Almost every feeding mistake starts in the same place: the weight you type in. Use the animal in front of you and the sum returns the calories that keep it exactly the size it is. If it is carrying an extra fifth, you have just calculated how to maintain that.
The fix is one substitution — ideal weight instead of current — and on a Labrador it is worth about a cup and three quarters of food a day.
The six steps, in order
The order matters because step one feeds every other step. Get the weight wrong and the rest is arithmetic on a wrong number.
- Settle on the ideal weight, not the current one.
This is the whole guide in one line. A body-condition score is the usual way to it: you should feel ribs under light pressure without seeing them, and there should be a waist behind the ribs when you look down. If the animal is overweight, every later step uses the target number.
- Work out the resting requirement.
Resting energy requirement is 70 × (ideal weight in kg)0.75 kilocalories a day. That is what the animal burns doing nothing. That three-quarter power is not just for show. It reflects metabolic scaling, which is why the per-kilogram requirement changes with an animal's size — a point we return to below.
- Multiply by the life-stage factor.
Maintenance energy is the resting figure times a factor for what the animal actually does. For dogs: 1.2 inactive, 1.6 neutered adult, 2.0 active, and 1.0 for weight loss. For cats: 1.0 lean, 1.2 neutered, 1.4 active, and 0.8 for weight loss. The weight-loss factors are intentionally below maintenance. This is not a default setting; you are choosing to create a calorie deficit.
- Turn calories into food, using your bag.
Divide the daily calories by the energy density printed on the packaging: cups per day is calories divided by calories per cup; grams per day is calories times 100 divided by calories per 100 g. This is the step nobody can do for you, because energy density varies enormously between foods and a cup is a measure of volume, not of food.
- Split it across meals, and weigh it once.
Divide the daily amount by the number of meals. Then weigh one portion on a kitchen scale and see what it looks like in your usual scoop, because a scoop filled by eye is the second-largest source of error after the weight in step one.
- Check water separately.
Water is not part of the calorie sum and is easy to forget. A reasonable daily estimate is 55 ml per kg for dogs and 50 ml per kg for cats. An animal on dry food drinks noticeably more than one on wet.
What the substitution is worth
These are computed rather than illustrative. Each row compares two scenarios for an animal that is 20% overweight: feeding to maintain its current size, against feeding for its target weight.
| Animal | Fed at current weight | Fed for loss at ideal | Difference | In food |
|---|---|---|---|---|
| Dog, 30 kg → 25 kg | 1,436 kcal | 783 kcal | 653 kcal/day | 1.72 cups |
| Dog, 9.6 kg → 8 kg | 611 kcal | 333 kcal | 278 kcal/day | 0.73 cups |
| Cat, 6 kg → 4.5 kg | 322 kcal | 173 kcal | 149 kcal/day | 0.5 cups |
Look at the two dogs. One is three times the other, the calorie gap is more than twice as large in absolute terms, and the proportional error is identical at 83.4% in both. This is not a coincidence. The size of the overfeed is a function of the life-stage factor and the percentage of excess weight, and not of the animal at all. A Chihuahua and a Labrador that are each a fifth too heavy are being overfed by exactly the same percentage.
Why the chart on the bag cannot be right for your animal
Resting requirement rises with weight to the power of three quarters, not in proportion to it. So calories per kilogram fall as animals get bigger:
| Ideal weight | Resting requirement | Per kilogram |
|---|---|---|
| 2 kg | 118 kcal | 58.9 kcal/kg |
| 10 kg | 394 kcal | 39.4 kcal/kg |
| 20 kg | 662 kcal | 33.1 kcal/kg |
| 40 kg | 1,113 kcal | 27.8 kcal/kg |
A 2 kg cat needs 2.1 times as many calories per kilogram as a 40 kg dog. Any rule of thumb expressed per kilogram is therefore wrong at one end or the other. A printed chart has to average across sizes, life stages and neuter status it cannot know — a generic starting point for a population, where this calculation is specific to your animal.
What this audit found in our own tools
Every guide here reads the source of the tools it links. This time the arithmetic in all our linked tools was correct. We report a clean bill of health as plainly as we would a defect.
All the tools passed the audit. The calorie calculator correctly implements the formula and shows its work. The portion tool handles the density arithmetic in all units, and the water calculator uses the right rates. Crucially, the ideal-weight tool works from a body-condition score, not a breed-based guess.
The missing piece is the sequence. Each tool does its job and gestures to the next — the calorie tool mentions ideal weight, the portion tool needs calories — but none connects the whole process from body condition to food scoop. This guide exists to fill that gap by laying out the steps in order and pointing to the right tool at the right time.
Type in the weight you want, not the weight you have. Everything downstream inherits that one number, and it is the only step where the error compounds. Read the calories off the bag you actually buy, because energy density is a property of the food and no calculator can know it. Weigh one portion once so you know what the number looks like in your own scoop. Treat a weight-loss factor as a decision — it sits below maintenance on purpose and is not a setting to leave on indefinitely. And check the sum against the animal every few weeks: body condition is the measurement that matters, and the arithmetic is only a starting estimate.
Run your own numbers
Use the tools in sequence. Start with the pet ideal-weight and body-condition calculator, step one, and the only one needing your judgement rather than a number. Next, the pet calorie calculator handles steps two and three, showing its work. The pet food portion calculator then turns those calories into portions using the density from your bag. For water, see the pet water intake calculator, step six. For puppies, the adult-weight predictor gives you the target weight for step one.
- Every figure in this guide is computed by a script committed alongside it, using constants copied verbatim from the shipped tools so the guide quotes exactly what our calculators return: the resting requirement and life-stage factors from the pet calorie calculator, the portion arithmetic from the portion calculator, and the water rates from the shared kernel.
- The resting energy requirement is the standard 70 × kg0.75 relation. The life-stage multipliers are the ones our calculator ships — dog 1.2 inactive, 1.6 neutered, 2.0 active, 1.0 weight loss; cat 1.0 lean, 1.2 neutered, 1.4 active, 0.8 weight loss — and they are what the worked table is built from.
- The calories-per-cup figures used to turn calories into a portion, 380 for the dog food and 300 for the cat food, are illustrative mid-range values chosen only to make the arithmetic concrete. They are not recommendations, and energy density is the one input you must take from your own packaging.
- ⚠️ These are energy estimates for healthy adult animals, and estimates are where a feeding plan starts rather than where it ends. Growth, pregnancy, lactation, kidney and thyroid disease and many medications all change the answer, and none of them is something arithmetic can detect.
This explains how feeding calculations are done. It is not veterinary advice and it is not a weight-loss plan. If an animal needs to lose weight, or has lost weight unexpectedly, the matter requires a vet who can perform a physical examination. Unplanned weight loss, especially, is a potential symptom, not a victory.
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