FBMI = (Rib Cage / 0.7062 − Leg Length) / 0.9156 − Leg Length
The FBMI was developed by the Waltham Centre for Pet Nutrition to estimate cat body fat percentage from two simple measurements: rib cage circumference (at the 9th rib) and lower hind leg length (knee to ankle). Ideal FBMI is 15-30% body fat. Over 60% of cats in developed countries are overweight or obese, leading to diabetes, arthritis, hepatic lipidosis, and shortened lifespan. The Body Condition Score (BCS) on a 1-9 scale is the most common veterinary assessment: 4-5 is ideal. Weight management: reduce calories by 15-20%, increase play time, and use measured feeding rather than free-choice. Even a 1-pound loss in a 10-lb cat is significant (10% of body weight).