Millimeters to Inches Converter
Convert millimeters to inches and inches to millimeters instantly for precision work, sewing and engineering.
Last updated: June 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.
Precision reference: fasteners, nozzles, and tolerances
| 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 |
Standard paper, screen and product sizes in mm and inches
Many everyday size standards are defined in one system and commonly referenced in the other. The table below covers the dimensions you are most likely to look up when cutting, printing or ordering online.
| Item | Millimeters | Inches |
|---|---|---|
| A4 paper (international) | 210 x 297 mm | 8.27 x 11.69 in |
| US Letter paper | 216 x 279 mm | 8.5 x 11 in |
| Credit card (ISO 7810) | 85.6 x 54 mm | 3.37 x 2.13 in |
| SIM card (standard) | 25 x 15 mm | 0.98 x 0.59 in |
| Watch case (typical) | 38-44 mm diameter | 1.50-1.73 in |
| M6 bolt hole clearance | 6.4 mm | 0.252 in |
| 1/4-inch camera tripod thread | 6.35 mm | 0.25 in |
| US standard door width | 813-914 mm | 32-36 in |
Converting fractional inches to millimeters
US rulers, drill bits and hardware are marked in fractions of an inch rather than decimals, which is where most millimeter conversions trip people up. Each fraction has an exact millimeter value, because one inch is defined as exactly 25.4 mm. The table below covers the common sixteenths you meet on a tape measure or a drill bit set.
| Fraction | Decimal inch | Millimeters |
|---|---|---|
| 1/16" | 0.0625" | 1.588 mm |
| 1/8" | 0.125" | 3.175 mm |
| 3/16" | 0.1875" | 4.763 mm |
| 1/4" | 0.25" | 6.350 mm |
| 3/8" | 0.375" | 9.525 mm |
| 1/2" | 0.5" | 12.700 mm |
| 3/4" | 0.75" | 19.050 mm |
| 1" | 1.0" | 25.400 mm |
For machining and toolmaking you also meet the "thou" (also called a mil), which is one thousandth of an inch. One thou equals 0.0254 mm, or 25.4 microns, and it is the unit engineers reach for when a tolerance is too tight to write neatly as a fraction.
Frequently Asked Questions
My drill bit set is in fractional inches, so 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, so 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.
What is a thou or mil in millimeters?
A thou (also called a mil) is one thousandth of an inch, used for machining tolerances. One thou equals 0.0254 mm, or 25.4 microns, so a part specified at 5 thou is 0.127 mm. Note that a "mil" here means a thousandth of an inch, not a millimeter.
How do I round a millimeter value to the nearest fractional inch?
Divide the millimeters by 25.4 to get decimal inches, then multiply by 16 and round to the nearest whole number to find the number of sixteenths. For example, 10 mm is 0.3937 inches; times 16 is 6.3, which rounds to 6, so 10 mm is about 6/16, or 3/8 of an inch.