Using Numbers and Unit Symbols in Measurements

Every measurement includes two parts: a numerical value and a unit of measurement. The name of the unit of measurement can be given in full (2 kilograms, 6 feet) but often a symbol is used as an abbreviation (2 kg, 6 ft).

Numerical values

Decimal Points and Markers: In English-speaking countries (except South Africa), in Mexico and in most Asian countries the period (.) is used as the decimal marker (decimal point) to separate the whole number and fractional portions of a number. In continental Europe and most South American countries the decimal marker is the comma (,). The International System (SI) allows either the period or the comma to be used; the Dictionary uses the period. Wikipedia's decimal separator article has a lists of countries using each marker.

Digit grouping: Since the comma is often used as a decimal marker, it should not be used to separate groups of digits. The SI recommends that numbers having many digits be represented as groups of three digits separated by narrow spaces, like this: 2 675 491. The Dictionary observes this convention. The SI also allows large numbers to be written without any grouping, as in 2675491, but it does not allow the use of commas or any other grouping marks other than narrow spaces.

Large numbers: How many is a billion? In America, the word "billion" means the number 1 000 000 000, or 109. In Britain, this number is (or was) traditionally called "one milliard" or "one thousand million," and "billion" means the number 1 000 000 000 000 or 1012, which is what Americans call a "trillion." Most other European languages have similar expressions, such as mil millones for 109 and billón for 1012 in Spanish.

The Dictionary uses the American definitions for large number names. Although the General Conference on Weights and Measures endorsed the European names in 1948, the American definitions seem to be gaining ground in Britain and perhaps elsewhere. See Names for Large Numbers for more on this question.

There's always interest, especially among school children, in knowing the names of still larger numbers: quadrillions, quintillions, etc. These names are also listed in on the Names for Large Numbers page.

Using symbols in the International System (SI)

In the International System of Units (SI), the units do not have "abbreviations". They have symbols. The unit symbols do not follow the grammatical rules for abbreviations, because they follow the mathematical rules for symbols instead. These rules include the following.

Using symbols for English customary units

In the English customary systems there are no "official" symbols or abbreviations. For many English units a variety of abbreviations are used. Sometimes these abbreviations duplicate metric symbols; for example, "A" is sometimes used in English for the acre instead of the ampere. It would be better to use the symbol "ac" for the acre. In this dictionary, the first symbol listed is either the official one or the one that should be preferred in order to minimize confusion of units.

The policy of this dictionary is to define symbols for the traditional English units and to apply the SI rules to their use. This is done for both consistency and clarity. It avoids a number of outstanding problems caused by the traditional abbreviations for the English units, especially the following.

If there is anything radical in this dictionary, here it is: applying SI rules to the symbols for English units means writing mi/hr instead of MPH, mi/gal instead of MPG, lb/in2 instead of PSI, and ft3/min instead of CFM. There's a small loss of space in doing this but a great gain in clarity.

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All material in this folder is copyright 2020 by Russ Rowlett and the University of North Carolina at Chapel Hill. Permission is granted for personal use and for use by individual teachers in conducting their own classes. All other rights reserved. You are welcome to make links to this page, but please do not copy the contents of any page in this folder to another site. The material at this site will be updated from time to time.

February 14, 2000; last updated April 24, 2018.