When it comes to numbers, you may know that our standard style book, the Chicago Manual of Style, calls for spelling out whole numbers from one through one hundred. The Associated Press Stylebook calls for spelling out whole numbers only up to nine. Here are some more rules about numbers in your text.
Use figures for numbers accompanied by abbreviations.
Abbreviations used with numbers usually are for units of measurement: lb., in., mm, mph, hrs., rpm, and so on. Writing that contains such abbreviations is very likely to make heavy use of numbers anyway, and thus to require some special style rules that you might establish; perhaps the rule can be a simple one for your text, such as to use figures for all units of measurement, or perhaps it has to be more complicated.
Occasionally an abbreviation is spoken--that is, used in dialogue. However, usually it is better to avoid figures in dialogue [see next rule]. Therefore, when an abbreviation is spoken, this rule is not followed: "It begins to knock at about four thousand rpm," he said; "Give her ten cc's now and ten more in an hour," the doctor said.
Spell out numbers in dialogue unless they are excessively awkward.
"You owe me one hundred and fifty-five dollars," he said is preferable to "You owe me $155," he said. Numbers, and also the dollar sign and percent sign, somehow do not look right in dialogue, although we do accept them in quotations in newspaper accounts; newspapers do not follow this rule.
"The materials were $122.36, the labor comes to $88.50 plus $43 for overtime, and payoffs were $1,250 to the city and $10 for the doorman, giving us a grand total of $1,413.86," he said would be very tedious if all the sums were spelled out. They could be spelled out if the writer wants to stretch them out for effect, but they are easier to absorb as figures, and most readers would be less put off by the figures than they would be by one hundred and twenty-two dollars and thirty-six cents. This rule permits figures when spelled-out numbers are unacceptably awkward.
The only general exception is years--they are always in figures in dialogue unless the writer wants them said in an unusual way: "It was in nineteen-ought six," he said, "and long before anyone foresaw the ruckus that came in nineteen and fourteen." Depending on the requirements of what they are writing, writers can decide to make any other exceptions they choose. "We're expecting the probe to be closest to Titan at exactly 2236 hours" and "Don't use this stimulant if the temperature is below 96.5 or above 101.5" could both spell out the numbers without excessive awkwardness, but if military time occurs constantly in the context of the first example and body temperature occurs constantly in that of the second, a writer may justifiably decide to use figures.