Write out numbers that require more than two words to express. For example, you would write out "twenty-three" or "nine" but use digits when writing "289." Don't forget the hyphens for compound numbers (from twenty-one to ninety-nine).
Use numerals (digits) in the following cases: dates and years (decades can be expressed as "the eighties or the 80s), decimals, percentages, fractions, scores, addresses, scores and complicated monetary units (thirty-three dollars would be okay, but $53.34 should be in digits). Write out numbers at the beginning of a sentence. In many of these cases, it would be cumbersome to write these numbers out, so you should rearrange the sentence so it doesn't begin with that number.
Remember that different style guides have different rules for expressing numbers. Some style guides prefer using digits for all numbers greater than nine. If your writing must conform to a specific style guide, be sure to research the rules. Numbers rules for two of the most popular style guides, Chicago and APA, are linked in the Resources Section.
Be consistent and clear. If you need to write out a number or use digits for the sake of clarity, breaking the rule will serve a greater purpose. For example, instead of writing "The forty forty-year-old men ate lunch," you may choose to write "The forty 40-year-old men ate lunch" to avoid the repetition.
Cite your references and double-check for accuracy when using numbers in an essay. Make sure the numbers you are using reflect the most up-to-date information. Certain figures, for example the population of a swiftly growing country, should only be included if they can accurately reflect the current reality. Outdated numbers may actually contradict or weaken the argument you are presenting through your essay.