Cat BMI Calculator
Use our cat BMI calculator to estimate FBMI (Feline Body Mass Index) from rib cage circumference and lower back leg length. Compare your result to a simple cat BMI chart to see if your cat may be underweight, normal weight, overweight, or obese.
What Is Cat BMI (FBMI)?
Cat BMI (often called FBMI) is a measurement-based way to estimate whether a cat may be underweight, at a normal weight, overweight, or obese. Unlike a human BMI calculator (height and weight), FBMI uses body measurements that better reflect a cat’s frame and fat distribution.
This calculator uses two measurements taken while your cat is standing: rib cage circumference (around the 9th rib area) and the length of the lower back leg (knee to ankle). It then returns an FBMI value you can compare to a cat BMI chart.
If you’re asking “is my cat overweight?”, FBMI is one helpful screening tool. It does not replace a veterinarian’s assessment, but it can help you track trends over time.
Cat BMI Formula
This calculator uses a common FBMI equation based on rib cage circumference and lower back leg length.
R is rib cage circumference and L is lower back leg length (use the same unit for both, like inches).
Cat BMI Chart
Compare your FBMI result to these common ranges.
| FBMI | Interpretation |
|---|---|
| < 15 | Underweight |
| 15 – 29.9 | Normal weight |
| 30 – 42 | Overweight |
| > 42 | Obese |
How to Measure and Calculate Cat BMI
- 1
Have your cat stand normally with legs under the body.
- 2
Measure rib cage circumference (in) around the chest near the front legs (around the 9th rib area).
- 3
Measure lower back leg length (in) from the knee to the ankle (not the paw).
- 4
Enter both measurements to calculate FBMI.
Frequently Asked Questions
FBMI can help estimate weight category, but also look for signs like difficulty feeling ribs, no visible waist from above, and a rounded belly. If you’re unsure, a vet can confirm with a body condition assessment.
There is no single number that fits all cats because frame size and breed vary. That’s why tools like FBMI (and body condition scoring) can be more useful than a fixed weight cutoff.
Cats have different body proportions, coats, and frame sizes. A measurement-based index like FBMI can be more informative than weight alone when comparing cats of different builds.
It’s a helpful estimate and a good way to track changes over time, but it’s not perfect. Hydration, posture, measurement errors, and individual body shape can affect the result.
Start with a vet check to rule out medical causes and to set a safe plan. Small changes like measured portions, fewer treats, and more daily play can help with gradual weight loss.