Below are general rules for using numbers when writing. Each citation style has specific instructions for using numbers and you should always double-check the style manual for documents that require you to strictly or exactly follow a particular style.
Beginning of a Sentence
Numbers that are at the start of a sentence must be spelled out. The only exception are years.
Four hundred twenty-five years ago…
1995 marked the first year of the…
Numbers between twenty-one and ninety-nine are the only numbers hyphenated. Written-out fractions must also be hyphenated.
Seventy-five penguins were taken to the wildlife park.
Two-thirds of the cake were left.
Using am and pm
AM and PM can also be written as A.M. and P.M., a.m. and p.m., and am and pm. It is most important to be consistent. Generally, it is recommended to use noon and midnight instead of 12:00pm and 12:00am respectively.
I left at noon for an appointment at 12:30pm.
I woke up as the clock struck midnight, reminding me of the fortune-teller’s words.
Mixed fractions are written as numbers unless it starts the sentence.
The wild cat was 3 ½ feet long.
Three and a half cans of soup were cooked for the party.
Numbers with Decimals
Numbers with decimals are written using numerals. If the number is less than one, a zero is usually placed before the decimal place.
324.32 pounds of gold were found in the sunken ship.
The bullet missed the major artery by 0.12825 inches.