When used in written text, spell out numbers one through nine and general numbers (dozens, a thousand); use figures for 10 and above. (An exception is when numbers are used in a headline.) Example:
If a number is the first word of a sentence, always spell it out.
Percentages are expressed as figures, and spell out the word “percent.” Example:
For very large sums of money, use figures with a dollar sign; spell out million or billion. Example: