To count the number of words in your MS, you simply use your computer's word count facility. Right?

Uh, wrong.  

Why? 

Because computers and publishers define "word" differently.

• To a computer, the encyclopaedias = 2 words.  
• To a publisher, the encyclopaedias = 3 words.

If you're a publisher, a word = 6 spaces, including empty spaces.

Publishers came up with this definition because they want to know how much space your MS will take up when printed. 

And, yes, that's the reason behind the format requirements for RWA writing contests:

• A4 paper
• non-proportional or mono-spaced font
• 3cm margins
• 25 lines per page

30 cm margins on A4 paper will give you approximately 60 spaces per line.  

If you adjust your right and left margins so that you have exactly 60 spaces per line, then:

10 words x 25 lines = 250 words per page. This count includes empty spaces and empty lines.

Use of a non-proportional or mono-spaced font is essential.  This formula won't work with proportional fonts, because proportional fonts will not give you a uniform number of spaces per line.  

Calculating the word count for your MS is then easy.  Simply divide the number of your pages by 4.

• 200 pages divided by 4 = 50 (thousand) words
• 220 pages divided by 4 = 55 (thousand) words
• 320 pages divided by 4 = 80 (thousand) words
• 340 pages divided by 4 = 85 (thousand) words

There is a chance that you might stumble over a publisher who is happy to accept computer word count, or proportional fonts.  Check individual publisher's requirements. 

Until then, use the above method.