Most owners can tell a cat is very thin or very round. The tricky part is the middle, where a cat that is a little too heavy just looks "fluffy". Vets use a simple hands-and-eyes check called a body condition score. You can do the same three checks at home in a minute, and no scales are needed.
Veterinary groups such as WSAVA score cats from 1 to 9, where 5 is ideal. The score comes from three things you can feel or see:
Long fur hides all of this, so use your hands first and your eyes second.
If you can't feel your cat's ribs and can't see a waist, it is time to talk to your vet, not to cut the bowl in half.
A cat is small, so even a little extra weight is a big share of its body. Vets link extra weight in cats to joint strain and a higher risk of illnesses such as diabetes. Royal Canin's vet guidance counts a cat as overweight at roughly 10–20% above its ideal weight, so for a 4 kg cat that is around half a kilo of extra padding.
Do not put a cat on a crash diet. Cats that eat too little or lose weight too fast can develop hepatic lipidosis, a serious liver problem. Weight-loss guidance from pet-obesity groups suggests a gentle pace of around 2% of body weight a month as a starting point, and says your vet's plan comes first.
See a vet if your cat is gaining or losing weight without a change in food, if the belly looks swollen rather than just round, if appetite or thirst changes, or if you can't feel the ribs at all. Never give human medicines or diet products. This guide helps you notice; only a vet can diagnose.