How to Style Numbers as Physical Dimensions
How to treat numbers in writing in general is a complicated issue dealt with in this DailyWritingTips post and others. The current post focuses on a subcategory of number style: numbers that refer to physical dimensions — an object’s size or the proportion thereof — or to nonphysical scientific measurement.
Occasional, casual references to dimensions are usually best treated by spelling them out (“The footbridge is fifty-four feet long”; “The temperature dropped overnight to twenty-three degrees”). However, numbers in content (generally nonfiction) that frequently details measurements, especially in a technical context, are better displayed in numeral form (“The respective mile-per-gallon performance for the three models is 67, 84, and 53”). In such a case, earlier or subsequent references to the units in question — and, ideally, all measurements — should be styled consistently, even if they otherwise appear in isolation.
Simple fractions (those describing less than a whole, such as one-third) and short mixed fractions (“one and three-eights,” for example) are easily read in word form, but a concentration of fractions is best styled with numerals (“The table is 34 1/2 inches high, 24 inches wide, and 42 1/4 inches long”); again, the form should be consistent throughout a particular piece of content and preferably in a recurring print or online publication.
Abbreviations and symbols for units of measure are always accompanied by numerals and never appear in association with spelled-out numbers; the shorthand is often but not always separated from the numeral by a letter space (consult a style guide about the distinctions).
Also, when unit terms are spelled out with numerals, a dimension used as a phrasal adjective is usually hyphenated before the noun but never after (“a 24-inch waist”; “her waist is 24 inches”), but hyphens are omitted when abbreviations or symbols appear (“a 10 km race”; “a 120 V system”). Note, too, that terms of units of measurement should be abbreviated only when associated with a numeral (“The lightbulbs differed in actual wattage,” not “The lightbulbs differed in W.”)
Number ranges can be indicated by the word to or an en dash (here, as on many Web sites, represented by a hyphen). To is suitable for numerals and spelled-out numbers alike (“The temperature range is 45 to 60 degrees” or “The temperature range is forty-five to sixty degrees”), but the en dash is appropriate only with numerals (“The temperature range is 45-60 degrees”).
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