When you write a number that will appear in print or online, do you use figures, or spell it out? If you want to follow an authoritative source to produce professional-looking content, the answer is both more complicated and simpler than you think (we already covered part of this topic with the article 10 Rules for Writing Numbers and Numerals).
The bible of the mainstream book-publishing industry, The Chicago Manual of Style, devotes 18 pages to the topic, while The AP Stylebook, the authority of record for newspapers, is appropriately more concise. (Various magazines generally use one style or the other, but Web sites tend toward AP style.) Other style manuals abound, but unless you’re writing for scholarly journals, you can count, so to speak, on Chicago or AP.
The more formal the writing, the more likely you’ll follow Chicago style, which originally evolved from guidelines developed for the University of Chicago Press but has since been adopted by most book publishers as the authority for grammar, usage, punctuation, and, yes, numbers.
The basic Chicago rule is to spell out numbers from one to one hundred but use numerals for 101 and up. But if you refer to two amounts in the same category, default to numerals. (“I found 137 mistakes, compared with only 89 last time.”) Major exceptions to the basic rule include a number as the first word in a sentence, larger round numbers (“five hundred”) and orders of magnitude (“millions,” “billions,” etc.). The point is to maintain consistency as much as possible.
Technical, statistically dense text, meanwhile, is better served by numerals, so in that case use digits for physical dimensions, degrees (both of temperature and angle), scores and percentages, money, time, and other references to quantity.
Newspaper style and less formal writing (and much of online content) hews closer to AP style, which derived from the rules for Associated Press newspaper articles: Spell out numbers only to ten, use numbers for 11 and up, and don’t be concerned about matching style when you refer to quantities on both sides of the tipping point. (“In a classroom poll conducted recently, only seven of 29 students agreed with that statement.”)
And what about those pesky hyphens? Don’t hyphenate a physical dimension to the unit name unless those two terms modify a noun (“10 feet,” but “10-foot pole”). Hyphenate double-digit numbers by themselves — and within larger numbers — if they aren’t multiples of ten (“sixty-four,” “one hundred twenty-eight”), but don’t hyphenate all the elements of a large number like a chain.
Simple and mixed fractions should be styled, depending on your preferred policy, either like “1/2” and “1 3/4,” or like “one-half” or “one and three-fourths.” Don’t bother setting case fractions (in which the numbers are reduced in size and placed on either side of a diagonal line); if you’re writing for a publication, the fraction will be formatted during the production stage according to its style.
To establish a style for your Web site or blog, keep in mind that gurus of online content advise using numerals, which are easier to scan (and most site visitors scan before they read, if they read at all), but note that the AP rule about using numerals for numbers you can count on your fingers still applies: “1 day, I’ll see with my own 2 eyes that you can beat 3 people in a row in 4-square” is going a bit far.