How the number is worked out
Two steps, and every veterinary nutrition service uses the same ones.
Resting needs come first: RER = 70 × (weight in kg)0.75. The three-quarter power is not decoration — energy use does not scale straight with body weight, which is why a Great Dane does not eat ten times what a beagle eats.
Then a multiplier for what the animal actually is. These come from the Pet Nutrition Alliance:
| Dog | Cat | |
|---|---|---|
| Neutered adult | 1.6 × RER | 1.2 × RER |
| Intact adult | 1.8 × RER | 1.4 × RER |
| Inactive or gains easily | 1.2–1.4 × RER | 1.0 × RER |
| Losing weight | 1.0 × RER | 0.8–1.0 × RER |
| Growing | 2–3 × RER | 2–3 × RER |
Notice how far neutering moves it. A neutered dog needs roughly 11% less than an intact one at the same weight, and almost nobody changes the food after the operation. That single gap is behind a lot of the weight gain that shows up in the year afterwards.
Why the bag overfeeds
The feeding guide on a bag is written for one whole weight band — often "30 to 50 lb" as a single row — and it assumes an active, unneutered animal, because that is the safe direction for a manufacturer to err in. It is not lying to you. It is just answering a different question from the one you are asking.
The number you want is the kcal per cup or kcal per kg on the same bag. That one is a fact about the food rather than a guess about your animal.
What this cannot tell you
Individual requirements at the same body weight vary by a wide margin — two identical-looking neutered labradors can differ by half again. The equation gives you a starting point that is much better than the bag, and the animal gives you the correction. Feed it for a month, weigh them, score their body condition, then adjust.
One hard rule for cats: never crash-diet one. Rapid weight loss in a cat causes hepatic lipidosis, a liver failure that can kill. Slow is not a preference here.