How the calculation works
Resting energy (RER) = 70 × body weight (kg)^0.75 kcal a day; daily energy = RER × the life-stage factor; grams a day = daily energy ÷ kcal per kg × 1,000; cost a day = pack price ÷ pack weight (kg) × grams a day ÷ 1,000
A worked example
A 20 kg neutered adult dog needs about 70 × 20^0.75 × 1.6 = 1,059 kcal a day: 286 g of a 3,700 kcal/kg food. From a 15 kg bag at $59.99 that is $1.14 a day, about $34.85 a month, and the bag lasts 52 days.
Why calories, not grams
Foods differ in energy: a light dry food may have 3,200 kcal per kg, a rich one 4,200, and wet food a quarter of that. Your pet needs a number of calories, so the grams it eats, and what they cost, follow from the food's energy. A cheaper food per kg can cost more per day if your pet needs more of it.
Where the formula comes from
Veterinary nutrition estimates a pet's resting energy as 70 × body weight in kg to the power 0.75, then multiplies it by a factor for life stage and activity: about 1.6 for a neutered adult dog, 1.2 for a neutered adult cat, more for puppies, kittens and working dogs. It is a starting point; individual pets vary.
Compare foods by price per 1,000 kcal
The cost per day depends on the price per kg and the energy per kg together. Dividing the price by the energy gives the price per 1,000 kcal, which compares dry and wet food on one scale: the lower it is, the less a day of food costs.
Common questions
My bag's feeding guide says something else. Which is right?
Both are estimates. Feeding guides are written for an average pet of each weight; the formula here is the one vets start from. Weigh your pet every few weeks and adjust the amount to keep a healthy body condition.
Where do I find the kcal per kg?
On the label, usually as "Calorie content (ME, calculated)" or "Energie". If it states kcal per 100 g, multiply by 10; if it states kcal per can or cup only, use the kcal per kg converter.
Is this veterinary advice?
No. It is arithmetic on the numbers you enter. Ask your vet about your pet's diet, especially for puppies, kittens, pregnant or nursing animals, seniors and pets with a health condition.
Method: arithmetic from the label values you enter. Reviewed .