What Is a Millimeter?
A millimeter (symbol mm) is a metric unit of length equal to one thousandth of a meter, exactly 0.001 m. Ten millimeters make a centimeter and 1,000 make a meter. One inch is exactly 25.4 mm, which makes the millimeter the bridge most engineers use between metric and customary drawings.
The millimeter is the working unit of engineering and manufacturing. International drawing standards give dimensions in millimeters by default, so a part marked “120” on a technical drawing is 120 mm long. It is also used for rainfall in most countries, camera lenses and film (35 mm), the caliber of firearms (9 mm), bolt sizes (an M8 bolt is 8 mm across), and the thickness of phones, glass and sheet metal. A standard bank card is 0.76 mm thick. Even countries that otherwise use inches, including the United States, specify most cars, bicycles, cameras and electronics in millimeters, because the parts come from a global supply chain.
Because the millimeter is defined from the meter, it shares the meter’s definition based on the speed of light, as set out by the BIPM. Precision work goes further still: machining tolerances are often given in hundredths of a millimeter, or 10 micrometers.
What Is a Mile?
The mile and the millimeter share a root. Mille is Latin for “thousand”: the Roman mille passus was a thousand paces, and the metric milli- means a thousandth. The two units went in opposite directions, though: the Roman mile counted a thousand steps forward, while the millimeter divides the meter into a thousand parts.
The modern mile is the international mile of 5,280 feet, fixed in length by a 1593 English statute. Since 1959 it has been exactly 1,609.344 meters, so in millimeters a mile is the whole number 1,609,344.
Metric Prefixes Explained: Milli-
Milli- means one thousandth (10⁻³). It was one of the original prefixes of the 1795 French metric system, alongside centi- and deci-, and it is the first step of the “engineering” prefixes that move in factors of 1,000: milli-, micro- (10⁻⁶), nano- (10⁻⁹) and so on, with kilo- and mega- going the other way. The NIST table of SI prefixes lists them all.
Milli- appears across the metric system in units people use every day. A milliliter (mL) is a thousandth of a liter and exactly one cubic centimeter. A milligram is a thousandth of a gram, the unit for most medicine doses. A millisecond is a thousandth of a second. In each case the prefix means the same thing, which is what makes metric conversions a matter of moving the decimal point.
Rainfall shows how practical the millimeter is. One millimeter of rain means one liter of water fell on every square meter of ground, because a layer 1 mm deep over 1 m² is exactly 1 dm³. Weather services outside the United States report rain in millimeters for that reason, while US forecasts use inches (25.4 mm each).
| Size | Millimeters (mm) | Miles (mi) |
|---|---|---|
| Bank card thickness | 0.76 | 4.72242 × 10⁻⁷ |
| Pencil lead (standard) | 2 | 0.00000124274 |
| 9 mm cartridge | 9 | 0.00000559234 |
| One inch | 25.4 | 0.0000157828 |
| 35 mm film width | 35 | 0.000021748 |
| A4 paper (short side) | 210 | 0.000130488 |
| One foot | 304.8 | 0.000189394 |
For length, three steps of milli- take you from the millimeter to the kilometer: 1 mm × 1,000 = 1 m, and 1 m × 1,000 = 1 km. That is why a million millimeters is a kilometer, or 0.621371 miles, and why a millimeter-to-mile result has so many decimal places.
A stack of paper makes the scale vivid. One sheet of ordinary printer paper is about 0.1 mm thick, so a stack one mile high would need about 16,093,440 sheets, more than 32,000 reams of 500. Slow processes work the same way: fingernails grow about 3.5 mm a month, so growing one a mile long would take roughly 38,000 years.
How Many Millimeters in a Mile?
There are exactly 1,609,344 millimeters in a mile, about 1.6 million. To convert miles to millimeters, multiply by that number. A tenth of a mile is 160,934.4 mm, and a 26.2-mile marathon is about 42,164,813 mm. Going the other way, a 1-meter step is 1,000 mm, or 0.000621371 miles, and 300 mm, about the length of a ruler, is 11.81 inches.