Thanks. For me, I never gave this much thought, because I always received instructions from my vet as to how much to give my cat, and she always took into account the exact kind of food I'm using (and also recommended or prescribed the kind of food to use). My cat always had a sensitive stomach, which limited our options.Just go look at a bag of cat food. Calories are calculated in grams (weight) but for the nutritional info they're listed in cups (volume) because people use measuring scoops I presume? I don't know. I know my 1/4 cup scoop holds between 0.6 and 0.8 oz by scale depending on how the kibble sits. I know when my vet was discussing feeding amounts with me she kept saying "measuring cup" which made no sense to me b/c that's volume based and not accurate.
Here is an example, and it's an RX food so it's held to a higher standard than regular food. Purina Pro Plan Veterinary Diets UR Urinary St/Ox Feline Formula They key is "Using a standard 8 oz./ 250 ml measuring cup which contains approximately 105 g of UR Urinary St/Ox feline formula." Weight based, not volume.
For dry, my unit of measurement is cups, and for wet, my "unit" is cans (of a specific wet food). I give a mixture of both. It's probably not the most accurate way to go about it, but I do weigh my cat monthly now, as he is 10, and I want to watch out for weight changes that may indicate any age-related health issues as he gets older. He's been stable.