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.

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.

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.

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.

Use a raised dot between units combined by multiplication,
to avoid ambiguity. *N**m* 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.*

**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.
*

The term

When applied to a base unit other than bit, byte or pixel,

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!

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.

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.

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.*