An overweight cat does not need a crash diet — it needs a slow, safe plan and a realistic timeline. Cats are different from dogs and people in one critical way: if a cat eats far too little, or stops eating, its body can flood the liver with fat faster than the liver can process it, causing a dangerous condition called hepatic lipidosis (fatty liver disease). That is why vets insist feline weight loss must be gradual — typically around 0.5% of body weight per week — and why “just feed half as much” can be genuinely harmful. Our free Cat Weight Loss Calculator builds that safe pace in from the start. Enter your cat’s current weight and the target (ideal) weight your vet has suggested, and it tells you how much your cat needs to lose, roughly how many weeks and months the journey will take at a safe rate, and the daily calorie target to feed — calculated as the resting energy requirement at the target weight, the starting point vets commonly use for a weight-loss ration.
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Why Use a Cat Weight Loss Calculator?
Most owners know their cat is “a bit chunky” but have no idea what that means in practice: how much needs to come off, how long it will honestly take, or how much food to put in the bowl tomorrow morning. Guessing leads to the two classic mistakes — feeding so little that the cat is hungry, miserable and at risk of liver trouble, or feeding only slightly less than before and seeing no change for months, then giving up. A calculator turns a vague worry into three concrete numbers you can act on and track: the total amount to lose, a realistic timeline measured in weeks and months, and a daily calorie target. That timeline matters for motivation, too. Safe feline weight loss is slow by design — a cat that needs to lose 2 kg may need well over a year — and owners who expect that pace stick with the plan, while owners who expect a transformation in six weeks abandon a plan that was actually working perfectly.
- Safe pace built in: the timeline assumes about 0.5% of body weight per week, the gradual rate vets recommend for cats.
- A real calorie number: the feeding target is the resting energy requirement at the target weight — not a guess or a fraction of the current food.
- Honest expectations: seeing “about 15 months” up front prevents the discouragement that kills most weight-loss attempts.
- A target to weigh against: weigh every two weeks and compare progress with the plan, adjusting only with your vet’s guidance.
Cat Weight Loss Explained — The 0.5% Rule and the RER Formula
Two pieces of maths run this calculator. The first is the safe rate of loss. Veterinary guidance for cats is to aim for roughly 0.5% to 1% of body weight per week, and this calculator uses the cautious end of that range: 0.5% of the current weight each week. Because the cat gets lighter every week, each week’s 0.5% is slightly smaller than the last, so the weight follows a gentle curve rather than a straight line. Working that curve backwards gives the number of weeks: weeks = ln(target ÷ current) ÷ ln(0.995). For a 7 kg cat heading to 5 kg, that is ln(5 ÷ 7) ÷ ln(0.995) ≈ 67 weeks — about 15 to 16 months — and the daily loss in the first week is only about 35 grams. That smallness is the point: it is what “slow and safe” actually looks like on a scale.
The second piece is the feeding target. Vets base calorie needs on the Resting Energy Requirement (RER): RER = 70 × (body weight in kg)0.75. For weight loss, the standard starting approach is to feed the RER calculated at the target weight — the calories a resting cat of that ideal size needs — rather than slashing the current ration by an arbitrary percentage. At a 5 kg target, RER = 70 × 50.75 ≈ 234 kcal per day. Your vet may adjust this up or down after the first weigh-ins, may recommend a specific therapeutic weight-loss diet (which delivers full protein and nutrients in fewer calories), and will want to rule out medical causes of weight gain such as an underactive thyroid before the plan starts.
| Example | To Lose | Safe Timeline at 0.5%/Week | Daily Target (RER at Target) |
|---|---|---|---|
| 5 kg → 4.5 kg | 0.5 kg | About 21 weeks (~5 months) | About 216 kcal/day |
| 6 kg → 5 kg | 1 kg | About 36 weeks (~8 months) | About 234 kcal/day |
| 7 kg → 5 kg | 2 kg | About 67 weeks (~15 months) | About 234 kcal/day |
| 8 kg → 6 kg | 2 kg | About 57 weeks (~13 months) | About 268 kcal/day |
Notice how the timeline grows faster than the amount to lose: the last kilogram always takes longer than the first, because 0.5% of a shrinking body is a shrinking amount. Plan for the full journey, celebrate each 100 grams, and recheck the plan with your vet every month or two — small calorie adjustments along the way are normal and expected.
How to Use This Calculator
- Weigh your cat accurately. Weigh yourself holding the cat, then without, and subtract — or use a pet scale. Use the same scale, at the same time of day, for every check.
- Get a target weight from your vet. The ideal weight depends on your cat’s frame, breed and body condition score — not just a breed average. If you are unsure, our Cat Weight Calculator and Cat BMI Calculator give a useful first estimate to discuss at the appointment.
- Enter both weights and click “Calculate Weight Loss Plan” to see the total to lose, the estimated weeks and months, and the daily calorie target.
- Feed the target with a kitchen scale. Grams, not handfuls or “about a cup” — a few grams of kibble a day is the difference between losing and stalling. Count treats inside the daily total (keep them under about 10% of calories).
- Weigh every two weeks and log it. Loss much faster than the plan, no loss for 6–8 weeks, or any refusal to eat all mean the same thing: check in with your vet and adjust the plan together.
Why Slow Is Non-Negotiable: Hepatic Lipidosis
The safety warning on this page is not fine print — it is the most important paragraph here. When an overweight cat stops eating or eats far too little, the body mobilises large amounts of fat to the liver for energy. A cat’s liver handles that fat flood poorly, fat accumulates inside liver cells, and the liver begins to fail: this is hepatic lipidosis, also called fatty liver disease, and overweight cats are the highest-risk group. Early signs include refusing food, drooling, vomiting, lethargy and yellowing of the eyes, ears or gums. It is life-threatening without treatment, yet almost entirely preventable with gradual weight loss. That is why every reputable feline weight plan caps the rate of loss, why a cat that will not eat for 24–48 hours needs a vet call (sooner for an overweight cat), and why the calorie target here is a measured RER figure rather than a drastic cut. Add daily gentle play — our Cat Exercise Calculator helps you plan it — because activity protects muscle while fat is lost, and muscle is what keeps the metabolism healthy at the new weight.
Educational & Veterinary Disclaimer
This calculator provides an educational estimate using a 0.5%-per-week loss model and the standard RER formula (70 × target weight in kg0.75). It does not replace veterinary care: the right target weight, diet and calorie level for your cat depend on its body condition, age, health and medications, and an overweight cat’s plan should be agreed with a vet before you start. Never starve a cat or cut food drastically — rapid restriction risks hepatic lipidosis. If your cat stops eating, loses weight much faster than planned, or seems unwell at any point, contact your veterinarian promptly.
High-Authority References & Sources
- American Animal Hospital Association (AAHA) — nutrition and weight management guidelines for cats.
- American Veterinary Medical Association (AVMA) — pet obesity and healthy-weight resources for owners.
- Merck Veterinary Manual — veterinary reference on feline obesity and hepatic lipidosis.
- Cornell Feline Health Center — cat obesity, feeding and weight-loss information.
- International Cat Care — guides on helping overweight cats lose weight safely.
Related Pet Calculators
- Cat Exercise Calculator
- Cat Feeding Calculator
- Cat BMI Calculator
- Cat Weight Calculator
- Dog Weight Loss Calculator
- All Pets Calculators
Frequently Asked Questions (FAQ)
How fast should a cat lose weight safely?
About 0.5% to 1% of body weight per week is the range vets commonly recommend, and this calculator plans at the cautious 0.5% end. For a 7 kg cat that is only about 35 g in the first week. Faster loss — and especially any plan where a cat stops eating — risks hepatic lipidosis (fatty liver disease), so slow is not a flaw in the plan; slow is the plan.
How many calories should an overweight cat eat to lose weight?
A common veterinary starting point is the Resting Energy Requirement at the target weight: RER = 70 × (target kg)^0.75. For a 5 kg target that is about 234 kcal per day, including treats. Your vet may adjust the figure after weigh-ins or prescribe a therapeutic weight-loss diet, so treat the calculator’s number as the starting point for that conversation.
How long will it take my cat to reach its ideal weight?
Longer than most owners expect. At 0.5% per week, losing 1 kg from 6 kg to 5 kg takes about 36 weeks, and losing 2 kg from 7 kg to 5 kg takes about 67 weeks — roughly 15 months. Enter your own cat’s numbers above for its personal estimate, and weigh every two weeks to confirm you are on the curve.
Can I just feed my cat half as much to speed things up?
No — please do not. Halving food suddenly can leave a cat eating far too little, and in an overweight cat that can trigger hepatic lipidosis, a dangerous liver condition. Feed the measured daily calorie target instead, weigh portions with a kitchen scale, and if progress stalls for 6–8 weeks ask your vet to adjust the target slightly. Never let an overweight cat go without food for more than a day without veterinary advice.
How do I know what my cat’s ideal weight should be?
Ask your vet to set it from your cat’s body condition score and frame — ribs easy to feel under a thin fat cover, a visible waist from above and the side. Breed averages and our Cat Weight Calculator and Cat BMI Calculator give a helpful first estimate, but the target in this calculator should be the one your vet agrees, especially for large breeds, seniors, or cats with any health condition.