Underweight cat signs — how do you tell, and what should you do about it? A cat that is underweight does not look dramatically different from one that is not — the change is gradual, a few hundred grams at a time, and owners often do not notice it until the ribs are hard to feel or the bones are easy to see. This guide covers the signs to look for, how to check at home with your hands and eyes, what usually causes it, and what to do next. Every cat is different, and the right weight for an individual cat is the one that keeps it healthy and moving well — not a single number on a chart.

The short version is: feel the ribs, look for a waist from above, and look for a tummy tuck from the side. You should be able to feel the ribs easily under a thin layer of fat, see a waist from above, and see a tummy tuck from the side. If you cannot feel the ribs, the cat is carrying too much weight; if the ribs are easy to see, the cat is carrying too little. Weigh the cat monthly, write the number down, and if the weight is moving in the wrong direction, or moving fast, see your vet — especially if the cat is a senior, a young animal, or has a health condition. The cat body condition score chart shows the nine-point scale vets use, and the how to tell if your cat is overweight guide gives the hands-on check in detail.

Signs of an underweight cat

The signs of an underweight cat are mostly things you can see and feel at home, without any equipment. Work through them one at a time, with the cat standing, not lying down, and in good light:

  • The ribs are easy to see, not just to feel, and the spine and hip bones are visible.
  • The waist is very obvious from above, and the tummy tuck is very obvious from the side.
  • There is almost no fat to feel over the ribs — the bones are sharp under your fingers.
  • The pet has lost weight over weeks or months, not just a few days.
  • The coat looks dull, and the pet seems tired or weak.
  • The pet eats well but still loses weight, or it has gone off its food.
  • The pet is a young animal that is not gaining weight as it grows.

If two or more of those signs apply, the cat is probably underweight — and the next step is to weigh it and check its body condition score, not to guess. A single sign on its own can be misleading: a cat with a thick coat can look heavier than it is, and a cat with a deep chest can look heavier from the side. Hands beat eyes for this job, and scales beat both. The how to weigh your cat at home guide explains the bathroom-scale method, and the cat ideal weight calculator works out a target weight range.

What causes an underweight cat?

An underweight pet is sometimes simply not getting enough food — a young animal growing fast, a nursing mother, or a very active pet can all need more than they are given. But weight loss in a pet that is eating normally is a warning sign: dental pain, parasites, thyroid problems, kidney disease, diabetes, and some other conditions all cause weight loss, and they need a vet to find them. An older pet that is losing weight needs a vet check promptly, because weight loss in a senior pet is never normal on its own.

Some breeds are more prone to weight problems than others — food-motivated breeds such as the Labrador Retriever and the Beagle gain weight easily, and small breeds with crowded teeth can lose weight because eating hurts. But breed is not destiny: a cat of any breed can be kept at a healthy weight with measured food and daily exercise, and a cat of a breed that is prone to weight gain needs measured food all the more. The cat weight loss diet plan guide puts food and exercise together, and the obesity in cats guide covers the health risks of carrying too much weight.

How to check at home: ribs, waist, and tummy

Run your hands along the cat's ribs, with flat fingers and light pressure. You should feel each rib easily under a thin layer of fat, like feeling the bones in the back of your hand. If you have to press hard to find the ribs, the cat is carrying too much fat; if the ribs are sharp and easy to see, the cat is carrying too little. Now look down at the cat from above: you should see a waist — a narrowing behind the ribs — and from the side you should see a tummy tuck, where the belly rises from the chest to the back legs. A cat with no waist from above and no tummy tuck from the side is carrying too much weight; a cat with a very obvious waist and a very obvious tuck may be carrying too little.

Do the check monthly, at the same time as the weigh-in, and write down what you find — a sentence in a notebook is enough. The line over months is what matters, not any single reading: a cat that is slowly gaining weight needs a small food cut, and a cat that is slowly losing weight needs a vet check, even if it still looks well. The cat body condition score chart gives the full nine-point scale, from 1 (emaciated) to 9 (obese), with 4 to 5 as the ideal range — a useful reference to keep with the notebook.

What to do if your cat is underweight

If the cat is underweight and otherwise well, start with measured food and a monthly weigh-in. Feed a little more — about 10% more than the normal amount — split into two or three meals a day, and re-weigh in a month. If the pet is eating well and still losing weight, or if it has gone off its food, see a vet promptly rather than feeding more. Do not change the food itself unless the vet advises it: the amount matters more than the brand, and a food change on top of a weight change makes it impossible to tell what worked.

See a vet promptly if the cat is losing weight without a change in food, if it is lethargic or off its food, if it is a senior, if it is a young animal that is not growing, or if it needs to lose or gain a lot of weight. At the vet, expect a weigh-in, a body condition score, and questions about food, treats, exercise, and other pets in the house — and possibly a blood test, because some medical conditions cause weight change, and those need treating, not just more or less food. The how to help a cat gain weight safely guide gives the full plan, and the cat calorie calculator works out daily calories for any weight.

Mistakes to avoid

The most common mistake is guessing: a cat fed by eye is almost always fed too much, and a cat weighed by eye is almost always weighed wrong. Measure food with scales, weigh the cat monthly, and write the numbers down. The second is forgetting treats: treats, table scraps, and food from other pets' bowls all count, and they often add up to more than the meal itself. The third is changing everything at once — food, treats, exercise, and feeding times — so that if the weight moves, you do not know what moved it. Change one thing at a time, wait a month, and then decide.

The fourth mistake is a crash diet, or the opposite — feeding a lot more all at once. A pet that is fed a lot more all at once can get an upset stomach, and a pet that is losing weight needs a vet check first, not just more food. The fifth is giving up after a plateau: weight change is rarely a straight line, and a month with no change is normal. Cut or add about 10% and keep going, rather than abandoning the plan. The best exercise for overweight cats guide gives a safe build-up plan for a heavy cat, and the cat weight loss diet plan guide puts it all together.

For related reading, see the cat body condition score chart, the how to weigh your cat at home guide, the cat ideal weight calculator, the how to help a cat gain weight safely guide, the cat weight loss diet plan guide, the obesity in cats guide.

Frequently asked questions

How do I know if my cat is underweight?

Feel the ribs — you should feel them easily under a thin layer of fat — look for a waist from above, and look for a tummy tuck from the side. If the ribs are easy to see and the waist is very obvious, the pet is carrying too little. Weigh the cat monthly and check its body condition score on the nine-point scale, where 4 to 5 is ideal.

What causes an underweight cat?

Sometimes not enough food, but weight loss in a pet that is eating normally is a warning sign: dental pain, parasites, thyroid problems, kidney disease, and diabetes all cause weight loss, and they need a vet to find them.

What should I do first if my cat is underweight?

Feed about 10% more, split into two or three meals a day, and re-weigh in a month. If the pet is eating well and still losing weight, or if it has gone off its food, see a vet promptly. See a vet before starting if the cat needs to lose or gain a lot of weight, or if it is a senior or a young animal.

How fast should a cat gain weight?

Slowly — about 10% more food at a time, re-weighed monthly. A pet that gains weight fast is gaining fat, not muscle, and a pet that is losing weight despite eating well needs a vet check, not just more food.

Do treats count?

Yes — treats, table scraps, and food from other pets' bowls all count, and they often add up to more than the meal itself. Keep treats under 10% of daily calories, use the cat's own food as treats or a single low-calorie treat, and agree in the household who feeds the cat so it is not fed three times by three people.

When should an underweight cat see a vet?

See a vet promptly if the cat is losing weight without a change in food, if it is lethargic or off its food, if it is a senior, if it is a young animal that is not growing, or if it needs to lose or gain a lot of weight. A vet check first is sensible because some medical conditions cause weight change, and those need treating, not just more or less food.

Results on this page are estimates for general information only and are not veterinary advice. If your pet is ill, underweight, overweight, pregnant, or has a health condition, consult your veterinarian before changing diet or care.