SI alphabet

The characters used as symbols for units in the SI system are as shown in Table 48, the meanings followed by a hyphen, e.g. femto-, being usable only as prefixes to the others. Only one prefix should be used for any one unit to produce a compound unit; such expressions as millimicrogram are outmoded and unacceptable. The one-time millimicrogram is now correctly called the nanogram, to be written ng; note that any prefix directly adjoins the elementary measure-unit symbol, making an unpunctuated composite symbol for the compound unit of measure. The symbol for each unit of measure, whether a simple unqualified symbol else a composite, should be separated from others, by individual spaces or the elevated point or, where appropriate, by the solidus indicating ‘per’, and the resulting string of them should be separated by a space from any preceding number (else a hyphen in English adjectival form, but then applicable only for one unit, e.g. ‘a 2-mm clearance’). For example, for 8 kilopascals, which is identically 8 kilonewtons per square metre, any of the following can be used:
8 kPa 8 000 Pa 8 × 10³ Pa 8 kN m^-2 8 kN·m^-2 8 kN/m².Note that, though a pluralizing ‘s’ is added to the full words in text in accordance with normal rules of the language, it is never added to any symbol. The letter ‘s’ has its own distinct role of representing the second of time; mostly, this appears in a quotient role, ‘per second’. As part of the endeavour to make the scheme invariable across languages, the term ‘per’ should not be used with the symbols; any such quotient should be written without controlling words, either by using the ‘/’ or by using negative indices, e.g. s^-1. Likewise the words ‘square’, ‘squared’, ‘cubic’, etc., and their abbreviations should not be used except within normal English text; most notably, cc for what is properly mL is deprecated.

		a-	= atto-
A	= ampere,	a	= are
Å	= angstrom
Bq	= becquerel
		c-	= centi-
C	= coulomb
Ci	= curie (temporarily)
	see also °C below
		cd	= candela
		d-	= deci-
D	= dalton (unofficially)	d	= day
		da-	= deca-, deka-
E-	= exa-
		eV	= electronvolt
		f-	= femto-
F	= farad
G-	= giga-
		g	= gram
Gy	= gray
		h-	= hecto-
H	= henry	h	= hour
Hz	= hertz
J	= joule
		k-	= kilo-
K	= kelvin
L	= litre	l	= litre
		lm	= lumen
		lx	= lux
M-	= mega-	m-	= milli-
		m	= metre
		min	= minute of time
		mol	= mole
		n-	= nano-
N	= newton
P-	= peta-	p-	= pico-
Pa	= pascal
		rad	= radian
S	= siemens	s	= second
		sr	= steradian
Sv	= sievert
T-	= tera-
T	= tesla	t	= tonne
		u	= unified atomic mass unit
V	= volt
W	= watt
Wb	= weber
Y-	= yotta-	y-	= yocto-
Z-	= zetta-	z-	= zepto-
		μ-	= micro-
Ω	= ohm
°	= degree
°C	= degree Celsius
′	= minute (of arc)
″	= second (of arc)

Technically any prefix can be added as needed to any unit. For mass this means to gram, not to kilogram, although the latter is the actual base unit in the SI (relative to coherent derived units); however, it is not usual to apply a larger prefix than kilo- to the gram. It is likewise incorrect to apply them to the tonne, which is the megagram, but it is common practice to do so (this application being to a unit that does not incorporate a prefix). The alternative of explicit multiplicative powers of 10 is always available. Positive and negative indices of any magnitude can be used as appropriate with any unit. Powers of ten should be placed immediately in front of the space before the first (unqualified else composite) measure-unit symbol in the string. The preferred practice now is to restrict such uses of powers to exponents that are multiples of 3, i.e. to integer powers of 1 000. Thus a value that might be written as 1.732 × 10¹¹ should be written as 173.2 × 10⁹ else 0.173 2 × 10¹². For mass, such multipliers should apply to the kilogram; for most other circumstances they should preferably apply directly to an unqualified unit else to a string of them.

Unless there is special cause, a compound expression should be resolved into standard unit terms and prefixed by powers of 10³ as needed, but, if the numerator is not compound, this can reasonably be prefixed to remove the powers, as in the kilopascal example above.