The formula
RER = 70 × weight^0.75 · daily need = RER × factor
Where it comes from
Resting energy requirement is not proportional to weight but to weight to the power three quarters, because metabolism scales with body surface rather than mass. That means a forty kilo dog does not eat twice what a twenty kilo one does: it eats sixty-eight per cent more. On top of that resting requirement goes a factor for the animal situation, from one for a neutered cat up to three for a puppy in full growth. Converting to grams then depends on the food, and there the differences between brands are enormous.
How to work it out by hand
- Work out the resting requirement: 70 times the weight to the power 0.75
- Multiply it by the factor for its situation
- Divide by the calories per hundred grams of the food and multiply by a hundred
- Split the result between the meals of the day
What is worth knowing
Calories per hundred grams are printed on the bag and vary a great deal: from 320 in a light food to 420 in a high-energy one, which changes the portion by nearly a third for the same animal. That is why switching brands while keeping the grams the same is one of the commonest reasons a dog puts on weight with nobody understanding why. Treats count too: they should not exceed ten per cent of daily calories, and that ten per cent has to come off the food.