Millimetre

Informal terminology

The term "mil" is sometimes used colloquially for millimetre. However, in the United States, "mil" traditionally means a thousandth of an inch, which may cause confusion.

Unicode symbols

To support layout compatibility with East Asian scripts (CJK), Unicode includes square symbols for:

• Millimetre – U+339C ㎜ SQUARE MM
• Square millimetre – U+339F ㎟ SQUARE MM SQUARED
• Cubic millimetre – U+33A3 ㎣ SQUARE MM CUBED

These symbols are often used in Japanese typography to align unit symbols with text characters.

Measurement

• On a standard metric ruler, the smallest divisions are typically millimetres.
• Precision engineering rulers may show increments of 0.5 mm.
• Digital calipers often measure to 0.01 mm accuracy.

Examples:

• Microwaves with a frequency of 300 GHz have a wavelength of 1 mm.
• Using frequencies from 30–300 GHz for millimetre-wave communications allows high-speed data transfer, such as 10 Gbps.
• The smallest visible object to the human eye is around 0.02–0.04 mm, such as a thin human hair.
• A typical sheet of paper is between 0.07 mm and 0.18 mm thick; copy paper is about 0.1 mm.
• Mechanical pencil leads are commonly produced in 0.5 mm and 0.7 mm sizes.
• Printed circuit boards, automotive components and precision-machined engineering parts are frequently specified using millimetres.

Millimetres are also commonly converted into imperial units such as inches for engineering, manufacturing, construction and international product specifications. One inch is defined as exactly 25.4 millimetres.

Additional metric and imperial conversion references:

• SI Units – NIST
• Millimetres to inches converter
• Inches to millimetres converter
• International System of Units (BIPM)
• Engineering Toolbox