What this calculator does
A mil is a unit of length equal to one thousandth of an inch, commonly used in engineering drawings, wire gauge specifications and coating or plating thickness. This mil to mm calculator converts between mils and millimetres in either direction, using the fixed definition that 1 mil equals 0.0254 mm.
It is worth being clear about what a mil is not: it has nothing to do with a millilitre, despite the similar-sounding abbreviation "mL". A mil is a unit of length, one thousandth of an inch, while a millilitre is a unit of volume, one thousandth of a litre. Mixing the two up is a common source of confusion, particularly since both are sometimes shortened informally.
The formula
One mil equals exactly 0.0254 mm, since one inch is defined as exactly 25.4 mm and a mil is one thousandth of an inch. Converting mil to mm multiplies by 0.0254; converting mm to mil divides by the same figure.
| Term | Meaning |
|---|---|
| Mil | A unit of length equal to one thousandth of an inch (0.001 in), used in engineering, wire gauge and coating-thickness contexts. |
| Millimetre (mm) | A metric unit of length equal to one thousandth of a metre. |
| Millilitre (mL) | A metric unit of volume, unrelated to the mil despite the similar abbreviation. |
The inputs explained
| Field | What to enter |
|---|---|
| Convert | Choose the conversion direction: mil to mm, or mm to mil. |
| Value | The value to convert, in whichever unit you selected as the starting point. |
When to use it
Reading a wire gauge or coating spec
Wire diameters and coating or plating thicknesses in American engineering documents are frequently given in mils, and converting to millimetres makes them comparable with metric drawings and tools.
Checking a plastic film or sheet thickness
Plastic sheeting, garbage bags and packaging film are commonly rated in mils in the United States; converting to millimetres gives a figure that lines up with metric thickness gauges.
Avoiding a mil-versus-millilitre mix-up
When a specification uses "mil" for thickness, converting it explicitly to millimetres removes any risk of it being misread as a volume figure in millilitres.
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.
Common mil to mm conversions
A range of mil values converted to millimetres.
Common mm to mil conversions
A range of millimetre values converted to mil.
| Millimetres | Result |
|---|---|
| 0.5 mm | 19.69 mil |
| 1 mm | 39.37 mil |
| 2 mm | 78.74 mil |
| 5 mm | 196.85 mil |
| 10 mm | 393.70 mil |
| 25.4 mm | 1,000.00 mil |
Questions
Is a mil the same as a millimetre?
No, and the similar spelling is a common trap. A mil is one thousandth of an inch (0.0254 mm), while a millimetre is one thousandth of a metre, a considerably larger unit: one millimetre is roughly 39.4 mils.
Is a mil the same as a millilitre?
No. A mil is a unit of length; a millilitre (mL) is a unit of volume. They are unrelated, but the abbreviations are close enough that specifications are sometimes misread, so it is worth checking context when "mil" appears.
Why is 1 mil exactly 0.0254 mm?
An inch is defined as exactly 25.4 mm, and a mil is one thousandth of an inch, so 1 mil works out to exactly 25.4 / 1000 = 0.0254 mm. This is a fixed definition, not an approximation.
Where are mils commonly used?
Mils are common in American engineering and manufacturing contexts: wire and cable diameters, sheet and film thickness, and coating or plating thickness specifications are all frequently given in mils.
For converting millimetres to inches, both as a decimal and the nearest fraction, see the mm to imperial converter.