Three numbers set the bowl: resting energy requirement (RER), one lifestyle factor, and the calories per gram of the food you actually bought. Get those 3 and portion size stops being a guess. A 4.5 kg neutered indoor cat lands at 260 kcal a day, which is 68 g of a 380 kcal kibble or 260 g of a 100 kcal pâté.
The RER formula, and the shortcut
RER in kcal per day = 70 × body weight in kilograms raised to the power 0.75. The exponent is there because energy scales with surface area, not mass, so a 9 kg cat does not need twice the food of a 4.5 kg cat. For 4.5 kg: 4.5^0.75 = 3.09, and 70 × 3.09 = 216 kcal.
The shortcut 30 × kg + 70 is within 5 percent between 3 and 6 kg, which covers most pets: 30 × 4.5 + 70 = 205 kcal against the true 216, a 5 percent gap, or 13 kcal. Below 3 kg the shortcut overshoots by up to 6 percent, and above 8 kg it drifts low, so use the exponent for kittens and giant breeds.
The 3 factors that matter
RER is what the cat burns lying still. Multiply it by one factor:
1.2 for a neutered adult with normal activity, the default for most indoor cats. 1.4 for an intact adult or a very active one that climbs and runs daily. 0.8 for weight loss, applied to the RER of the target weight, never the current weight.
Those 3 cover roughly 90 percent of adult cats. Kittens, pregnancy, and illness have their own numbers, and the table below keeps them in one place.
| Body weight | RER (kcal) | 30 × kg + 70 | Maintenance 1.2 × RER | Loss 0.8 × RER |
|---|---|---|---|---|
| 3.5 kg | 179 | 175 | 215 | 143 |
| 4.0 kg | 198 | 190 | 238 | 158 |
| 4.5 kg | 216 | 205 | 260 | 173 |
| 5.0 kg | 234 | 220 | 281 | 187 |
| 5.5 kg | 251 | 235 | 302 | 201 |
| 6.0 kg | 268 | 250 | 322 | 215 |
One full worked sum
Cat: 4.5 kg, neutered, indoor, body condition score 5. Step 1, RER = 70 × 4.5^0.75 = 70 × 3.09 = 216 kcal. Step 2, neutered adult factor 1.2: 216 × 1.2 = 260 kcal per day, call it 260.
Step 3, convert to grams. Read the label for kcal ME. A kibble at 380 kcal per 100 g: 260 ÷ 380 × 100 = 68 g. A pâté at 100 kcal per 100 g: 260 ÷ 100 × 100 = 260 g, which is 2 cans of 130 g. Step 4, split: 3 meals of 23 g kibble, or 4 meals of 65 g pâté.
Mixed feeding works the same way in calories. Half the ration from each food is 130 kcal + 130 kcal: 34 g of the 380 kcal kibble plus 130 g of the 100 kcal pâté, and the two together still weigh 164 g, not 130 g.
Where the label hides the math
Some labels print kcal per cup or per can only. A cup of kibble is 8 oz by volume and about 100 g by weight, so per-cup and per-100 g are close but never identical, and a dense pellet at 110 g per cup adds 10 percent. Some print ME per kg: divide by 10 to get per 100 g, so 3,800 kcal per kg is 380 per 100 g. If the label gives only guaranteed analysis and no kcal line, estimate with 3.5 × protein + 8.5 × fat + 3.5 × carbohydrate in grams per 100 g, which lands within 10 to 15 percent.
Why the number is a starting point, not a verdict
Individual cats vary by 20 percent either side of the formula, because activity, coat, and muscle mass all move the total. Feed the calculated amount for 14 days, then weigh. Flat weight means the number is right. Up 1 to 2 percent of body weight means cut 10 percent, which is 26 kcal here, not 100. Down more than 1 percent a week means add 10 percent.
Cold weather is the one seasonal shift worth planning for: maintenance can rise 10 to 20 percent in an unheated home in winter, so 260 kcal becomes 286 to 312 kcal, and the how and why sit in winter calorie needs. A cat that eats 260 kcal and still loses weight needs a vet, not a bigger bowl.
Life stages that break the 3 factors
Kittens under 4 months run 2.5 to 3.0 × RER and eat 4 meals a day; from 4 to 12 months the factor drops to 2.0. A pregnant queen reaches 1.6 to 2.0 × RER in the last 3 weeks, and a nursing queen can hit 2 to 6 × RER at peak lactation, which is 1,300 kcal or more on a 4 kg cat. Senior cats over 11 often need 1.1 to 1.4, because digestion of fat and protein falls off, and a thin old cat should not be put on a light diet.
Treats and the 10 percent line
Treats belong inside the 260 kcal, not on top. Keep them at or under 10 percent, so 26 kcal: 7 g of the same kibble, 1 freeze-dried cube at about 5 kcal, or 20 g of plain cooked chicken at 130 kcal per 100 g, which is 26 kcal. Three 30 kcal dental chews would be 90 kcal, a third of the day, and that alone erases a loss plan. The full split is in the treat calorie budget.
Cheap food, expensive food, same math
Price per 100 kcal is the only fair comparison between a 4 kg bag at 20 dollars and a 2 kg bag at 35 dollars. A 4 kg bag at 380 kcal per 100 g holds 15,200 kcal, which is 58 days for this cat, so 34 cents a day. The 2 kg bag holds 7,600 kcal, 29 days, at 1.21 dollars a day. The cost tool does this per year once you enter the bag weight and the kcal line.
When to stop calculating and call
Book a visit if weight moves more than 5 percent in 2 months with no change in intake, if the cat eats 300 kcal a day and still loses, if intake drops below 60 percent of RER for 3 days, or if a cat over 8 suddenly demands more food. Raised thyroid hormone, diabetes, and intestinal disease all present as a cat that cannot hold weight. Bring the grams fed per day and 3 weights with dates; that is what turns a guess into a diagnosis.
Five steps to a settled bowl
- Weigh the cat, score body condition, and set the target weight before any calorie math.
- Compute RER = 70 × kg^0.75, then multiply by 1.2, 1.4, or 0.8.
- Read kcal per 100 g off the label, divide the daily kcal by it, multiply by 100.
- Weigh the daily ration in grams, split into 3 or 4 meals, and log 14 days.
- Reweigh at day 14 and adjust by 10 percent, never by 30.
Terms mentioned in this article
Frequently asked
Why is the exponent 0.75 and not a simple per-kilo rule?
Energy use tracks body surface area, which grows slower than mass, so the exponent 0.75 keeps a 9 kg cat from being fed twice a 4.5 kg cat. The simple 30 × kg + 70 rule is within 5 percent from 3 to 6 kg, which is why clinics use it as a quick check, but it drifts on kittens and very large cats.
Do I use current weight or ideal weight for a fat cat?
Use ideal weight when the cat is over body condition score 6. A 6.5 kg cat with a 5.0 kg ideal gets RER from 5.0 kg, which is 234 kcal, and the 0.8 loss factor brings it to 187 kcal a day. Feeding a diet on current weight gives a ration that keeps the cat fat.
The label only lists protein and fat, no calories. Now what?
Estimate metabolizable energy with the modified Atwater values: 3.5 kcal per gram of protein, 8.5 per gram of fat, 3.5 per gram of carbohydrate, all per 100 g of food. The estimate usually lands within 10 to 15 percent of the real figure, and weighing the cat every 14 days corrects the rest.