What is millimetre?
A millimetre is a metric unit of length equal to one thousandth of a metre. It is used in precision manufacturing, medical imaging, engineering tolerances, and detailed technical specifications.
Real-world uses
Millimetres are essential in precision engineering, machining tolerances, and manufacturing specifications. Rainfall is reported in mm by meteorological services worldwide. Dentists measure cavity depths and orthodontic adjustments in millimetres.
History
Part of the original French metric system from the 1790s. The prefix "milli-" derives from the Latin "mille" meaning thousand, indicating one-thousandth of a metre. It became critical with the rise of precision manufacturing in the Industrial Revolution.
Common mistakes
Confusing millimetres with centimetres when reading rulers—each small division on a metric ruler is 1 mm, not 1 cm. Also, mixing up mm with mils (thousandths of an inch) in engineering contexts.
What is inch?
An inch is a unit of length in imperial and US customary systems equal to 1/12 of a foot. It is widely used for screen sizes, display resolutions, pipe diameters, and small-scale measurements in everyday contexts.
Real-world uses
Inches are used throughout the United States, Canada (partially), and the UK for screen sizes, pipe diameters, lumber dimensions, and screw/bolt specifications. Display sizes for monitors, TVs, and phones are universally quoted in diagonal inches.
History
The inch has ancient origins, traditionally defined as the width of a man's thumb or three barleycorns laid end to end. It was standardized internationally in 1959 as exactly 25.4 mm, reconciling slight differences between British and American definitions.
Common mistakes
Assuming inch fractions are decimal—1/8 inch is 0.125 inches, not 0.18. Also, confusing nominal lumber sizes (a "2x4" is actually 1.5 × 3.5 inches) with actual inch measurements.
When is this conversion used?
Converting between millimetre and inch is common when working across metric and imperial systems, such as international trade, travel between countries with different measurement standards, or following instructions from a different region. Screen sizes, paper dimensions, and clothing measurements often require converting between centimetres and inches, especially when shopping internationally.
Worked examples
1 millimetre = 0.03937008 inch
1 inch = 0.0254 metre
How to convert millimetre to inch
To convert millimetre to inch, multiply the value by 0.03937008.
To convert inch back to millimetre, multiply by 25.4.
Measurement standards
The metre is one of seven SI base units, maintained by the International Bureau of Weights and Measures (BIPM). Since 2019, it is defined by fixing the numerical value of the speed of light in vacuum to exactly 299,792,458 metres per second.
Did you know?
A human hair is roughly 70 micrometres wide, while a single gold atom measures about 0.144 nanometres in diameter — meaning you could line up nearly half a million atoms across the width of one hair.
Quick reference: millimetre to inch
| millimetre | inch |
|---|---|
| 0.1 | 0.00393701 |
| 0.5 | 0.01968504 |
| 1 | 0.03937008 |
| 2 | 0.07874016 |
| 5 | 0.19685039 |
| 10 | 0.39370079 |
| 25 | 0.98425197 |
| 50 | 1.96850394 |
| 100 | 3.93700787 |
| 250 | 9.84251969 |
| 500 | 19.68503937 |
| 1,000 | 39.37007874 |
Common values
| millimetre | inch | |
|---|---|---|
| Height of a door | 2,100 millimetre | 82.67716535 inch |
| Basketball court length | 28,000 millimetre | 1,102.36220472 inch |
| Football field (soccer) | 105,000 millimetre | 4,133.85826772 inch |
| Marathon distance | 42,195,000 millimetre | 1,661,220.47244094 inch |
| Altitude of a cruising airplane | 10,668,000 millimetre | 420,000 inch |