14. Quantities, units, and letter symbols

14.1 Quantity

The word quantity has many meanings; in this subclause, the word refers to physical quantities, which are described in units of measure such as length, mass, time, and temperature. A unit is a particular sample of a quantity, chosen so that an appropriate value may be specified. Meter, kilogram, hour, and degree Celsius are some of the units used for the four quantities noted previously.

The value of a quantity is generally expressed as the product of a number and a unit. Quantities and units may be represented in text by letter symbols and are always so represented in equations. If a number and unit cannot be identified for a quantity, the discussion may concern an amount rather than a quantity, in which case the term amount should be used.

14.2 Numbers

The following rules should be observed:

1. The decimal marker should be a dot on the line (decimal point).
2. For numbers of magnitude less than one, a zero should be placed in front of the decimal point (e.g., 0.5).
3. In general text, isolated numbers less than 10 should be spelled out. However, in equations, tables, figures, and other display elements, Arabic numerals should be used.
4. The value of a quantity shall be expressed by an Arabic numeral followed by a space and the appropriate unit name or symbol. An upright (Roman) type font should be used for the unit symbol even if the surrounding text uses a sloping (italic) font.
5. If tolerances are provided, the unit shall be given with both the basic value and the tolerance (150 m ± 5 mm). Ranges should repeat the unit (e.g., 115 V to 125 V). Dashes should never be used because they can be misconstrued as subtraction signs.
6. Percent is indicated by the sign (%) and directly follows the number, without a space.

14.3 Metric system

IEEE Policy 9.16 calls for measured and calculated values of quantities to be expressed in metric units [SI (Système International d’Unités)] in IEEE publications.¹ (See IEEE/ASTM SI 10 for guidance on metric practice.)² Proposed new standards and revised standards submitted for approval should use metric units exclusively in the normative portions of the standard. Inch-pound data may be included in parentheses after the metric unit if the standards committee believes that the audience for this document would benefit from the inclusion of inch-pound data, based on concerns for safety or clarity. Metric units shall always be the primary unit of measurement.

IEEE Policy 9.16 recognizes the need for some exceptions and contains the following statement: “Necessary exceptions to this policy, such as where a conflicting world industry practice exists, must be evaluated on an individual basis and approved by the responsible major board of the Institute for a specific period of time.” Standards Coordinating Committee 14 (SCC14), as part of the coordination process, shall review requests for individual exceptions, including those noted below, and shall report its recommendations to the IEEE SA Standards Board.

Exceptions:

1. A specific exception is given for trade sizes, such as the AWG wire series and inch-based standards for fasteners. Such data need not be translated into metric terms.
2. Also excepted are those cases, such as plugs and sockets, where a mechanical fit to an inch-based product is required.
3. The metric policy does not require metric products to be substituted for inch-based products. For further information, see IEEE/ASTM SI 10, IEEE Std 260.1, and IEEE Std 270.

14.4 Letter symbols

In IEEE standards, letter symbols should be used rather than abbreviations. Letter symbols include symbols for physical quantities (quantity symbols) and symbols for the units in which those quantities are measured (unit symbols). The quantity and its unit can usually be separated by a non-breaking space to avoid unfortunate line breaks. Unlike common abbreviations, letter symbols are invariant in singular and plural, are not followed by a period, and maintain their case independent of the surrounding text (see IEEE Std 260.1).

For example, standard quantity symbols for length, mass, and time are l, m, t. They are set in italic letters. Unit symbols for the same three quantities are m, kg, and s, set in Roman (upright) letters. Note especially that V is the symbol for the unit “volt,” and V (italic) is the symbol for the quantity “voltage.” Unit symbols may not be used to stand for the quantity being measured, as follows:

Incorrect: “The km between the substations is 20.”

Correct: “The distance between the substations is 20 km.”

Incorrect: “The amperes that flow into the ground.”

Correct: “The current that flows into the ground.”

Incorrect: “Polarity shall be additive for all kVA transformers rated at 200.”

Correct: “Polarity shall be additive for all transformers with an apparent power rating of 200 kVA.”