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Rules and style conventions for expressing values of quantities
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Summary
Unit symbols
Unit names
Rules and style conventions for expressing values of quantities

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Related articles
Section 5.3.1
Section 5.3.3
Section 5.3.4
Section 5.3.5
Section 5.3.6
Section 5.3.7
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SI brochure, Section 5.3 cont.


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.

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.

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.

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 to the Expression of Uncertainty in Measurement [ISO, 1995]. The standard uncertainty (i.e. estimated standard deviation, coverage factor k = 1) 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 28 (29) times 10–27 kg

where mn is the symbol for the quantity (in this case the mass of a neutron), and the number in parenthesis 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 29 times 10–27 kg. If any coverage factor, k, different from one, is used, this factor must be stated.

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.



Related articles

Section 5.3.1
Section 5.3.3
Section 5.3.4
Section 5.3.5
Section 5.3.6
Section 5.3.7
     

Note: For the official text, please refer to the PDF files available at:
  • http://www.bipm.org/utils/common/pdf/si_brochure_8_en.pdf (in English) and
  • http://www.bipm.org/utils/common/pdf/si_brochure_8_fr.pdf (in French).
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