A Concise Summary Of The International System Of Units (Page 4 of 4) in pdf

letter or a capital L may be used, the capital being allowed in

shows a table of vapour pressure as a function of temperature,

this case to avoid confusion between the lower case letter l and

and the logarithm of vapour pressure as a function of reciprocal

the number one, 1.

temperature, with the columns labelled in this way.

The symbol for a nautical mile is given here as M; however there

T/K

K/T

p/MPa

ln(p/MPa)

is no general agreement on any symbol for a nautical mile.

216.55

4.6179

0.5180

−0.6578

273.15

3.6610

3.4853

1.2486

The language of science:

304.19

3.2874

7.3815

1.9990

using the SI to express the values of quantities

Algebraically equivalent forms may be used in place of

The value of a quantity is written as the product of a number

K/T, such as kK/T, or 10

(T/K)

−1

and a unit, and the number multiplying the unit is the numer-

ical value of the quantity in that unit. One space is always left

In forming products or quotients of units the normal rules of

between the number and the unit. For dimensionless quantities,

algebra apply. In forming products of units, a space should be

for which the unit is the number one, the unit is omitted. The

left between units (or alternatively a half high centred dot can

numerical value depends on the choice of unit, so that the same

be used as a multiplication symbol). Note the importance of

value of a quantity may have different numerical values when

the space, for example, m s denotes the product of a metre and

expressed in different units, as in the examples below.

a second, but ms denotes a millisecond. Also, when forming

The speed of a bicycle is approximately

complicated products of units, use brackets or negative expo-

v = 5.0 m/s = 18 km/h.

nents to avoid ambiguities. For example, the molar gas constant

The wavelength of one of the yellow sodium lines is

R is given by:



= 5.896 × 10

−7

m = 589.6 nm.

/T = R = 8.314 Pa m

mol

−1

= 8.314 Pa m

/(mol K).

Quantity symbols are printed in an italic (slanting) type, and

they are generally single letters of the Latin or Greek alphabet.

When formatting numbers the decimal marker may be either

Either capital or lower case letters may be used, and additional

a point (i.e. a stop) or a comma, as appropriate to the circum-

information on the quantity may be added as a subscript or as

stances. For documents in the English language a point is usual,

information in brackets.

but for many continental European languages and in some other

There are recommended symbols for many quantities, given

countries a comma is usual.

by authorities such as ISO (the International Organization for

When a number has many digits, it is customary to group the

Standardization) and the various international scientiﬁc unions

digits into threes about the decimal point for easy reading. This

such as IUPAP and IUPAC. Examples are:

is not essential, but it is often done, and is generally helpful.

for temperature

When this is done, the groups of three digits should be sepa-

for heat capacity at constant pressure

rated only by a (thin) space; neither a point nor a comma should

for the mole fraction (amount fraction) of species i

be used. The uncertainty in the numerical value of a quantity

for relative permeability

may often be conveniently shown by giving the uncertainty in

m (K ) for the mass of the international prototype of the

the least signiﬁcant digits in brackets after the number.

kilogram K .

Example: The value of the elementary charge is given in

the 2002 CODATA listing of fundamental constants as

Unit symbols are printed in a roman (upright) type, regardless

e = 1.602 176 53 (14) × 10

−19

of the type used in the surrounding text. They are mathematical

where 14 is the standard uncertainty in the ﬁnal digits quoted

entities and not abbreviations; they are never followed by a stop

for the numerical value.

(except at the end of a sentence) nor by an s for the plural. The

use of the correct form for unit symbols is mandatory, and is

illustrated by the examples in the SI Brochure. Unit symbols

may sometimes be more than a single letter. They are written

in lower case letters, except that the ﬁrst letter is a capital when

For further information

the unit is named after an individual. However when the name

see the BIPM website,

of a unit is spelled out, it should begin with a lower case letter

or the SI Brochure 8th edition,

(except at the beginning of a sentence), to distinguish the unit

which is available at

from the man.

In writing the value of a quantity as the product of a numerical

value and a unit, both the number and the unit may be treated

This summary has been prepared by the Comité Consultatif

by the ordinary rules of algebra. For example, the equation

des Unités (CCU) of the Comité International des Poids et

T = 293 K may equally be written T/K = 293. This procedure

Mesures (CIPM), and is published by the BIPM.

is described as the use of quantity calculus, or the algebra of

March 2006

quantities. It is often useful to use the ratio of a quantity to its

unit for heading the columns of tables, or labelling the axes of

Ernst Göbel, President of the CIPM

graphs, so that the entries in the table or the labels of the tick

Ian Mills, President of the CCU

marks on the axes are all simply numbers. The example below

Andrew Wallard, Director of the BIPM

A Concise Summary Of The International System Of Units Page 4

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