35mm) | 0.35mm | ⚠️ Will NOT interchange |
| M8 bolt (8mm) | 5/16″ bolt (7.94mm) | 0.06mm | ⚠️ Looks like it fits — it doesn’t |
| M10 bolt (10mm) | 3/8″ bolt (9.52mm) | 0.48mm | ⚠️ Obviously different |
| M12 bolt (12mm) | 1/2″ bolt (12.7mm) | 0.70mm | ⚠️ Common confusion |
| 8mm wrench | 5/16″ wrench (7.94mm) | 0.06mm | ✅ Usually works in a pinch |
| 10mm wrench | 3/8″ wrench (9.52mm) | 0.48mm | ❌ Too loose, will round the bolt |
| 13mm wrench | 1/2″ wrench (12.7mm) | 0.30mm | ✅ Often works |
| 19mm wrench | 3/4″ wrench (19.05mm) | 0.05mm | ✅ Virtually identical |
Related: Engineering Unit Converter | Master Drill Size Chart | Sheet Metal Gauge Chart