Resting energy requirement in a cat scales as 70 x body weight in kilograms raised to the 0.75 power. For a 4 kg cat that is 198 kcal per day; for 6 kg it is 268 kcal. Multiply by an activity factor of 1.2-1.4 for a normal indoor cat and the daily target lands at 238-277 kcal for the 4 kg animal.
The formula is a starting estimate, not a measurement. Oxygen consumption of 0.2-0.3 L per kg per hour is what a resting cat actually takes up, and that number and the formula disagree by a wide margin. Knowing which one you are holding stops most feeding mistakes.
Why the exponent is 0.75, not 1.0
Metabolic rate tracks body surface and heat loss more than mass, so it scales allometrically. Doubling body weight raises resting energy by about 68%, not 100%, because 2 raised to the 0.75 is 1.68. A 2 kg cat needs 118 kcal, a 4 kg cat 198 kcal, an 8 kg cat 333 kcal: 1.68x each time weight doubles.
This is why small cats eat more per kilogram than large ones. A 3 kg Siamese and a 7 kg Maine Coon sit at opposite ends of the scale, and a breed quiz helps set the target weight. Per kg of body weight, a 2.5 kg cat uses 56 kcal/kg/day of resting energy, a 4 kg cat 50, and a 7 kg cat 43. A 10 kg cat is down to 35 kcal/kg/day, which is also why obese cats gain faster on the same bowl as a lean one.
Oxygen: 0.2-0.3 L/kg/h and 4.7-5.0 kcal per liter
Indirect calorimetry measures oxygen uptake and carbon dioxide output. A resting cat consumes roughly 0.2-0.3 L of oxygen per kilogram per hour, with a respiratory quotient between 0.7 and 1.0 depending on whether fat or carbohydrate is being burned. Each liter of oxygen consumed releases about 4.7-5.0 kcal, so a 4 kg cat burning 0.25 L/kg/h uses 1.0 L per hour, or 24 L a day, worth roughly 115 kcal.
That is 115 kcal against the 198 kcal the formula predicts, a gap of close to 40%. The gap is real and it has two sources: calorimetry readings come from resting, fasted, often sedated cats, while the 70 x kg^0.75 figure is a clinical convention that already carries built-in margin. Treat the formula as a starting point and the scale as the referee.
Activity factors of 1.2-1.4 in practice
Multiply resting energy by 1.2 for a sedentary indoor cat, 1.3 for a normal active adult, and 1.4 for a cat that hunts or climbs daily. A 4 kg indoor cat therefore needs 238 kcal, an active one 257, an outdoor hunter 277. For weight loss, do not use the current weight; use the ideal weight and feed 0.8 times its resting energy, which for a 4 kg target is 158 kcal a day.
Two adjustments override the arithmetic. Kittens under 4 months need 2.0-2.5 times resting energy, and a cat recovering from illness or surgery may need 1.2-1.5 times. Pregnant and nursing queens at peak lactation can require 2-3 times resting energy, which is the highest demand in the species.
Neutering cuts demand 20-30%
Gonadectomy removes the gonadal hormones that raise basal metabolism and drive activity, and resting energy expenditure falls by roughly 20-30% within 4-12 weeks. Food intake often rises 10-20% over the same window. The combination is why neutered cats gain 15-30% of body weight within 12 months on an unchanged ration.
The fix is arithmetic, applied before the weight arrives. Take the resting energy at ideal weight, multiply by 1.2, then reduce by 20-25% if the cat was neutered as an adult. For a 4 kg cat that moves the daily ration from 238 kcal to about 180-190 kcal, roughly a 20% cut in kibble grams.
Table: resting energy by body weight
| Weight (kg) | RER (kcal/day) | x1.2 (kcal) | x1.4 (kcal) |
|---|---|---|---|
| 2.5 | 139 | 167 | 195 |
| 3.0 | 160 | 192 | 223 |
| 4.0 | 198 | 238 | 277 |
| 5.0 | 234 | 281 | 328 |
| 6.0 | 268 | 322 | 376 |
| 7.0 | 301 | 362 | 422 |
When the rate itself moves
Disease changes the multiplier before it changes the formula. Hyperthyroidism raises metabolic rate substantially, often 20-40% above normal, and a cat that eats 300 kcal a day and still loses weight is the classic picture. Chronic kidney disease tends to lower appetite before it lowers metabolic rate, so the cat loses muscle while total energy need stays near normal. Diabetes mellitus leaks energy through the urine: each gram of glucose carries about 4 kcal, so 10 g spilled costs roughly 40 kcal.
Age shifts things slowly. Cats over 11 years often need 10-20% more energy than middle-aged cats because digestion of fat and protein becomes less efficient, which is why unexplained weight loss in an older cat is a reason for blood work rather than a diet change. Smell sets intake too: a cat with a blocked nose from an upper respiratory infection often eats less, and the vomeronasal organ works with the main olfactory system on that signal.
Measuring instead of guessing
Weigh food in grams, not cups, and weigh the cat on the same scale every 2 weeks. A 4 kg cat eating 238 kcal a day that gains 100 g per month is eating about 25-30 kcal a day more than it needs; that is 4-5 g of dry food. Those margins are invisible by eye and obvious on a scale.
Body condition is scored 1-9 by rib and spine palpation: 5 is ideal, 6-7 is overweight, 8-9 is obese. A cat at 7/9 should lose 1-2% of body weight per week, no faster, because rapid loss over 2% per week risks hepatic lipidosis.
When to see a vet
Book an exam if weight changes more than 5% in 1 month or 10% in 6 months with no change in ration. Call within 24-48 hours for a cat that eats well and still loses weight, drinks more than 45 mL/kg per day (about 180 mL for a 4 kg cat), or has vomiting more than 2 times a week. Stop free-feeding and switch to 2-3 measured meals a day if two cats in the house eat from one bowl, because you cannot calculate a rate you cannot measure.
5 steps to set a real ration
- Weigh the cat today in kilograms to one decimal, and score body condition at the same time.
- Pick the ideal weight, compute 70 x kg^0.75, then multiply by 1.2.
- Cut 20-25% if the cat is neutered and sedentary; do not cut below 0.8 times resting energy.
- Weigh the daily portion in grams and split it into 2-3 meals.
- Recheck weight every 14 days; adjust by 10% up or down only if the direction is wrong for 3 checks.
Frequently asked
Why do the oxygen figures and the 70 x kg^0.75 formula disagree?
They measure different things. Calorimetry gives oxygen uptake in a resting, fasted cat, and 0.2-0.3 L per kg per hour converts to roughly 115 kcal a day for a 4 kg cat. The formula returns 198 kcal because it is a clinical convention with margin built in. Use the formula to set a starting ration, then let 2-week weight checks correct it.
Should I feed a neutered cat less from day one?
Plan for it rather than react to it. Resting energy falls 20-30% within 4-12 weeks of gonadectomy while intake often rises 10-20%, and many cats gain 15-30% of body weight in the first year. Cutting the ration by 20-25% at the time of surgery, measured in grams, avoids most of that gain without a starvation diet.
Is 238 kcal a day right for my 4 kg indoor cat?
It is the arithmetic answer, not a guarantee: 70 x 4^0.75 is 198 kcal, times an activity factor of 1.2 gives 238. Weigh the cat every 14 days. Stable weight means the number is right, a gain of 100 g a month means roughly 25-30 kcal too much, and steady loss means too little. Adjust by 10%, not by a third.