What this calculator does
A mil, also called a thou, is one thousandth of an inch: a small unit used mainly for wire diameter, sheet and foil thickness, and coating thickness in engineering and manufacturing contexts, especially in the US where the inch is still the working unit. It is easy to confuse with the metric millimetre, which is roughly 40 times larger, so mixing the two up in a spec sheet is a common and costly mistake.
This calculator converts between mils and millimetres in either direction. Because a mil is a fixed fraction of an inch, and an inch is a fixed 25.4 mm, the conversion factor never changes: 1 mil is always exactly 0.0254 mm, so there is nothing to look up or estimate, only the arithmetic to run.
The formula
To go from mils to millimetres, multiply by 0.0254. To go the other way, from millimetres to mils, divide by 0.0254. The result is also shown in micrometres (µm), since coating and film thicknesses are often specified in that unit too.
| Term | Meaning |
|---|---|
| Mil (thou) | One thousandth of an inch: 0.001 in, equal to 0.0254 mm. |
| mm | Millimetre, one thousandth of a metre, the metric unit most commonly compared against mils. |
| µm | Micrometre, one thousandth of a millimetre, shown as an extra reference point. |
The inputs explained
| Field | What to enter |
|---|---|
| Value | The value to convert, in whichever unit you are starting from. |
| Direction | Choose the direction of the conversion: mils to millimetres, or millimetres to mils. |
When to use it
Reading a US wire gauge or foil spec
US datasheets for wire insulation, foil and film thickness are often given in mils. Converting to millimetres lets you compare directly against a metric spec or measuring tool.
Checking a coating or plating thickness
Paint, powder coating and electroplating thickness is frequently quoted in mils; converting to micrometres or millimetres makes it comparable with quality standards written in metric units.
Avoiding a mil/mm mix-up
Because 1 mil (0.0254 mm) is so much smaller than 1 mm, treating the two as interchangeable understates a thickness by roughly 40 times, which matters when a tolerance is tight.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
What do common mil values convert to in mm?
A range of mil values converted straight to millimetres.
What do common millimetre values convert to in mils?
The same range worked the other way, from millimetres back to mils.
Questions
How many mm is 1 mil?
1 mil equals 0.0254 mm. This is a fixed conversion, since a mil is defined as one thousandth of an inch and an inch is fixed at 25.4 mm.
Is a mil the same as a millimetre?
No, and mixing them up is a common error. A millimetre is roughly 39.4 times larger than a mil, so a spec of "10 mil" and "10 mm" describe very different thicknesses.
What is a mil used for?
Mils are commonly used for wire diameter, plastic film and foil thickness, and coating or plating thickness, particularly in US engineering and manufacturing documentation.
Is a mil the same as a millilitre?
No. "Mil" here is a unit of length (a thousandth of an inch); it is unrelated to the millilitre, a unit of volume, despite the similar-sounding abbreviation.
For general inch to millimetre conversions at larger scales, see the decimal to fraction calculator for working with imperial fractions.