Never start a sentence with a numeral. Spell out the number:
One hundred and thirty-five people attended the conference.
In General
Spell out numbers one to nine in text.
Use numerals for 10 and higher.
Thousands take a comma: 2,450 not 2450.
Large, round, even numbers used as approximations are spelled out:
The history spans some four thousand years of Western civilization.
Very large numbers (million and higher) may be expressed with a combination of numerals and spelled-out numbers.
2 million, 10 million
These same rules apply to ordinals:
seventh place, sixth position, 135th award
Clusters of Numbers
Numbers within a sentence or paragraph that cluster together and are used in the same context should maintain consistency. If one of the numbers would normally be written as numeral, use numerals for all in that same category. It is fine to have one category written with numerals and another with numbers spelled out:
There are 14 graduates, 25 alumni, 3 first-year students, and 1 senior in the program.
Middlebury faculty published 20 books in 1998; 5 were on the bestseller list, and they will be publishing 7 books next month.
When mother came, we found seven dead pigeons outside. That didn’t deter us from enjoying our snack. She served 5 kinds of cookies, 11 new beers, and 7 types of goat cheese.
Decimals
Use numerals, even in text:
We are expecting to harvest 5.4 tons of corn
Fractions
Simple fractions: Spell out in text. Hyphenate the fraction if it represents a single quantity.
five-sixths of the population
He received two-thirds majority
But: I’m dividing my estate into five fifths to distribute to my heirs.
Whole numbers plus fractions: These can be spelled or expressed in numerals, whichever is most readable. (Do not link whole numbers to the fraction with a hyphen)
Three and three-fourths cups of flour should be enough to make pizza.
The recipe calls for 3 ¾ cups of flour.
Measurements
Hyphenate measurements that serve as adjectives preceding a noun:
The bandage is a two-inch-long strip of gauze.
Place this two-foot block of wood in the fireplace.
Connect measurements with hyphens when the numbers represent a range, and they function as an adjective preceding a noun:
We knew that a tsunami might create 80-to-90-foot tidal waves.
Money
References to money may be written as numerals or spelled out. If spelled, also spell the unit of currency, except when using very large numbers.
fifty cents; six dollars
$.50; $125
$1 million; $10.3 billion
Percentages
Always express percentages as a numeral-word combination, except in charts and scientific copy, where % is used:
25 percent, 4 percent
Reunion
Years
Use numerals for specific reunions: 20th reunion, 10th reunion
Reunion events may be expressed this way: Reunion 2008, Reunion Weekend
Temperature
45 degrees F (no period after the F within a sentence)
45°F (no spaces)