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SI Brochure: The International System of Units (SI) [8th edition, 2006; updated in 2014]

Section 5.3: Rules and style conventions for expressing values of quantities

Section 5.3.1: Value and numerical value of a quantity, and the use of quantity calculus

The value of a quantity is expressed as the product of a number and a unit, and the number multiplying the unit is the numerical value of the quantity expressed in that unit. The numerical value of a quantity depends on the choice of unit. Thus the value of a particular quantity is independent of the choice of unit, although the numerical value will be different for different units.

Symbols for quantities are generally single letters set in an italic font, although they may be qualified by further information in subscripts or superscripts or in brackets. Thus C is the recommended symbol for heat capacity, Cm for molar heat capacity, Cm,p for molar heat capacity at constant pressure, and Cm,V for molar heat capacity at constant volume.

Recommended names and symbols for quantities are listed in many standard references, such as the ISO and IEC 80000 series, Quantities and units, the IUPAP SUNAMCO Red Book Symbols, Units, Nomenclature and Fundamental Constants in Physics, and the IUPAC Green Book Quantities, Units and Symbols in Physical Chemistry. However, symbols for quantities are recommendations (in contrast to symbols for units, for which the use of the correct form is mandatory). In particular circumstances authors may wish to use a symbol of their own choice for a quantity, for example in order to avoid a conflict arising from the use of the same symbol for two different quantities. In such cases, the meaning of the symbol must be clearly stated. However, neither the name of a quantity, nor the symbol used to denote it, should imply any particular choice of unit.

Symbols for units are treated as mathematical entities. In expressing the value of a quantity as the product of a numerical value and a unit, both the numerical value and the unit may be treated by the ordinary rules of algebra. This procedure is described as the use of quantity calculus, or the algebra of quantities. For example, the equation T = 293 K may equally be written T/K = 293. It is often convenient to write the quotient of a quantity and a unit in this way for the heading of a column in a table, so that the entries in the table are all simply numbers. For example, a table of vapour pressure against temperature, and the natural logarithm of vapour pressure against reciprocal temperature, may be formatted as shown below.

T/K 103 K/T p/MPa ln(p/MPa)
216.55 4.6179 0.5180 –0.6578
273.15 3.6610 3.4853 1.2486
304.19 3.2874 7.3815 1.9990

The axes of a graph may also be labelled in this way, so that the tick marks are labelled only with numbers, as in the graph below.

example graph axes

Algebraically equivalent forms may be used in place of 103 K/T, such as kK/T, or 103 (T/K)–1.

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Section 5.3.2: Quantity symbols and unit symbols

Just as the quantity symbol should not imply any particular choice of unit, the unit symbol should not be used to provide specific information about the quantity, and should never be the sole source of information on the quantity. Units are never qualified by further information about the nature of the quantity; any extra information on the nature of the quantity should be attached to the quantity symbol and not to the unit symbol.

Section 5.3.3: Formatting the value of a quantity

The numerical value always precedes the unit, and a space is always used to separate the unit from the number. Thus the value of the quantity is the product of the number and the unit, the space being regarded as a multiplication sign (just as a space between units implies multiplication). The only exceptions to this rule are for the unit symbols for degree, minute, and second for plane angle, °, ', and ", respectively, for which no space is left between the numerical value and the unit symbol.

This rule means that the symbol °C for the degree Celsius is preceded by a space when one expresses values of Celsius temperature t.

Even when the value of a quantity is used as an adjective, a space is left between the numerical value and the unit symbol. Only when the name of the unit is spelled out would the ordinary rules of grammar apply, so that in English a hyphen would be used to separate the number from the unit.

In any one expression, only one unit is used. An exception to this rule is in expressing the values of time and of plane angles using non-SI units. However, for plane angles it is generally preferable to divide the degree decimally. Thus one would write 22.20° rather than 22° 12', except in fields such as navigation, cartography, astronomy, and in the measurement of very small angles.

Section 5.3.4: Formatting numbers, and the decimal marker

The symbol used to separate the integral part of a number from its decimal part is called the decimal marker. Following the 22nd CGPM (2003, Resolution 10), the decimal marker "shall be either the point on the line or the comma on the line." The decimal marker chosen should be that which is customary in the context concerned.

If the number is between +1 and –1, then the decimal marker is always preceded by a zero.

Following the 9th CGPM (1948, Resolution 7) and the 22nd CGPM (2003, Resolution 10), for numbers with many digits the digits may be divided into groups of three by a thin space, in order to facilitate reading. Neither dots nor commas are inserted in the spaces between groups of three. However, when there are only four digits before or after the decimal marker, it is customary not to use a space to isolate a single digit. The practice of grouping digits in this way is a matter of choice; it is not always followed in certain specialized applications such as engineering drawings, financial statements, and scripts to be read by a computer.

For numbers in a table, the format used should not vary within one column.

Section 5.3.5: Expressing the measurement uncertainty in the value of a quantity

The uncertainty that is associated with the estimated value of a quantity should be evaluated and expressed in accordance with the Guide JCGM 100:2008 (GUM 1995 with minor corrections), Evaluation of measurement data – Guide to the expression of uncertainty in measurement. The standard uncertainty (i.e. the estimated standard deviation) associated with a quantity x is denoted by u(x). A convenient way to represent the uncertainty is given in the following example:

mn = 1.674 927 351(74) times 10–27 kg

where mn is the symbol for the quantity (in this case the mass of a neutron), and the number in parentheses is the numerical value of the combined standard uncertainty of the estimated value of mn referred to the last two digits of the quoted value; in this case:

u(mn) = 0.000 000 074 x 10–27 kg.

If an expanded uncertainty U(x) is used in place of the standard uncertainty u(x), then the coverage factor k must be stated.

Section 5.3.6: Multiplying or dividing quantity symbols, the values of quantities, or numbers

When multiplying or dividing quantity symbols any of the following methods may be used: ab, a b, a · b, a times b, a/b, italic a over italic b, a b–1.

When multiplying the value of quantities either a multiplication sign, times, or brackets should be used, not a half-high (centred) dot. When multiplying numbers only the multiplication sign, times, should be used.

When dividing the values of quantities using a solidus, brackets are used to remove ambiguities.

Section 5.3.7: Stating values of dimensionless quantities, or quantities of dimension one

As discussed in Section 2.2.3, the coherent SI unit for dimensionless quantities, also termed quantities of dimension one, is the number one, symbol 1. Values of such quantities are expressed simply as numbers. The unit symbol 1 or unit name "one" are not explicitly shown, nor are special symbols or names given to the unit one, apart from a few exceptions as follows. For the quantity plane angle, the unit one is given the special name radian, symbol rad, and for the quantity solid angle, the unit one is given the special name steradian, symbol sr. For the logarithmic ratio quantities, the special names neper, symbol Np, bel, symbol B, and decibel, symbol dB, are used (see 4.1 and Table 8).

Because SI prefix symbols can neither be attached to the symbol 1 nor to the name "one", powers of 10 are used to express the values of particularly large or small dimensionless quantities.

In mathematical expressions, the internationally recognized symbol % (percent) may be used with the SI to represent the number 0.01. Thus, it can be used to express the values of dimensionless quantities. When it is used, a space separates the number and the symbol %. In expressing the values of dimensionless quantities in this way, the symbol % should be used rather than the name "percent".

In written text, however, the symbol % generally takes the meaning of "parts per hundred".

Phrases such as "percentage by mass", "percentage by volume", or "percentage by amount of substance" should not be used; the extra information on the quantity should instead be conveyed in the name and symbol for the quantity.

In expressing the values of dimensionless fractions (e.g. mass fraction, volume fraction, relative uncertainties), the use of a ratio of two units of the same kind is sometimes useful.

The term "ppm", meaning 10–6 relative value, or 1 in 106, or parts per million, is also used. This is analogous to the meaning of percent as parts per hundred. The terms "parts per billion", and "parts per trillion", and their respective abbreviations "ppb", and "ppt", are also used, but their meanings are language dependent. For this reason the terms ppb and ppt are best avoided. (In English-speaking countries, a billion is now generally taken to be 109 and a trillion to be 1012; however, a billion may still sometimes be interpreted as 1012 and a trillion as 1018. The abbreviation ppt is also sometimes read as parts per thousand, adding further confusion.)

When any of the terms %, ppm, etc., are used it is important to state the dimensionless quantity whose value is being specified.

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