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FBMI uses body weight divided by the square of rib cage circumference (or nose-to-tail length in some protocols). The result is compared against breed-adjusted reference ranges.
FBMI = Weight(kg) / (Rib_cage_circumference(m))²FBMI correlates with the 9-point BCS scale: BCS 4–5 is ideal (FBMI ~40–45), BCS 6–7 is overweight (FBMI ~46–55), BCS 8–9 is obese (FBMI >55).
Ideal FBMI range: 38–46 for domestic shorthairLarge breeds (Maine Coon, Norwegian Forest) have higher normal FBMI than small breeds (Singapura, Devon Rex). Use breed-specific charts when available.
Target weight = breed_ideal_FBMI × rib_cage²Updated: July 2026
A 4.5 kg cat with 25 cm rib cage circumference at ideal body condition.
→ FBMI: 72 — note: use rib cage in meters; clinical FBMI ~42–45 at BCS 5
A 6.8 kg cat with 28 cm rib cage, BCS 7/9, showing abdominal fat pad.
→ FBMI elevated above ideal; target weight reduction of 15–20% recommended
Same cat after 12-week diet program, now 5.9 kg with unchanged rib cage.
→ 13% weight loss achieved; continue gradual reduction — safe rate maintained
Weigh at consistent times, ideally before morning feeding. A full bladder adds 50–100 g and a large meal adds 30–50 g, skewing FBMI calculations.
Measure rib cage at the 9th rib behind the front leg, snug against skin but not compressing tissue. Long-haired cats require parting fur for accurate measurement.
Cats must lose weight slowly — 0.5–1% of body weight per week maximum. Faster loss risks hepatic lipidosis (fatty liver disease), which is life-threatening.
Feline body mass index (FBMI) estimates body fat composition using weight relative to body frame size. Veterinarians and pet owners use FBMI alongside the 9-point body condition score to identify overweight and obese cats, which are predisposed to diabetes and joint disease.