Spell out in nontechnical contexts whole numbers from one through 100, round numbers and numbers beginning a sentence. For example, "Last year I ate one hundred hot dogs and drank 1,537 glasses of water;" "Thirty-three thousand dogs and 107 poets live in my neighborhood."
Spell out in nontechnical contexts physical quantities represented by whole numbers from one through 100, round numbers and numbers beginning a sentence. For example, "Our taxi driver raced down Seventh Avenue at forty-nine miles per hour;" "The rain-swollen stream was three hundred feet wide;" "The Marianas Trench is 11,033 meters deep;" "The temperature last night fell below 107 degrees;" "Two feet seven is tall for a mouse."
Use numerals when usage and tradition suggest this use. For example, "a 90-watt bulb;" "a size 7 dress;" "a recommended inflation of 40 psi;" "a no. 7 filbert brush."
Spell out simple fractions. For example, "Four-fifths of the day I spend wishing I were doing something else;" "With two-thirds of his match left to play, the chess champion gave up and went home."
Spell out whole numbers and simple fractions if they are short, but use numerals if this will make your expression clearer. For example, "The race lasted one and one-third miles," but "The tallest beanstalk in the world is 8 feet 7 1/2 inches high."
Use numerals followed by an abbreviated unit in technical contexts. For example, "Dry forest precipitation averaged 3.2 cm during March;" "Average lizard gross body weight was 5.5 g;" "The 21 ha submontane plot was designated study area C."
Spell out any unit of measurement without a numeral. For example, "We used Newtons and not foot-pounds to measure the force of the rhino's impact with the brick wall."
Close up a unit to its number if the unit is repeated. For example, "31%-41%," but "2 x 5 mm."
Use a numeral and the word "percent" in nontechnical contexts. For example, "Less than 67 percent of all sunsets are worth painting;" "By afternoon, the job was 70-75 percent finished."
Use a numeral and a percent sign without a space in technical contexts. For example, "At least 23% of the difference can be attributed to evaporative loss."