Loona Calculators
Convert millimetres to inches instantly — type any value and the result appears as you type. Below you will find the exact formula, a mm to in conversion table and a worked example.
Formula: Inches = millimetres ÷ 25.4. One inch is exactly 25.4 millimetres, so dividing by 25.4 gives the exact result.
Question: How many inches is 100 mm — the width of a standard credit-card-sized object is about 85.6 mm; 100 mm is a common engineering size?
Steps: Take the value in millimetres: 100 mm. Apply the formula: Inches = millimetres ÷ 25. That gives 100 → 3.937 in (rounded to four decimal places).
Result: 100 mm = 3.937 in.
| Millimetres (mm) | Inches (in) |
| 1 mm | 0.0394 in |
| 2 mm | 0.0787 in |
| 5 mm | 0.1969 in |
| 10 mm | 0.3937 in |
| 20 mm | 0.7874 in |
| 25 mm | 0.9843 in |
| 50 mm | 1.9685 in |
| 100 mm | 3.937 in |
| 500 mm | 19.685 in |
| 1000 mm | 39.3701 in |
Need a value that is not in the table — or the reverse direction? Use the converter at the top of this page, or the full Unit Converter for every unit in one place.
Multiply the value in millimetres by 0.0394: mm × 0.0394 = in. For example, 10 mm × 0.0394 = 0.3937 in. The converter above does this instantly.
Exactly 0.0394 in (to four decimal places). The full relationship: Inches = millimetres ÷ 25.
Exactly 1 inch — 25.4 mm is the definition of the inch.
Reverse the formula above. For a quick check, divide by the same factor instead of multiplying.
In engineering drawings, drill and screw sizes, and tyre and rim markings, where US drawings use inches and most of the world uses millimetres.
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