MM to Inches — Millimeter Conversion Chart & Formula
Convert millimeters to inches with the exact formula, a detailed chart, and practical examples. 1 mm = 0.03937 inches. Covers engineering, jewelry, and more.
Millimeters to inches is essential for precision work — machining, 3D printing, jewelry sizing, bullet calibers, and engineering drawings all require accurate conversion between these units. The CalcHub Length Converter converts any value instantly.
The Formula
Inches = Millimeters / 25.4Or equivalently: Inches = Millimeters x 0.03937
The number 25.4 is exact — one inch is defined as exactly 25.4 millimeters.
Worked Example 1 — Drill Bit
You need a 6 mm drill bit but only have imperial bits. 6 / 25.4 = 0.236 inches, which is closest to a 15/64" bit (0.234").
Worked Example 2 — Ring Size
A ring has an inner diameter of 18.2 mm. In inches: 18.2 / 25.4 = 0.717 inches, corresponding to approximately a US ring size 7.5.
MM to Inches Conversion Chart
| MM | Inches | Fraction (nearest) |
|---|---|---|
| 1 | 0.039 | ~1/32" |
| 2 | 0.079 | ~5/64" |
| 3 | 0.118 | ~1/8" |
| 4 | 0.157 | ~5/32" |
| 5 | 0.197 | ~13/64" |
| 6 | 0.236 | ~15/64" |
| 8 | 0.315 | ~5/16" |
| 10 | 0.394 | ~25/64" |
| 12 | 0.472 | ~15/32" |
| 15 | 0.591 | ~19/32" |
| 20 | 0.787 | ~25/32" |
| 25 | 0.984 | ~63/64" |
| 25.4 | 1.000 | 1" (exact) |
| 30 | 1.181 | 1-3/16" |
| 40 | 1.575 | 1-37/64" |
| 50 | 1.969 | 1-31/32" |
Common Precision References
| Item | MM | Inches |
|---|---|---|
| Credit card thickness | 0.76 | 0.030 |
| Pencil lead (mechanical) | 0.5 / 0.7 | 0.020 / 0.028 |
| USB-C connector width | 8.4 | 0.331 |
| 9mm bullet diameter | 9.01 | 0.355 |
| Standard wrench set | 8–19 | 5/16"–3/4" |
| Phone thickness | 7–9 | 0.28–0.35 |
Practical Use Cases
3D printing: Nozzle diameters are specified in mm (0.4 mm is standard). Layer heights of 0.2 mm = 0.008 inches. Tolerances matter — a 0.1 mm error (0.004") can mean the difference between a press fit and a loose fit. Woodworking: European tools and hardware use metric. A 35 mm hinge cup is standard for cabinet doors. That is 1.378 inches — you need a 1-3/8" Forstner bit, which is 34.93 mm. Close enough. Electronics: PCB trace widths, component leads, and connector pin spacing are all specified in mm. A 2.54 mm pin pitch = 0.100 inches = 100 mil, the standard DIP spacing.How do I convert mm to fractional inches?
Divide mm by 25.4 to get decimal inches, then find the nearest fraction. For 1/16" precision: multiply the decimal by 16 and round. Example: 10 mm = 0.394" x 16 = 6.3, round to 6, so 10 mm is about 6/16" = 3/8".
What is the difference between mm and mil?
A mil (or thou) is 1/1000 of an inch = 0.0254 mm. It is used in PCB design and thin-film applications. Do not confuse mil with millimeter — they differ by a factor of 25.4.
Which is more precise, mm or fractional inches?
Millimeters. A measurement of 12.7 mm is precise to 0.1 mm (0.004"). The nearest fractional inch (1/2") is only precise to 0.5" (12.7 mm). For equivalent precision in imperial, you need decimal inches or thousandths.
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