This article is from the misc.writing Writing FAQ, by Wendy Chatley Green email@example.com with numerous contributions by others.
Start at the beginning. Point at the first word and
say "One." Point at the second word and say "Two." Repeat,
increasing the count by one integer for each word at which
you point. <g>
Now, some more professional answers:
1. You could use the "Word Count" feature of your
word processor. Note that all word processors do not use
the same algorithm to compute this--Word may give a
different figure than WordPerfect.
2. You can multiply the number of pages in the
manuscript by 250. This gives a very rough estimate.
3. Figure that 1.5 typewritten/computer-printed
pages equal one page of a book (another rough estimate)
4. Count the words on five random pages of the
manuscript. Find the average number of words per page
(divide the count by five) then multiply this number by the
number of pages in the manuscript.
You will be paid by the publisher's word-count, not
yours; the publisher's algorithm may differ. (And padding
word-count is like double-parking in front of Police
Headquarters; you *will* get caught.)