Spell numbers that begin a sentence. It would be, "Thirty cars were in the car show," not, "30 cars were in the car show." This rule trumps all other rules regarding numbers in writing. Never start a sentence with a numeral figure.
Spell out numbers smaller than 10. If your number is less than ten, always write it out instead of using a numeral figure. An example is, "My son ate five pieces of candy," not, "My son ate 5 pieces of candy."
Be consistent with numbers in a series. You should never write, "I had four dogs and 10 cats." You would write, "I had four dogs and ten cats," or "I had 4 dogs and 10 cats."
Use figures for decimals and percentages. The correct way to write using this rule is, "My grade point average was a 3.8 in school," not, "My grade point average was a three point eight in school."
Use figures for years and days, except when you are referring to a specific decade or era. "I was born in 1982," is a better way to express a date than, "I was born in nineteen eighty two." When a decade or era is being referred to, it is considered okay to spell numbers. It would be, "I was born in the eighties," not, "I was born in the 80s."
Spell out simple fractions, but use numeral figures for complex fractions. A simple fraction is any faction that is less than one. If the faction is above one and contains a whole number and a fraction, it should be a numeral. For example, "There is only one-half of a pizza left, but there are 1 1/2 pies left."