Cat BMI calculator explained
Feline Body Mass Index (FBMI) uses rib cage girth and lower-hind-leg length to approximate a cat's percent body fat. The method, developed by Hawthorne and Butterwick, correlates well with veterinary body condition scoring and gives cat owners a way to monitor progress between clinic visits. This calculator runs the FBMI equation in any unit set and reports the resulting index instantly.
How the conversion works
FBMI is computed from two tape measurements in centimeters or inches. Let be rib cage circumference and be lower hind leg length (from the patella to the calcaneus). Hawthorne and Butterwick derived:
Measurements entered in inches are converted to centimeters before the formula runs. Scores around 15 indicate a lean adult, while values above 20 suggest overweight status.
Units and conversions
| Measurement | Units | Notes |
|---|---|---|
| Rib cage | cm, in | Wrap the tape just behind the front legs while the cat exhales. |
| Leg length | cm, in | Measure from the midpoint of the patella to the calcaneus while the cat stands. |
| FBMI | dimensionless | Interpreted relative to published cutoffs (lean approx 15, overweight approx 20, obese ≥ 29). |
Worked examples
- Average indoor cat
Rib cage 35 cm, leg length 20 cm.
Score 12.3 suggests a lean to ideal body condition.
- Weight-management candidate
Rib cage 44 cm, leg length 19 cm.
An FBMI near 21 lines up with overweight status, signaling that a veterinary weight-management plan is warranted.
Tips and pitfalls
- Take measurements twice and average them; small tape errors lead to large FBMI swings.
- Measure after the cat exhales and while standing to avoid inflated rib cage readings.
- Use FBMI as a trend line; track the number monthly to confirm diet and enrichment changes are working.
- Always confirm findings with a veterinarian, especially for brachycephalic or long-haired breeds where palpation remains essential.