Trying to work out how much carbohydrate is in a dog food is achieved by using the analytical constituents (the percentages of protein, fibre, ash, and fat) which are written on the dog food packaging. Moisture may be declared on the packaging and if not then a default value of 8% is used in the allaboutdogfood website.
To work out the amount of carbohydrate in the food you start with 100%. Subtract the protein percentage, subtract the fat percentage, subtract the ash percentage, subtract the moisture percentage. The remainder is the carbohydrate percentage in the food.
So as an example a dog food with 20% protein, 8% fat, 7.5% ash, 3.5% fibre has 56.5% carbohydrate of which the fibre (the indigestible part of carbs) is 3.5%. Here is how that is worked out:
Using default (of 8% moisture) equates the carbohydrate level as follows: 100% minus 20% protein minus 8% fat minus 7.5% ash minus 8% moisture =56.5% carbohydrate of which the fibre (the indigestible part of carbs) is 3.5%.