Appendix A: SI Units
‘‘SI’’ is an abbreviation for Le Syste
`me International d’Unite
´s, an international system of units
adopted by many national and international authorities, associations, professional societies, and agencies.
SI is closely related to but not identical with the former cgs, mks, and mksa systems of metric units.
Official information, development history, and more detail on SI can be found in Bureau of Standards
Special Publication 330 共1974兲and in the SEG Subcommittee on Metrification 共1981兲publication.
SI is based on seven base units listed in Table 1 and two supplementary units 共the last two兲.
SI derived units are formed by combining the base and supplementary units. Some of the derived
units are listed in Table 2.
Table 3 shows how larger or smaller units can be made by adding prefixes. When prefixes are used,
the first syllable is accented. Note that k and M stand for 10 3and 106, whereas M and MM 共or m and
mm兲are sometimes used in the oil industry for designating thousands and millions 共of gas volumes兲.
Prefixes are raised to the power of the unit employed; for example, km3means cubic kilometers, not
thousands of cubic meters. Prefixes are not compounded 共GW rather than kMW兲.
SEG allows the forms in Table 4 in addition to those in Tables 1 to 3.
Table 5 relates cgs electromagnetic and electrostatic units to SI units; see also Figure E-8. Figure M-1
relates cgs and SI magnetic units.
Rules about writing units
Symbols are written in Roman 共not italics兲type. They are never pluralized.
Unit names, including prefixes, are not capitalized except at the beginning of a sentence or in titles.
Unit names are pluralized in the usual manner, as 100 meters, 70 henries, except for lux, hertz, and
siemens. Fractional values require the singular form.
Periods are not used after symbols, that is, symbols are not abbreviations.
Symbols are lower case except when named for a person 共exception: L for liter兲.
A space separates a numerical value and the unit symbol 共except for °C兲; thus, 10 m, 0.112 s,
1.5 g/cm3, 20°C. A hyphen separates value and symbol when used as an adjective; thus, 35-mm film. No
space separates a prefix and the symbol; thus, ms for milliseconds, kW for kilowatt.
The symbols ‘‘/’’ or ‘‘.’’ are used to indicate the compounding of symbols 共for example, km/s or N.m
for kilometers per second and newton-meter兲, but are not used when units are written out. Where
symbols are compounded, parentheses should be used to avoid ambiguity, as W/共m.k兲. ‘‘P’’ is not
acceptable as an abbreviation for ‘‘per.’’ ‘‘Per’’ should not be compounded; thus, ‘‘meters per second
squared,’’ not ‘‘meters per second per second.’’ Use⫻rather than •for products of numbers; thus
6.2⫻5, not 6.2.5. A space should be used on each side of symbols for multiplication, addition, subtrac-
tion, convolution 共⫻,⫹,⫺,*兲and for the division symbol ⫼but not for /.
Numbers with many decimal places should be grouped by threes separated by a space rather than by
a comma 共which Europeans read as a decimal point兲; thus, 4 720 525 or 0.528 75. For numbers smaller
than one, a zero should be shown in the units place. A space is not necessary for four-digit numbers.
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