Keeping your cat at the correct weight is crucial to maintaining the animal's health throughout its lifetime. A cat that is too heavy will suffer some of the same problems as a person who is obese, while a cat that is too thin is no healthier. Such cats can be more susceptible to disease and illness, and when they do get sick they may not have the strength to fight the disease. Cats may lose weight for a number of reasons, and any sudden change in your cat's appearance should be noted.
Keep track of the cat's normal weight. While this isn't necessarily possible for a newly adopted cat, one that has been a member of the family for years should have had numerous veterinarian checkups. Keep the number in mind, as well as the body shape and appearance the veterinarian has given approval to.
Weigh the cat. Most cats are at a healthy weight when they are between 3 and 4.5 Kgs (7 and 10 pounds). While there are some breeds that may be heavier or lighter than this, the standard is a good guideline to keep in mind.
Look at the cat from above when she is standing straight. There should be a distinct, hourglass shape to the cat, with a slightly narrowed waist behind the rib cage and before the hips. If the cat is too thin, this indentation at the waist will be severe instead of gradual, and the edge of the ribcage will be very clear. This will be a more effective method to use on short-haired cats.
Run your fingers down the cat's back. This works on short- and longhair cats. If the cat's spine if easily felt and especially if you can feel each one of the vertebrae, the cat is too thin. There should be a thin layer of fat along the cat's back.
Run your hands along the cat's sides. You should be able to feel the ribs on a cat with a healthy weight, but if the cat has a bony feel and each one of the ribs is prominent, she is too thin. The ribs are one of the best ways to measure whether or not a cat is at a healthy weight.
Look at the cat from the side when she is standing. If the bottom of the rib cage is clearly visible and there is a severe tuck up into the cat's body behind the ribs, she is too thin. While there should be a distinct waist, the bottom of the ribcage should blend into the belly of the cat.
Any sudden change in your cat's weight or appearance can signal illness. Cats will often hide any ill feelings, so they may not act sick even if they are.