Food portion calculator
Most people feed by the scoop, and most scoops are wrong. This works out your pet's daily calorie need using the standard veterinary formula, then converts it into grams of the food you actually have in the cupboard.
Calories per 100g is the number that makes a portion accurate. It is on the packaging as metabolisable energy, and it varies more between foods than owners expect. How feeding amounts are worked out →
How this calculator works
Feeding guidance on a bag of food is written for an average animal that does not exist. It assumes a middle weight, a middle activity level and an un-neutered adult. Change any one of those and the number moves, sometimes by a third.
This tool uses the calculation vets actually use. It starts with resting energy requirement — the calories an animal burns doing nothing at all — and then multiplies it by a factor for that animal's life stage and activity.
The formula
Resting energy requirement is calculated as 70 × (body weight in kg)0.75. The exponent matters: energy need does not rise in a straight line with weight. A 40 kg dog does not need twice the calories of a 20 kg dog. It needs about 1.7 times as many.
That figure is then multiplied by a life-stage factor. A neutered adult dog sits around 1.6. An entire adult is nearer 1.8. A puppy under four months can be as high as 3.0 because growth is expensive. A cat on a weight-loss plan drops to 0.8.
Turning calories into grams
Calories alone do not help you at the bin. Every food has a different energy density, printed on the bag as kcal per 100 g or sometimes kcal/kg. Divide the daily calorie need by that density and you have the weight of food to serve.
This is where most feeding errors happen. A rich, high-fat food might carry 400 kcal per 100 g while a light formula carries 320. Serving the same scoop of both means over-feeding by a quarter without changing anything you can see.
What this cannot account for
Pregnancy, lactation, thyroid disease, diabetes, kidney disease and several medications all change energy requirements, sometimes dramatically. Prescription diets come with their own feeding protocol that overrides any general formula. If your animal is on one, follow the vet's plan rather than this number.
Treat the result as a starting point, then watch the animal. Body condition over a few weeks tells you more than any calculation. If the waist disappears, reduce; if ribs become sharp, increase.
Common questions
Why is this different from the number on the bag?
Bag guidance is built for an average animal at an average activity level. This calculation uses your pet's actual weight and life stage, so a neutered indoor cat and an active working dog of the same weight get different answers.
Should I split the daily amount into meals?
The figure is a daily total. Most adult dogs do well on two meals; puppies under six months usually need three or four. Cats often prefer several small meals. Splitting does not change the total.
Do treats count?
Yes, and they add up faster than people expect. Treats should stay under about 10% of daily calories. If you give a lot of treats, subtract them from the meal portion rather than adding them on top.
My pet is losing or gaining weight on this amount. What now?
Adjust by roughly 10% and hold for two to three weeks before judging. Weight change is slow and daily fluctuations are mostly water. If weight moves sharply in either direction without a diet change, see a vet.
Is the same formula used for cats?
The resting energy calculation is identical. The multipliers differ — cats have a narrower range and are more prone to obesity when neutered, so the neutered adult factor is lower than a dog's.