When your cat food packaging states ‘crude protein’, this does not mean the actual protein level because it includes the moisture. If you see the words ‘crude protein’ you should also see the word ‘moisture’ in the same listing.
This allows you to calculate the actual level of protein in the ‘dry matter’ of the food, which excludes the moisture content.
E.g. Given that Crude Protein is 9%, Moisture is 80%, actual dry level Protein can be calculated as follows:
Protein in dry matter = [9 / (100-80)] * 100% = 45%
This is especially useful in calculating the actual protein level in various brands of canned food, as canned food is mostly moisture. Hope this helps!