A few simple rules and tips will help you use numbers like an expert.
Overall, write out the numbers zero through ten as words. Numbers 11 and greater can be written as numerals.
Four easy, important guidelines
(1) Never begin a sentence with a numeral. Instead, write a number in text format when it begins a sentence.
“Twelve people comprise a jury.”
(2) Be mindful of ranges of numbers.
“The company indicated that production would increase by 50 to 100 million barrels.”
This sentence means that production would increase from 50 barrels to 100 million barrels—not from 50 million barrels to 100 million barrels. When you’re writing a range of numbers, make sure the lowest number is fully written out.
(3) Use numerals, not words, whenever you’re pairing a quantity with a sign, such as the percent or dollar sign.
“The profits grew 5%.”
“You owe me $2.”
(4) Avoid ordinal numbers (1st, 2nd, 5th), even—and especially—when writing out dates.
“The board met on October 5, 2015.”
“She was the third person in line.”
A SPECIAL EXCEPTION FOR RESUMES:
I haven’t found research that supports these claims, but the advice is worth following regardless. So, even if the number is between zero and ten, I write it as a numeral on resumes.