Here are the general rules†:
1) spell out the numbers one through hundred
2) spell out round numbers with hundreds, thousands, hundred thousands, millions
So you would use:
seventy-eight; 3,589; 762; three million
However, percentages always take numerals:
25% or 25 percent
not: twenty-five percent or twenty-five %
(Note that there is no space in 25%.)
In fact, abbreviations always take numerals:
47¢ (and not .47¢—unless of course you really mean less than half a cent!)
On the other hand, numbers with centuries are always spelled out:
the twenty-first century
not: the 21st century
Finally, there’s a third general rule that’s often flouted:
3) don’t begin a sentence with a numeral
Thus, you will want to avoid something like this:
97 Tips for Canadian Real Estate Investors is a national bestseller.
You could change this to:
Ninety-seven Tips for Canadian Real Estate Investors is a national bestseller.
Another solution (which avoids tampering with the actual title of the book) is to recast the sentence so it doesn’t start with the number:
Don Campbell’s 97 Tips for Canadian Real Estate Investors is a national bestseller.
Following these three simple rules will help you steer clear of 90 percent of the potholes with numbers!
*I am using the word numeral in the restricted sense that the Chicago Manual of Style uses it as non-spelled out forms. Other references (like the Canadian Oxford Dictionary) use the word numeral to refer to both the spelled out and non-spelled out forms.
†taken from the Chicago Manual of Style chapter 9
(Image at top courtesy of scottchan.)