Millimeters to Inches Converter
Convert millimeters to inches and inches to millimeters instantly for precision work, sewing and engineering.
Last updated: May 2026
Enter a value to see the conversion instantly.
How to convert millimeters to inches
Enter a value in millimeters to instantly see the equivalent in inches. The formula is: inches = millimeters x 0.0393701. The reverse: millimeters = inches x 25.4. One inch equals exactly 25.4 millimeters.
Common reference values
| Millimeters | Inches | Typical use |
|---|---|---|
| 1 mm | 0.0394 in | Wire thickness |
| 5 mm | 0.1969 in | Pencil lead diameter |
| 10 mm | 0.3937 in | 1 cm reference |
| 25 mm | 0.9843 in | About 1 inch |
| 50 mm | 1.9685 in | Small object size |
| 100 mm | 3.9370 in | 10 cm reference |
| 250 mm | 9.8425 in | Notebook width |
| 500 mm | 19.6850 in | Half meter |
Understanding millimeters and inches
Millimeters are part of the metric system, the smallest commonly used metric length. Inches are part of the imperial system, used in the US and UK. One inch equals exactly 25.4 mm. Precision work like 3D printing, sewing patterns and engineering drawings often need both.
When You'll Need This Conversion
Use this converter for: Reading European product dimensions, converting sewing patterns, 3D printing tolerances, engineering drawings, watch and jewelry sizing.
Frequently Asked Questions
My drill bit set is in fractional inches — how do these compare to mm?
Common sizes: 1/16" = 1.59 mm, 1/8" = 3.18 mm, 3/16" = 4.76 mm, 1/4" = 6.35 mm, 3/8" = 9.53 mm, 1/2" = 12.7 mm. When buying metric drill bits, look for the mm equivalent. For wood and plastic, ±0.1 mm is fine; for metal and precision fits, use the exact converted value.
My 3D printer slicer uses mm but the model file shows inches — how do I scale?
Multiply every inch measurement by 25.4 to convert to mm. A model designed at 1" scales to 25.4 mm exactly. In most slicers (Cura, PrusaSlicer, OrcaSlicer), you can set import units to inches or use the scale tool to multiply by 25.4. Confirm the final dimensions in the slicer preview before printing.
Why use millimeters instead of centimeters?
Millimeters provide more precision for small measurements. Engineering, manufacturing, sewing and 3D printing typically use mm because tolerances are tighter than 1 cm.
Are there any common mm-inch shortcuts?
Yes. 25 mm = 1 inch, 100 mm = 4 inches, 250 mm = 10 inches. For quick estimates, divide mm by 25.4. For precision, use this converter.
Why mix metric and imperial in modern projects?
Tools and materials from different regions use different systems. US-made parts use inches, European-made parts use mm. Converters bridge the gap.