So you have done the responsible thing: you sat down with the feeding table on your cat’s food packet, then you ran a calorie calculation for good measure — and the two numbers disagreed with each other. Perhaps the packet says 50 grams a day while the maths says 70. It is tempting to assume you have misread one of them, but you almost certainly have not. Understanding why cat food feeding guidelines differ from calorie calculations is one of those small lightbulb moments that makes feeding your cat properly so much easier, because the two figures were never trying to answer the same question in the first place.

The packet table is a public safety net, drawn up for the whole population of cats. Your calorie calculation is a personal plan for one individual cat. They are built from different assumptions, rounded in different ways, and aimed at different levels of precision. Once you see how each one is made, the gap between them stops looking like an error and starts looking like useful information.

Quick answer: Cat food feeding guidelines differ from calorie calculations because the packet table is written for an “average” adult cat and rounded into broad weight bands, while a calorie calculation uses your cat’s actual weight, neuter status and activity level through the RER/MER formulas. The guideline is a safe starting point for the population; the calculation is a personalised target for your individual cat.

How the feeding table on the packet is actually built

Pet food companies have to put one feeding table on a bag or pouch that works — roughly — for millions of cats they will never meet. That forces some heavy compromises.

First, the table cannot know your cat’s weight precisely, so it sorts cats into wide bands: 2–4 kg, 4–6 kg, 6–8 kg and so on. A 4.1 kg cat and a 5.9 kg cat land in the same box, even though one weighs almost half as much again as the other. Second, it cannot know whether your cat is a sleepy indoor senior or a tree-climbing young hunter, so it assumes a sort of middle-of-the-road adult. Third, the numbers get rounded to friendly figures — 45 g, 60 g, ¾ cup — because “47.3 grams” on a packet would look absurd.

The table also has to protect the manufacturer. If the guideline slightly overfeeds the average cat, owners complain about weight gain and blame the brand. So many tables are pitched a touch conservatively, toward the lower middle of the plausible range. None of this makes the table wrong; it makes it a blunt instrument, deliberately designed for safety across a whole population rather than accuracy for one cat.

How calorie calculations work instead

A calorie calculation throws away the idea of an average cat and starts with yours. It works in two steps.

Step one: Resting Energy Requirement (RER). This is roughly the calories your cat needs at rest in a thermoneutral environment, and it scales with body weight according to the formula RER = 70 × (body weight in kg)^0.75. The 0.75 power looks fiddly, but it simply reflects that energy needs rise more slowly than body weight — a cat twice as heavy does not need twice the calories.

Step two: Daily Energy Requirement, usually called MER. You multiply RER by a factor for your cat’s life stage and lifestyle. As a rough guide: a neutered adult indoor cat uses 1.2–1.4; an active adult or outdoor cat uses 1.6–1.8; an intact adult around 1.4–1.6; a growing kitten 2.5 or more relative to its current weight. Neutering genuinely lowers energy needs — typically by about a fifth — which is why the packet’s one-size table struggles with neutered indoor cats, the most common kind of pet cat in Britain.

The two numbers side by side: a worked example

Let us run both methods for one real cat: a 4.5 kg neutered adult who lives indoors and plays in short bursts.

The maths first. RER = 70 × (4.5)^0.75. Since 4.5^0.75 works out to about 3.09, RER ≈ 70 × 3.09 ≈ 216 kcal per day. For a neutered indoor adult, multiply by 1.2–1.4: 216 × 1.2 ≈ 259 kcal at the low end, 216 × 1.4 ≈ 302 kcal at the high end. So the calculation suggests roughly 260–300 kcal a day.

Now the packet. Take a typical dry food at about 390 kcal per 100 g. The feeding band for 4–5 kg cats on such a food commonly says something like 45–60 g per day. That converts to 175–234 kcal — comfortably below what the calculation says, even though we used the top of the band. On this food, the gap between “packet says” and “maths says” is around 25 g of kibble a day, which is a substantial part of a meal.

Does the packet underfeed, then? Not exactly. The band was drawn for a generic 4–5 kg adult, and its figures are averaged, rounded, and pitched a little cautiously. Our worked cat is neutered and indoor — but the maths we used already accounted for that, which is precisely the point. The table simply cannot know those details, so it guesses at the middle.

Which one should you actually trust?

Neither, on its own — and both, together. Think of the packet table as a safe harbour and the calorie calculation as a compass.

Start with the packet’s guideline when you first open a new bag or pouch. It is tested against the food’s own energy content, it keeps you out of trouble, and it is right there on the label. Then refine with the calculation: work out your cat’s personal daily target from their real weight and lifestyle, convert that into grams of the food you actually buy, and adjust from the packet’s figure toward the calculated one.

Finally — and this is the part neither the packet nor the formula can do for you — watch the cat. The ultimate arbiter is not a number at all but your cat’s body condition: can you feel (but not prominently see) the ribs, is there a visible waist from above and the side, is the weight stable on fortnightly weigh-ins? If you want to skip the arithmetic, our cat calorie calculator runs the RER/MER maths for you once you enter your cat’s weight and lifestyle, and gives you a personal daily target to feed against.

How to reconcile the two safely in practice

Here is a simple routine that uses both tools the way they were meant to be used:

  • Weigh your cat properly. Bathroom scales are fine: weigh yourself, then weigh yourself holding the cat, and subtract. Do this every two weeks, not daily — body weight naturally wobbles a few grams.
  • Use ideal weight, not current weight, in the calculation. If your vet has said your cat should be 4 kg rather than 4.8 kg, run the maths on 4 kg, or the formula will defend the extra weight.
  • Weigh the food, don’t scoop by eye. A “cup” of kibble can vary by 20% depending on the scoop and the kibble size. A cheap kitchen scale removes that error entirely.
  • Start at the packet guideline, drift toward the calculation. Change portions by about 10% at a time and give each change two to three weeks before judging it — weight shifts slowly in cats.
  • Adjust by body condition, not just the scales. A muscular 5 kg cat and a flabby 5 kg cat are different feeding projects. Learn the 1–9 body condition score and aim for 4–5.

One safety note: never crash-diet an overweight cat. Cats that lose weight too fast can develop hepatic lipidosis (fatty liver), a serious condition. Any reduction plan for an obese cat should go through your vet.

Common mistakes

  • Treating the guideline as an exact prescription. A range printed for a two-kilo-wide weight band cannot be exact for your cat. Use it as a starting line, not a finish line.
  • Running the maths on the wrong weight. For overweight cats, always calculate from the target weight, not the current one — otherwise you are budgeting calories to maintain the very weight you want to lose.
  • Forgetting that neutered means less. Neutering drops energy needs noticeably. If you kept feeding the same amount after the operation, that alone explains a slow drift upward.
  • Changing foods without recalculating. One dry food might be 350 kcal per 100 g and the next 430. The same scoop of each is a different dinner. Every new bag means new maths.
  • Adjusting every single day. Appetite varies day to day. Judge portions by fortnightly weight trends and body condition, not by Tuesday’s leftovers.
  • Ignoring treats and extras. Treats, dental chews and the odd bit of ham all count toward the daily total. The guideline assumed the packet was the whole diet.

Frequently asked questions

Is the packet feeding table ever just wrong?
Rarely, for the average cat it describes. It can be meaningfully wrong for cats at the edges — very small, very large, elderly, or unusually active — because the “average” it was written for looks nothing like them.

Should I just split the difference between the two figures?
Averaging is a reasonable emergency starting point, but a better approach is to feed the calculated figure and watch body condition for three weeks. The cat’s actual shape and weight trend will tell you which number was closer.

My cat is losing weight on the packet guideline. What should I do?
Move toward the higher calculated figure in 10% steps, and if weight loss continues or your cat seems unwell, see your vet — unexplained weight loss is always worth investigating rather than simply feeding more.

Do I need to recalculate after neutering or a big weight change?
Yes. Recalculate after neutering, after any weight change of more than about 10%, when switching foods, and at life-stage changes (kitten to adult, adult to senior). A two-minute recalculation beats months of drift.