In general, write out numbers zero through nine. Use numerals for numbers 10 and above. Common exceptions include age, weight, monetary amounts, temperature, page numbers, scores and percentages.
Write out any number that begins a sentence.
Write out ordinal numbers (a number that indicates rank) under 10; use numerals for ordinal numbers 10 and above.
Example: Drowning is the third leading cause of death of children ages 10-16. In 1965, it ranked 15th.
Use numerals for each number when two or more numbers appear in a sentence and 1 of them is 10 or larger.
Example: Last month, 8 of the 16 geothermal sites were evaluated.
Use numerals (not words) until one million.
Examples: 3,000 (not 3 thousand) 3 million (not 3,000,000)
Use Roman numerals (I, V, X, L, C, D and M) for wars and to show personal sequence for animals and people (World War II, Pope John Paul VI, King George III).