Writing SI units and symbols

Charles Poynton

This note explains how to write quantities and units of the Système international d'unités (SI), colloquially known as the metric system. I catalog the power-of-ten prefixes, and I list some important units.

Write a numeric value with units in either the journalistic style, using prefix and unit names (four kilohertz); or the scientific style, using prefix and unit symbols (4 kHz). Don't mix these styles: Do not mix a prefix name with a unit symbol (WRONG: kiloHz), or a prefix symbol with a unit name (WRONG: kHertz). Avoid "abbreviations" for units (WRONG: sec., amp); use the unit names or symbols instead.

If you are writing for an international audience, express values in the metric (SI) system used by the majority of the world's population. If appropriate, follow an SI value with the equivalent Imperial value in parentheses. Express the Imperial value with an accuracy comparable to the original: write 5 m (16 feet), not 5 m (16.4042 feet). Spell out inch, foot, pound and so on: Do not abbreviate to in, ft, and lb unless space is an overriding concern. Do not use " and ' symbols for inch and foot: These symbols are easily lost in reproduction, and they are unfamiliar to a large fraction of the world's population.

Journalistic Style

In free text, use journalistic style for units and measurements: Spell out numbers one through ten in words; express numbers larger than that in numerals. Follow a number by a space, then the prefix name and unit name spelled out entirely in lower case and without spaces: four megahertz, 2.2 microfarads, 3.5 megahertz, 75 ohms.

Use hundred, thousand, million, and so on, only for pure numbers. For a number with a unit, spell out the SI prefix: four kilowatts (not four thousand watts). Avoid using words for extreme quantities larger than a million, because billion, trillion, and so on, have different numerical values in different countries. If you absolutely must use words, follow the example of the BBC World Service: say thousand million or million million.

Use a hyphen between a numeral and its unit only when necessary to form a compound modifier, and only with a unit name, not a unit symbol: 3.5-inch diskette, 35-millimeter film. To avoid the confusion of two hyphens when a negative number is involved, as in -12-volt power, use a space instead of a second hyphen.

In many countries a comma indicates the decimal: in these countries the notation 10,000 indicates precisely ten, not ten thousand! Some of your readers will find it ambiguous if you use a comma as a separator between three-digit groups. In a numeric value having four or more consecutive digits, use a space to separate groups of three digits, both left and right of the decimal point.

Scientific Style

In a table, an illustration or a technical text, use the scientific style for measurements and units. Write the number in figures, followed by a nonbreaking space. Then write the prefix symbol and the unit symbol with appropriate capitalization and no spaces: 4 MHz, , . Separate the last digit from the unit with a nonbreaking space; this will prevent clumsy line breaks.

SI prefix symbols are capitalized for multipliers and larger, and lower case for multipliers and smaller.

A unit symbol is written in lower case, except that its initial letter is capitalized if the unit is named after a person. These are symbols, not abbreviations or contractions: Do not use periods or other punctuation. To avoid confusion with math symbols ("variables"), do not italicize unit symbols.

Use appropriate capitalization. The symbol k for kilo - a multiplier of 1000 - combines with hertz as kHz; the symbol for decibel is written dB. A popular computer in 1987 had a nameplate stating its memory capacity as 1 mb. In fact it had a megabyte of memory, properly written as 1 MB, not a millibit!

When you write a negative sign, use a nonbreaking hyphen instead of a regular hyphen. This prevents the sign from being left stranded at the end of the line: -
400 V power results from using a standard hyphen,
-400 V power results from a nonbreaking hyphen. The former is, at the very least, confusing to your reader. At its worst, it could compromise personal safety.

Dates

Different countries have different conventions for writing dates. A reader in the U.S.A. takes 08/04/50 to be August 4th, but a U.K. reader takes it to be the 8th of April. In the next century, will 01/02/03 be the first, second or third day of the month? Avoid ambiguity. Write dates in the ISO/IEC 8824 form: 1996-06-07.

Unit Combinations

Use a raised dot between units combined by multiplication, to avoid ambiguity. Nm for newtonmeter avoids potential confusion with nanometer, nm.

per

Use the per notation for everyday units formed by division, such as miles per hour, mph; revolutions per minute, rpm; and dots per inch, dpi.

slash

In a scientific or engineering unit formed by division, set off a single-element denominator with a slash: write m/s for meters per second. I write b/s for bits per second, although some people use bps.

exponents

For a compound unit having a complex denominator, use exponent notation: write for meters per second squared (NOT m/s/s).

ohm

Use ohm when the symbol is unavailable (as in ASCII character code).

degrees

The temperature unit kelvin, K, properly has no degree sign. The non-SI symbols for Celsius (C) and Fahrenheit (F) have degree signs in order to avoid ambiguity with coulomb C and farad F. The term centigrade is obsolete; the proper term is Celsius.

Computing

b, B

Use little b for bit, big B for Byte. Spell these out where necessary to avoid ambiguity.

k

Little k - pronounced KEY-loh or kill-oh, spelled-out kilo - is the standard SI prefix for (1000). It is not often used in computing.

K

Use big K for the multiplier (1024) common in computing. Do not write or pronounce big K as kilo; to do so invites confusion with little k, 1000. Simply write it as upper-case K and pronounce it kay.

baud

The term baud does not apply to data rate, but to symbol rate. When you see the unit baud used in computing, the unit b/s (bit per second) is nearly always meant.

mega, giga

When applied to a base unit other than bit, byte or pixel, M (mega) and G (giga) refer to the SI power-of-ten multipliers and . Standard data communication rates are based on powers of ten and use the SI multipliers, not power-of-two multipliers: 1.544 Mb/s denotes 1 544 000 bits per second; 19 200 bits per second is properly written 19.2 kb/s (not 19.2 Kb/s).

disk storage

When applied to bytes of disk storage capacity:

• M (mega) denotes (1000 K); and
• G (giga) denotes (1 000 000 K).

bits, bytes or pixels

When applied to raw bits, bytes or pixels:

• M (mega) denotes (1024 K); and
• G (giga) denotes .

In computing, M (mega) and G (giga) are ambiguous. M could denote 1 000 000, 1 024 000, or 1 048 576. G could denote 1 000 000 000, 1 024 000 000, 1 048 576 000, or 1 073 741 824. The value of the giga prefix in computing varies more than 7 percent depending on its context. If an exact value is important, write out the whole number!

SI Prefix Names, Symbols and Multipliers

This table contains a complete list of SI prefix multiplier names, symbols, and power-of-ten values, standardized by the Bureau International des Poids et Measures (BIPM, www.bipm.fr). The symbol alone, and the term micron, have been abolished: Use for micrometer. Use lower-case u for if the micro symbol is unavailable.

Basic SI Unit Names and Symbols

This table includes some important SI units and their derivations, and the names of a few individuals whose names have been given to units. The seven base SI units are m, kg, s, A, K, mol, and cd; the other units are derived.

Further information

Information is available at BIPM, http://www.bipm.fr/enus/3_SI/.

Information is available at NIST, http://physics.nist.gov/cuu/Units/. See Guide for the Use of the International System of Units (SI) [NIST Special Publication 811] (Acrobat PDF format, 400 KB), Typefaces for symbols in scientific manuscripts (Acrobat PDF format, 62 KB), and SI Unit rules and style conventions - Check List for Reviewing Manuscripts.