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Cat BMI Calculator

Calculate your cat’s body fat with the Feline Body Mass Index (FBMI): measure the rib cage and the back leg to see if your cat is underweight, ideal, overweight or obese, plus a target weight.

Units

Rib cage circumference (cm or in)

Lower back leg length (cm or in)

Current weight (kg or lb)

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How it works

body fat % = [(rib cage circumference ÷ 0.7062) − leg length] ÷ 0.9156 − leg length (both in cm) · ideal weight ≈ weight × (1 − body fat) ÷ 0.80

The Feline Body Mass Index was developed at the WALTHAM Centre for Pet Nutrition (Butterwick, 2000) and estimates body fat from two tape-measure readings checked against DEXA scans: the rib cage circumference at the 9th rib and the leg index measurement from the knee to the hock. A body fat of 15–29% is ideal, 30–42% overweight and above 42% obese. The ideal-weight estimate keeps the cat’s lean mass and assumes a target of 20% fat. Measure with the cat standing and the legs vertical, and repeat a few times for a steady reading.

Worked example

A 5 kg cat with a 34 cm rib cage and a 13 cm lower leg: 34 ÷ 0.7062 = 48.14; 48.14 − 13 = 35.14; 35.14 ÷ 0.9156 = 38.38; 38.38 − 13 = 25.4% body fat — ideal. Lean mass = 5 × 0.746 = 3.7 kg, so the weight at 20% fat would be about 4.7 kg.

Sources: Butterwick R. How fat is that cat? J Feline Med Surg 2000;2(2):91–94 · WSAVA Global Nutrition Guidelines — body condition

Frequently asked questions

What is a healthy FBMI for a cat?

A body fat of 15–29% is ideal. 30–42% is overweight and more than 42% is obese; below 15% the cat is underweight and should see a vet.

How do I measure my cat for the FBMI?

With the cat standing, wrap a soft tape around the chest at the 9th rib (just behind the shoulder blades, level with the front of the elbows). Then measure the back leg from the middle of the kneecap to the point of the heel. Use the same units for both.

Is the FBMI better than weighing my cat?

Weight alone does not show whether a cat is big-boned or fat. The FBMI estimates body fat, and the 9-point body condition score done by a vet is the other standard method; using both is best.

How fast should an overweight cat lose weight?

Slowly — about 0.5–2% of body weight per week under a vet’s plan. Crash diets can cause hepatic lipidosis (fatty liver), which is life-threatening in cats.

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An estimate only; ask your veterinarian before changing your cat’s diet or starting weight loss.