How to Calculate Carbs on a Dry Matter Basis
Looking at the Guaranteed Analysis on the label, add up the values for Protein, moisture, fat, fiber, and ash (if listed - (sometimes it is not). Then subtract from 100 - the difference is the wet matter carbs. So, if we have a food with 78% moisture, 11% protein, 5% fat, 2% fiber and 1.5% Ash, the calculation would look like this:
78.00 + 11.00 + 5.00 + 2.00 + 1.50 = 97.50 Subtract that from 100, and the remainder, 2.50, is the WET matter carbs.
However, when comparing the carbohydrate contents of any food, it must be done on a dry matter basis, even with dry food, as dry does contain some small amount of moisture. So, looking at the GA on the label once again, subtract the moisture content from 100 - in this case, the difference is 22. Then divide your wet matter carbs by this number. So...
2.50 divided by 22. = 11% carbs on a dry matter basis.
This is an easy formula, and once you've done it a few times, you will be able to look at the GA and pretty much know what the carb content is based on the values given. Remember, tho, that ingredients are just as important as the carb content - you want food with no grains (including rice or soy) no veggies, fruits, glutens, or cornstarch, and no gravy. Broth is fine, as long as it is not thickened with starches.