Cat BMI Calculator
Use our cat BMI calculator to estimate feline body mass index from rib cage and leg length. Convert units easily and get FBMI instantly.
Cat BMI Calculator
Result will appear here...
What this does
So you want to know if your cat is carrying too much weight, and the scales alone do not really answer that. This does better. It uses the Feline Body Mass Index, which estimates your cat's body fat from two simple measurements.
How to use it
- Measure your cat's rib cage circumference, and enter it with its unit.
- Measure the lower back leg length, and enter it with its unit.
- Tap Calculate to see the FBMI. Centimetres give the cleanest result.
A soft tape measure and a calm, standing cat make this much easier.
How it works
The Feline Body Mass Index was built by researchers to estimate body fat, and it was checked against DEXA scanning, the same body fat scan used in studies, which it matched closely. From your two measurements in centimetres, the formula is:
FBMI = ((rib cage ÷ 0.7062) - leg length) ÷ 0.9156 - leg length
The result is an estimate of your cat's body fat. Whatever units you enter are converted to centimetres first, and it all runs on your device.
How to take the measurements
Accuracy here comes down to measuring in the right spots, with your cat standing and their head upright. For the rib cage, measure the circumference all the way around the chest at the level of the 9th rib, which sits just behind the front legs. For the leg, measure the lower back leg, from the knee to the ankle, so the section between those two joints, not down to the paw. Use a flexible tape and take each measurement twice to be sure, since a centimetre out can move the result.
Reading the result
As a rough guide, an FBMI under about 15 suggests an underweight cat, somewhere between 15 and 30 is the healthy range, and above 30 points toward overweight, climbing into obese from the low 40s. Like any single measure, it is a guide rather than a diagnosis. Pair it with the simple rib test, you should be able to feel your cat's ribs under a light covering, and with a vet's view. Cat obesity is common and creeps up slowly, so a number like this is a handy early warning to act on.
Questions people ask
Is this a real measure?
Yes. The Feline Body Mass Index was developed and tested against DEXA body fat scans, so it is a genuine estimate of body fat, not just a weight to height ratio.
What is a healthy FBMI?
Roughly 15 to 30. Below that suggests underweight, and above it suggests overweight, with obese in the low 40s and up.
Where exactly do I measure the leg?
The lower back leg, from the knee joint down to the ankle. Do not include the paw.
Should I still see a vet?
Yes. Use this alongside the rib test and a vet's body condition assessment, especially before starting any weight loss plan.
References
- The Feline Body Mass Index (FBMI): a simple measure of body fat content in cats. https://www.felinediabetes.com/FelineBMI.pdf
Dr. Anup Paudel is an MBBS doctor and medical officer with expertise in general medicine and health information. Alongside his medical work, he has a personal interest in pet health and practical animal care information. At Eon Tools, he reviews pet tools.