How it's calculated
It's based on the standard veterinary formula for energy requirement:
RER (resting energy requirement) = 70 × (weight in kg)0.75 — this is how many calories a cat burns just by existing. RER is then multiplied by a factor that depends on age, neutering and activity:
| Situation | Multiplier to RER |
|---|---|
| Kitten under 4 months | 2.5 |
| Kitten 4-12 months | 2.0 |
| Adult, neutered, normal activity | 1.2 |
| Adult, intact | 1.4 |
| Senior (11+) | 1.1 |
| Couch potato (indoor, doesn't play) | ×0.9 to the normal portion |
| Active (plays a lot, outdoor access) | ×1.2 to the normal portion |
| Weight loss | ×0.8 to the normal portion |
| Weight gain | ×1.2 to the normal portion |
To get grams per day, the final calories are divided by your food's calorie density. The density is listed on the bag as "kcal/100 g" or "kcal/kg" (in that case divide by 10). If it's not on the bag, look for the specification on the manufacturer's website.
Cat specifics: water and urinary tract disease
Cats have evolved to drink very little — in the wild they get water from their prey. On a dry-only diet most cats take in too little fluid, which raises the risk of urinary tract disease and kidney problems.
If you feed dry food only, you must provide: clean water in several places, ideally a fountain (cats drink better from moving water). The ideal option is combined feeding: dry during the day + wet 1-2 times a day.
Body condition — the key final check
Any calculation is just a starting point. From there, look at the cat's body:
- Normal: ribs are felt under light pressure of the palm. A waist is visible from above. A small fat flap on the belly is normal — that's not excess weight.
- Underweight: ribs, spine and hip bones are easily visible → add 10% food and reassess in 2-3 weeks.
- Overweight: ribs can't be felt, no waist, fat deposits visible on the sides and belly → reduce the portion by 10%.
Treats count too: they should be no more than 10% of daily calories.
Frequently asked questions
I feed both dry and wet — how do I count it?
Count the calories separately for each food and add them up. For example: 50 g of dry × 380 kcal/100 g = 190 kcal; + 100 g of wet × 80 kcal/100 g = 80 kcal; total 270 kcal. Compare that with your target portion.
Can I leave dry food out for my cat all the time?
For kittens — usually yes (they're growing and self-regulate). For neutered adults — better not, they're prone to overeating. For cats that are gaining weight, switch to portioned feeding 2-3 times a day.
Do I need to change the portion after neutering?
Yes. After neutering, energy needs drop by ~25%. If you don't reduce the portion, the cat will gain excess weight. Recalculate the portion 2-4 weeks after the operation.
What if my cat has kidney disease or diabetes?
This calculator is for healthy cats. With medical conditions you need a therapeutic diet and an individual portion set by your vet.