What a feeding portion actually is
A cat's portion is a calorie number first and a volume second. Feeding guides printed on the bag are ranges built for the average cat of a given weight, and they routinely overshoot for the animal most likely to be reading them: a neutered, indoor, middle-aged cat that sleeps eighteen hours a day. The way to get an honest portion is to calculate the energy your cat needs, then divide by the calorie density of the food actually in your cupboard.
Everything starts with the resting energy requirement, or RER — the energy a cat burns lying still, awake, in a room where it is neither heating nor cooling itself. It is a function of body weight alone. Every other number on this page is RER multiplied by something: an activity factor to reach maintenance, or a reduction factor to produce weight loss.
The second half of the problem is unit conversion. A can and a cup are containers, not doses. A 3 oz pâté can might hold 70 kcal or 100 kcal; a cup of kibble might be 320 kcal or 500 kcal depending on how much fat is sprayed on it. Both numbers are printed on the label in the calorie content statement, which is exactly why this calculator asks for them rather than assuming.
The formula, term by term
The equation used across small-animal practice is:
RER = 70 × (body weight in kg)0.75
The exponent is the interesting part. Metabolic rate does not scale in proportion to body mass; it scales to roughly the three-quarter power, a relationship first described for mammals by Max Kleiber in the 1930s and adopted by the National Research Council in its 2006 Nutrient Requirements of Dogs and Cats. In practical terms, a 6 kg cat is 50% heavier than a 4 kg cat but needs only about 36% more resting energy: 60.75 ÷ 40.75 = 1.36. This is why doubling a portion because a cat looks twice the size of another cat overfeeds it every time.
The coefficient 70 converts the metabolic mass into kilocalories per day. There is an older linear approximation, RER = 30 × kg + 70, that is still taught for cats between roughly 2 and 45 kg; for a 5 kg cat it gives 220 kcal against 234 from the exponential form. This calculator uses the exponential form throughout, because it is the version that stays correct at kitten weights.
The factor k converts resting energy into maintenance energy. These are the standard feline multipliers used in small-animal practice and reproduced in the WSAVA Global Nutrition Guidelines toolkit: 1.0 for an obesity-prone or strictly inactive cat, 1.2 for a neutered indoor adult, 1.4 for an intact or genuinely active adult, 1.6 for gestation, 2.0 for a kitten of four to twelve months and 2.5 for a kitten under four months. Neutering lowers maintenance energy needs measurably, which is why the indoor neutered adult sits at the bottom of the adult range.
Weight loss works differently. The reducing ration is not maintenance minus a bit; it is calculated from the weight you want the cat to reach. This calculator uses 80% of the RER of the ideal weight, which is the conventional feline starting point. It deliberately ignores the activity factor, because applying an activity multiplier to a cat you are trying to slim down defeats the purpose.
Worked example: a 12 lb neutered indoor cat that should weigh 10 lb
Set the current weight to 12 lb, the ideal weight to 10 lb, the goal to weight loss, a 50/50 wet-to-dry split, 80 kcal per can, 400 kcal per cup, 110 g per cup, and two meals a day.
- Convert the ideal weight. 10 lb × 0.45359 = 4.536 kg.
- Raise it to the 0.75 power. 4.5360.75 = 3.108.
- Multiply by 70. 3.108 × 70 = 217.6 kcal — the resting requirement at the target weight.
- Take 80%. 217.6 × 0.8 = 174 kcal per day. That is the whole ration.
- Split it. Half is 87.0 kcal of wet and 87.0 kcal of dry.
- Convert to cans. 87.0 ÷ 80 = 1.09 cans a day — call it one can plus a spoonful.
- Convert to dry. 87.0 ÷ 400 = 0.218 cups, and 0.218 × 110 = 24 g a day on a kitchen scale.
- Per meal. 174 ÷ 2 = 87 kcal, so roughly half a can plus 12 g of kibble at each sitting.
- Timeline. Losing 1% of body weight a week from 5.443 kg to 4.536 kg takes ln(4.536 ÷ 5.443) ÷ ln(0.99) = (−0.1823) ÷ (−0.01005) = 18.1 weeks, a little over four months.
- Cost. 1.09 cans at $1.20 is $1.31, and 24 g is 0.053 lb at $3.00 a pound, so $0.16 — $1.47 a day. Across an average month of 30.44 days that is about $45.
Notice how small 24 g of kibble looks. That is the number owners find hardest to accept, and it is the reason a scale beats a scoop for any cat on a reducing plan.
How to read the result and adjust it
Treat the calorie target as a starting point and the scale as the referee. Individual maintenance requirements vary around the calculated value by a wide margin — the factor table is a population average, and your cat is one animal. Recheck the weight every two to four weeks and adjust the ration by roughly 10% at a time until the trend is right; that step size is the one used in practice because it is large enough to move the scale and small enough not to trigger a food refusal.
For a cat on a reducing plan, the safe range is 0.5% to 2% of body weight per week, and this calculator projects the midpoint of 1%. Below 0.5% a week the plan is not really working; above 2% you are into the territory where feline hepatic lipidosis becomes a real risk, particularly if the cat stops eating altogether. Cats mobilise fat to the liver faster than they can process it, and a fat cat that goes off food for a few days can become critically ill. This is the single most important difference between slimming a cat and slimming a dog.
Body condition score matters more than the number on the scale. On the 9-point scale used in practice, 5 is ideal, each point above 5 is roughly 10% over ideal weight, and 9 is severely obese. If your cat scores 7, its ideal weight is about 20% below today's weight — that is a defensible way to fill in the ideal weight field when nobody has given you a target. The kg to pounds calculator converts between the units your vet records and the ones your bathroom scale shows.
Two practical checks on the output. If the cans-per-day figure is not close to a whole or half can, buy a smaller can size or shift the split rather than trying to portion a third of a can twice a day. And if the grams-per-day figure is under about 20 g, the dry portion has become too small to measure with a scoop — either weigh it or drop dry food from the plan.
Daily calories by body weight
| Weight (lb) | Weight (kg) | RER (kcal/day) | Maintenance, ×1.2 | Reducing ration, 0.8 × RER |
|---|---|---|---|---|
| 6 | 2.72 | 148 | 178 | 119 |
| 8 | 3.63 | 184 | 221 | 147 |
| 10 | 4.54 | 218 | 261 | 174 |
| 12 | 5.44 | 249 | 299 | 200 |
| 14 | 6.35 | 280 | 336 | 224 |
| 16 | 7.26 | 310 | 371 | 248 |
| 18 | 8.16 | 338 | 406 | 270 |
| 20 | 9.07 | 366 | 439 | 293 |
Read the reducing column against the ideal weight, never the current one. A 16 lb cat that should be 12 lb gets the 12 lb row's reducing figure of 200 kcal, not the 16 lb row's 248.
Mistakes that wreck a feeding plan
- Sizing a reducing ration from the current weight. Feeding 80% of the RER of an obese weight is barely a restriction at all. The ideal weight is the input that matters.
- Using the bag's feeding guide as a dose. Those ranges assume an intact, moderately active cat. For a neutered indoor cat they commonly land above maintenance.
- Forgetting treats and the other household's bowl. Treats, dental chews, milk and the food a neighbour leaves out all count against the same daily total. Ten kcal of treats is 6% of a reducing ration.
- Measuring kibble with a mug or a random scoop. A cup is a defined 8 fluid ounce volume, not a weight. A level cup of a dense small-piece kibble can weigh 125 g while a light, airy one weighs 95 g — the same scoop, a third more food. Weigh one cup of your bag on a kitchen scale once and enter that figure above.
- Changing food and portion on the same day. Change one variable at a time, over seven to ten days, or you will not know which change caused a refusal or a loose stool.
- Applying this to a cat with a diagnosed condition. Kidney disease, hyperthyroidism, diabetes and pancreatitis all change energy needs and require a therapeutic diet at a prescribed amount. This calculator is for healthy cats.
- Ignoring water. Calories are only one half of intake. Wet food carries roughly three-quarters of its weight as water, which is a real contribution to daily hydration in a species with a weak thirst drive.
Never starve a cat thin
Feline hepatic lipidosis is the reason cat weight loss has to be slow and supervised. When a cat stops eating or loses weight too quickly, fat is mobilised into the liver faster than the liver can export it, and liver failure can follow within days. Any cat on a reducing plan should be weighed every two weeks, and any cat that refuses food for more than 24 hours needs a veterinarian, not a smaller bowl. The 2021 AAHA Nutrition and Weight Management Guidelines set the target loss rate for cats at 0.5% to 2% of body weight per week for exactly this reason.
Where this fits among the other ways to portion food
Three alternatives exist and each has a place. The manufacturer's feeding chart needs no calculation and is fine for a lean, active, intact cat at maintenance — the animal it was built for. Percentage-of-body-weight rules, such as feeding 2% to 3% of body weight in raw or fresh food, are common in the fresh-feeding world; they are quick but they ignore calorie density entirely, so they only work within one product line. A veterinary nutritionist's plan is the right answer for any cat with a medical condition, a food allergy, or a home-prepared diet, because those require nutrient balancing and not just calorie counting.
The RER-and-factor method used here is the one taught in veterinary curricula and used in practice for healthy animals, and it is the same skeleton as the canine calculation — only the multipliers differ. The feline maintenance factors sit lower than the canine ones (1.2 for a neutered indoor cat against roughly 1.6 for a neutered dog), so an identical body weight yields a smaller daily target for a cat. If you are working out where your other pet sits in its life, the dog age in human years calculator covers canine life stages, and the puppy adult weight calculator projects growth curves. For clinical weights and fluid volumes, the veterinary fluid therapy rate calculator works from the same body-weight basis, and the ounces to grams calculator handles label conversions when your food is sold by weight in one system and portioned in another.
Key terms
- RER
- Resting energy requirement — the calories a cat burns at rest, awake, in a thermoneutral environment, calculated from body weight alone.
- MER
- Maintenance energy requirement — RER multiplied by a life-stage and activity factor. This is the number that holds weight steady.
- Calorie content statement
- The line on a pet food label giving metabolisable energy, expressed as kcal per kilogram and per common household measure such as a cup or a can. Required on US labels under AAFCO model regulations.
- Body condition score
- A 9-point visual and hands-on assessment of fat cover. Five is ideal; each point above five corresponds to roughly 10% above ideal body weight.
- Metabolisable energy
- The energy in food that the animal can actually absorb and use, after losses in faeces and urine. Label calorie figures are metabolisable energy.