Hello! I realize the title sounds a bit clickbaity, but it's not hyperbole.

The short explanation is the nutritional facts on pet food includes its water weight, which makes the other percents like protein and fat on the packaging effectively pointless and impossible to compare from one product to the next, and they almost never include the ash and carbohydrate content. What you need is the dry matter basis, which is the percent breakdown excluding the water.

So what to do?

Well if you want a simple answer, just use https://catfooddb.com/ which has already done most of the leg work for you. Go find your preferred brand and check out the foods you're already feeding and pay attention to the pie charts to see how much protein, fat, and carbohydrates there are in the food you're currently using.

Is your food not listed? Don't fret!

If you are taking your pet food choices seriously, you can repeat her findings (often more accurately due to finding newer and more accurate data) on your own by figuring out the most current dry matter basis values for the particular food you’re looking at.

The math behind calculating dry matter basis

https://endocrinevet.blogspot.com/2014/01/how-to-calculate-carbohydrate-and.html

An online dry matter basis calculator

https://balance.it/convert

The catch is unless you know the exact ash content, which is almost never listed on the packaging sold to consumers, you have to guess, which greatly distorts the total carbohydrates. The best way I have found to get the exact ash content it to just go to chewy.com (not an endorsement. I use them strictly to get at information not disclosed to consumers) and look at the consumer questions because someone has likely already asked and use that value, or ask the question yourself and chewy will respond within a day or two.

So what do these values even mean?

At the end of the day these dry matter basis values are completely arbitrary unless you have something to compare them against, I recommend looking at data sheets put together by zoos where they have identified the dry matter basis of various prey species for use in feeding at zoos.

https://www.rodentpro.com/informationcenter/resources/nutrient-composition-of-whole-vertebrate-prey

https://www.semanticscholar.org/paper/NUTRIENT-COMPOSITION-OF-WHOLE-VERTEBRATE-PREY-FISH)-Dierenfeld-Alcorn/9119b1ba4e298635227d69da95636d920eb4b6e9

I am just a regular consumer like you, but my take on the subject is you want (in dry matter basis) a breakdown of something like 66-75% protein, 25-33% fat, and as little ash and carbs as you can get. Wet foods typically don’t have much ash while dry foods have a lot more. For the record, ash is the amount of bones burned into ash during the manufacturing process 💀. Cats are obligate carnivores so they should have zero carbohydrates.

Thanks for coming to my TED talk.