When writing a business or formal letter, the way you write numbers can influence how the recipient sees the readability and professionalism of your writing. Drawing on common writing style principles and accepted standards for particular topics, you can appropriately write out numerals and other quantities in a way that will solidify the quality of your writing. The main trick is to be consistent throughout.
Write addresses the same as you would for an envelope, with the street address on one line and the city, state and ZIP code on the next line. Example: 123 Elm Street on one line and Eagletown, LA 71234 on the next.
Write contact phone numbers as either (999) 123-4567 or 999-123-4567.
Write out the month in dates, such as January 1, 2012.
Stick to words for zero through one hundred and numerals for 101 and above. You could say your organization has eighty-five members or 185 members.
Use numerals for consistency, if your letter includes numbers above and below 100. This avoids awkward phrasing, such as "between ninety-five and 105."
Spell out large, round numbers with only a numeral at the beginning, as in 8 million. Use numerals for figures that would require many words, opting for 775,000 rather than seven hundred seventy-five thousand.
Use numerals for all percentages, as in 87 percent or 87.75 percent. You can use either the word percent or the sign % as long as you remain uniform. Do not include trailing zeroes (such as 55.0 percent) unless directing your letter to a math or science audience warranting significant digits. Even tens numbers can be in words, as in twenty percent, though keeping consistent with other percentages would trump this.
Use numerals for decimal figures such as 3.14, and include a zero at the beginning of a number less than one, such as 0.375. Again, omit trailing zeroes unless they are central to the accuracy of the data that you're discussing.
Write out fractions in words, such as two-fifths, only if the number is less than one. Use numerals for greater mixed numbers, such as 3 3/4.
Use the dollar sign and numerals for money amounts below 1 million, omitting the decimal and zeroes for whole dollars, such as $15 or $20,000.
Write round figures of 1 million or more with only the beginning numerals, as in $10 million. You can use a maximum of two decimals for a number like $12.75 million, but otherwise use only numerals, such as $12,755,500.