Here's the copyfitting process I've read about on this website:
step 1. Get out a pencil and scrap paper to write out the upcoming measurements and calculations--your head will thank you later.
step 2. Get a character count. In other words, count each letter, number, symbol, space and punctuation mark of the copy.
step 3. Look up the Character per pica (CPP) measurement, which is based on the typeface being used and the alphabet length.
step 4. Multiply the CPP by the line length. This will tell you how many characters will fit in one line of copy.
step 5. Divide the character count found at the beginning by the number of characters that fit in one line (the number found in the last step). This will tell you the total number of lines the given copy will take up.
step 6. Multiply that number by the leading to get the copy depth.
step 7. If the project is mostly text, divide the total number of lines (found in step 5) by the number of lines per page desired to get the number of pages the copy will take up. However, if there are headers, subheaders, or illustrations, this number will have to be changed slightly.
Now aren't you glad you got out that scrap paper?