Roughly, it's:

Subtract (fat grams * 9) from Total Calories
then
Subtract (protein grams * 4) from the number you get above
then
Divide the number you get out of that by 4 and that should approximately equal total effective carb count...

That's how the carb calculator link mentioned earlier in this thread and the even better one right here on this site (see over to your right, a quick link named Carbohydrate Counter?) work. This website's Carbohydrate Counter uses much more exact numbers (or factors) than 9/4/4 ...