Dictionary.com Word FAQs
What is the rule for determining whether or not to write out a number as a word?
In general, write out the first nine cardinal (1-9) numbers; use figures for 10 and above. Write out the first nine cardinal (1-9) numbers (except for address numbers 2-9, dates, decimals, game scores, highways, latitude/longitude, mathematical expressions, measurement/weight, money/financial data, percentages, proportion, scientific expressions, statistics, technical expressions, temperature, time, unit modifiers, votes, and numbers not written out in a proper noun) and any number that begins a sentence; use figures for 10 and above. The first nine ordinal (1st-9th) numbers are usually written out, especially when describing order in time or location.