Millimetre

Millimetre

Different lengths with respect to the electromagnetic spectrum. The microwave spans from 1 metre to 1 millimetre.

The millimetre (SI symbol: mm; international spelling) or millimeter (American spelling) is a unit of length in the International System of Units (SI), equal to one thousandth of a metre, the SI base unit of length.

• 1 metre = 1000 millimetres • 1 centimetre = 10 millimetres

• 1000 micrometres • 1000000 nanometres

Since an inch is officially defined as exactly 25.4 millimetres, 1 millimetre is precisely 5⁄127 inches (≈ 0.03937 inches).

Definition

Millimetre

Since 1983, the metre has been defined as "the length of the path travelled by light in vacuum during a time interval of ⁠1/299792458⁠ of a second".

A millimetre, being ⁠1/1000⁠ of a metre, is the distance light travels in ⁠1/299 792 458 000⁠ of a second.

Informal terminology

Millimetre

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

Millimetre

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

Millimetre

• 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.

• 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 (e.g., 10 Gbps). • The smallest visible object to the human eye is around 0.02–0.04 mm (e.g., 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.