#1

General Only units of the SI and those units recognized for use with the SI are used to express the values of quantities. Equivalent values in other units are given in parentheses following values in acceptable units only when deemed necessary for the intended audience.

#2

Abbreviations Abbreviations such as sec, cc, or mps are avoided and only standard unit symbols, prefix symbols, unit names, and prefix names are used.

proper: s or second; cm3 or cubic centimeter; m/s or meter per second

improper: sec; cc; mps

#3

Plurals Unit symbols are unaltered in the plural.

proper: l = 75 cm

improper: l = 75 cms

#4

Punctuation Unit symbols are not followed by a period unless at the end of a sentence.

proper: The length of the bar is 75 cm.

The bar is 75 cm long.

improper: The bar is 75 cm. long.

#5

Multiplication

& division A space or half-high dot is used to signify the multiplication of units. A solidus (i.e., slash), horizontal line, or negative exponent is used to signify the division of units. The solidus must not be repeated on the same line unless parentheses are used.

proper: The speed of sound is about 344 m·s-1 (meters per second)

The decay rate of 113Cs is about 21 ms-1 (reciprocal milliseconds)

m/s, m·s-2, m·kg/(s3·A), m·kg·s-3·A-1

m/s, m s-2, m kg/(s3 A), m kg s-3 A-1

improper: The speed of sound is about 344 ms-1 (reciprocal milliseconds)

The decay rate of 113Cs is about 21 m·s-1 (meters per second)

m ÷ s, m/s/s, m·kg/s3/A

#6

Typeface Variables and quantity symbols are in italic type. Unit symbols are in roman type. Numbers should generally be written in roman type. These rules apply irrespective of the typeface used in the surrounding text. For more details, see Typefaces for symbols in scientific manuscripts

proper: She exclaimed, "That dog weighs 10 kg!"

t = 3 s, where t is time and s is second

T = 22 K, where T is thermodynamic temperature, and K is kelvin

improper: He exclaimed, "That dog weighs 10 kg!

t = 3 s, where t is time and s is second

T = 22 K, where T is thermodynamic temperature, and K is kelvin

#7

Typeface Superscripts and subscripts are in italic type if they represent variables, quantities, or running numbers. They are in roman type if they are descriptive.

subscript category typeface proper usage quantity italic c p , specific heat capacity at constant pressure descriptive roman m p , mass of a proton running number italic

#8

Abbreviations The combinations of letters "ppm," "ppb," and "ppt," and the terms part per million, part per billion, and part per trillion, and the like, are not used to express the values of quantities.

proper: 2.0 µL/L; 2.0 x 10-6 V;

4.3 nm/m; 4.3 x 10-9 l;

7 ps/s; 7 x 10-12 t,

where V, l, and t are the quantity symbols for volume, length, and time.

improper: "ppm," "ppb," and "ppt," and the terms part per million, part per billion, and part per trillion, and the like

#9

Unit

modifications Unit symbols (or names) are not modified by the addition of subscripts or other information. The following forms, for example, are used instead.

proper: V max = 1000 V

a mass fraction of 10 %