This article is from the Fat-free Diet FAQ, by artemis@rahul.net (Michelle Dick) with numerous contributions by others.
There is no simple way to convert from a weight percentage to a
calorie percentage. The reason is that the conversion will depend
on how much water, fiber, and other non-caloric ingredients are in
the item. For instance, you could add a drop of oil to a glass of
water. By weight, it would have less than 1% fat, yet 100% of the
calories would be from fat. One can calculate a lower bound on the
%CFF, however. If a product has X% fat by weight, it must be at
least:
900X
%CFF = ------
400 + 5X
 
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