Numbers below 10 are spelled out in nonscientific text. The exceptions are percents, semester hours, grade-point averages and credit hours: 10 percent, 125 semester hours, 3.5 grade-point average, 4 credits, unless the number begins a sentence.
Treat numbers alike in a sentence if they refer to the same thing: “The class of 150 students was separated into smaller groups of 15 students each."
Always spell out numbers at the beginning of a sentence or reword the sentence to avoid spelling out a large number.
Spell out ordinal numbers in text: e.g., first, second, third—not 1st, 2nd, 3rd.
For numbers of four digits or larger, use a comma: 1,500; 35,000. Very large numbers should be expressed with a figure and word: 450 million. Exceptions are dates, temperatures, and SAT scores.
Spell out references to money under 100 and put in figures for references to 100 and more: “The ticket will cost five dollars.” In text referring to registration or conference fees, use figures and the dollar sign because these are easier to read: “Conference materials and lunch are covered in the $275 registration fee.”
Fractional amounts higher than one dollar are set in figures: “The course notes cost $15.95.” But whole dollar amounts use .00 only when in a sentence with a dollar/fraction amount: “The registration fee is $75.50 if you are staying on campus, but $55.00 if you stay off campus.”
Decimals and fractions are set in figures, although in some cases, a fraction may be spelled out: “The professor covered about two-thirds of the chapter during the class.” Note that hyphens are used when spelling out fractions.
Figures plus a.m. or p.m. (lowercase with periods) are used to designate time in both text and schedules for ease of reading. “The lecture begins at 10:30 a.m. Lunch is at noon.” If the text is a formal invitation, small caps are often used for AM and PM, with or without periods.
The plurals of numbers are formed by adding an “s”: 5s, 20s, 1980s.
Spell out references to centuries and decades: fifteenth century, the sixties. (See the Chicago Manual for more detail.)
Telephone numbers are written with parenthesis: (603) 862-1460.