Cat Body Condition Calculator
Never crash-diet a cat — one that stops eating can develop hepatic lipidosis within days.
The formula
ideal weight = current weight / (1 + 0.10 * (BCS - 5))
There is no pet BMI
Body mass index does not transfer to animals: a whippet and a bulldog of identical weight and height are differently shaped in ways no ratio captures, and the variation between dog breeds dwarfs anything seen in humans. What veterinary medicine uses instead is a body condition score — a nine-point scale assessed by looking and by feel.
What you are feeling for
At an ideal score of 5 the ribs are easy to feel under a thin layer of fat but not visible, there is a waist when viewed from above, and the belly tucks up when viewed from the side. Each point above 5 is worth roughly 10 % of body weight, which is what makes the arithmetic below possible at all.
Losing it slowly
Safe weight loss is about 1 % of body weight per week for dogs and slightly less for cats. Cats in particular must never be crash-dieted: a cat that stops eating can develop hepatic lipidosis within days, and that is a far more dangerous condition than the obesity being treated.
This does not replace a veterinarian
Every figure on this page comes from a published reference range, and a range is what it stays. The right dose for an individual animal depends on its kidney and liver function, on what else it is taking, on whether it is pregnant or very young or very old, and on what is actually wrong with it — none of which arithmetic can see. Use this to understand the scale of a dose or an exposure, and to have a better conversation with your vet. Do not use it to start, stop or change a medication.
If you believe your animal has swallowed something toxic, do not wait to see whether signs develop. Contact your veterinarian or an animal poison line immediately — in most poisonings the treatment that works is the one given early.