AP style is the style guide codified in The Associated Press Stylebook and Briefing on Media Law. AP style is primarily used by journalists and others in the news industry when writing and reporting news stories. However, this very specific professional writing style may be adhered to by other types of freelance and professional writers as well, to standardize their writing styles. AP rules on measurements are extensive.
Crucial to accurately using AP style on measurements is appropriate use of numbers, as most units of measurement are attached to a numeral. The basic AP style rule on numbers is that numerals less than 10 are spelled out, while 10 and greater are represented numerically. They are many exceptions to this rule, however. Numbers that begin a sentence should be spelled out. Dimensions ("a 5-by-5 square") are always represented numerically, as are dollars and cents, millions and billions ("7 billion"), time and speed measurements and percentages.
Because AP style is primarily an American style standard, use of the imperial system of measurement is favored, although AP style does make allowances for the growing popularity of the metric system. The Stylebook contains detailed listings for different imperial standards of measure, and approximate metric equivalences and methods for conversion, such as: cup = 8 fluid ounces, 240 milliliters and .24 of a liter. Other specified measurements include pounds and ounces, miles, inches and knots.
AP style indicates that metric terms should be included "when relevant," although it provides no strict rules for what defines relevance. Two guidelines for the use of metric measurements in a story: metric should be used when they are the primary form of measurement, and metric units can be provided, in parentheses, for more commonly known measurements -- like indicating measurements in kilometers per hour in a story about speed limits.
The AP Stylebook also specifies a number of miscellaneous rules for writing measurements. Hyphens are used when the measurement becomes a compound modifier -- "5-foot-3 woman." When indicating miles per gallon as a measurement, subsequent references become mpg; miles per hour should be consistently indicated as mph. Fractions should be spell and hyphenated -- "three-fourths" -- but prefers that they be converted to decimals when expressed with a whole number -- "1.75" instead of "one and three-fourths." The word percent should be spelled, and percentage units less than one should be preceded with a zero -- "0.5 percent." AP style also specifies some less common units of measurement, such as the "rad" -- a unit of measurement for absorbed radiation; "rem" -- the measurement for absorbed radiation in living tissue; and "caliber" -- a standardized measurement for a gun barrel's interior diameter.