Cat Calorie Needs Calculator (RER / MER)
Calculate how many calories your cat needs per day — resting energy requirement × the WSAVA/AAHA factor for neutered, intact, indoor, senior, kitten, pregnant, nursing, weight-loss or weight-gain cats — then turn it into grams or cups of dry food, cans of wet food or a mix, per meal, with a treat budget and a week-by-week weight-loss plan.
How it works
RER = 70 × weight(kg)^0.75 · MER = RER × life-stage factor (weight loss and gain use the ideal weight) · ideal weight ≈ current ÷ (1 + 0.1 × (BCS − 5)) · dry grams = kcal ÷ (kcal per 100 g) × 100 · cans = kcal ÷ kcal per can · treats ≤ 10% of kcal · WSAVA adult range = between 130 × kg^0.4 (obese-prone) and 100 × kg^0.67 (lean) · weeks to goal = ln(ideal ÷ current) ÷ ln(1 − weekly %)
A cat’s resting energy requirement (RER) rises with body weight to the three-quarter power rather than in proportion, so a 6 kg cat needs less than twice the calories of a 3 kg one. Multiplying the RER by a factor for reproductive status, age and activity — the multipliers used in WSAVA and AAHA nutrition guidance — gives the daily maintenance requirement. Neutering lowers it by around a quarter, the reason so many desexed indoor cats gain weight on the old ration. For weight loss or weight gain the ideal weight is used instead of the current one; if you don’t know it, the calculator estimates it from the 9-point body condition score, where each point above 5 is roughly 10% extra weight, and lays out a week-by-week plan at a safe 0.5–2% a week. For healthy adults it also shows the WSAVA reference range for that weight. Because food energy density varies enormously, the calorie target only becomes useful once converted into grams or cups of dry food and cans of wet food — split into meals and with treats budgeted at no more than 10% — and the body condition score, not the bag’s feeding chart, is the final arbiter.
Worked example
A 4.5 kg neutered indoor cat has a resting requirement of 70 × 4.5^0.75 ≈ 216 kcal, so about 260 kcal a day (WSAVA range for that weight: 236–274 kcal). Split half and half, that is 1.5 × 85 kcal pouches plus 34 g of a 380 kcal/100 g dry food, or two meals of 0.76 pouch and 17 g, with no more than 26 kcal of treats. If the same cat scores 7/9, its ideal weight is about 4.5 ÷ 1.2 = 3.75 kg; feeding 0.8 × 70 × 3.75^0.75 ≈ 151 kcal a day and losing 1% a week, it would reach that weight in about 19 weeks.
Sources: WSAVA — Calorie needs for healthy adult cats (2020) · Pet Nutrition Alliance — Calculating calories based on pet needs (RER/MER factors) · Pet Nutrition Alliance — Calorie calculator
Frequently asked questions
How many calories does a cat need a day?
A typical neutered adult needs about 70 × weight(kg)^0.75 × 1.2: roughly 200 kcal for a 3 kg cat, 238 kcal at 4 kg, 281 kcal at 5 kg and 322 kcal at 6 kg. Intact, young and active cats need more; inactive and overweight-prone indoor cats need less.
The bag says feed 60 g and you say 34 g — who is right?
Feeding guides are written for intact, active cats and err generous. If the cat is at ideal weight and stable on 60 g, keep it; if it is gaining, the calculated figure is the better starting point.
How fast can a cat lose weight safely?
About 0.5–1% of body weight per week, and no more than 2%. Faster loss, or a cat that stops eating, risks hepatic lipidosis — a medical emergency. Never fast a cat to slim it; the weight-loss option feeds 0.8 × the RER of the ideal weight and shows how many weeks the goal will take.
How do I know my cat’s ideal weight?
Your vet can tell you, or use the 9-point body condition score: at 5/9 you can feel the ribs under a light fat cover and see a waist from above. Each point above 5 is roughly 10% over ideal, so a 6 kg cat scoring 7 has an ideal weight of about 5 kg. Pick the score under the ideal weight and the calculator estimates it.
How should I split wet and dry food?
Any split works as long as the total calories match. Set the share from wet food and the calculator divides the calories, then converts each part into cans and grams. Wet food adds water, which helps cats that drink little, and dividing the day into two or more meals suits most cats.
How many treats can my cat have?
Keep treats to 10% of the daily calories — about 26 kcal for a cat that eats 260 kcal. Turn on “Keep 10% for treats” and the food amounts are reduced to make room for them.
Related calculators
General veterinary guidance and rules of thumb (WSAVA / AAHA nutrition and reproduction references). Every animal differs — confirm feeding amounts, pregnancy timing and any concern with your veterinarian, especially for kittens, puppies, seniors and animals with medical conditions.