How this instrument works
Human BMI works because it divides weight by height squared, and a bathroom scale plus a tape measure are all anyone needs. Cats break that shortcut: a cat's weight alone says very little about body fat without knowing its frame size, and most owners don't have an easy way to measure a cat's 'height' the way they would a person's. The Feline Body Mass Index (FBMI) works around this by using two circumference-and-length measurements instead — rib cage circumference and leg length (measured from the middle of the kneecap to the point of the hock) — both of which correlate strongly with a cat's actual body fat percentage as measured by dual-energy X-ray absorptiometry (DEXA), the imaging method researchers use to validate body-composition formulas.
This calculator applies that formula directly: FBMI equals rib cage circumference divided by 0.7062, minus leg length, all divided by 0.9156, minus leg length again. The two constants (0.7062 and 0.9156) are regression coefficients fitted to DEXA-measured body fat data, not arbitrary round numbers — they are what make the formula's output track real, imaging-confirmed body fat percentage rather than just producing a plausible-looking ratio.
The FBMI formula was developed by Hawthorne and Butterwick and published in 2000. It is not, as some later sources have claimed, an Ohio State University formula — that attribution is a documented mix-up. It is a genuinely feline-specific method, distinct from adapting a human or canine BMI formula, and the resulting FBMI score is designed to be read against feline-specific bands (roughly: under 15 underweight, 15 to 30 ideal, 30 to 42 overweight, above 42 obese) rather than the human BMI scale.
- Measure rib cage circumference with a soft tape measure wrapped around the widest part of the chest, just behind the front legs, and enter it in centimeters into Rib cage circumference (cm).
- Measure leg length from the middle of the kneecap to the point of the hock (the backward-facing 'ankle' joint on the hind leg) and enter it in centimeters into Leg length (cm).
- Read Feline BMI (FBMI) directly beneath both fields — it recalculates immediately as either measurement changes.
- Take both measurements on a calm, standing cat for consistency — a hunched or sitting cat can throw off the rib cage reading enough to shift the result into a different band.
- Treat the FBMI score as a screening estimate, not a diagnosis — a veterinarian assessing body condition in person, with hands-on palpation, remains the more reliable judge of an individual cat's weight status.
Worked example — a 32 cm rib cage, 12 cm leg length
A domestic shorthair has a rib cage circumference of 32 cm and a leg length of 12 cm — proportions in the range of an average adult cat. Dividing 32 by 0.7062 gives about 45.31; subtracting the 12 cm leg length brings that to about 33.31. Dividing by 0.9156 gives about 36.38, and subtracting the leg length one more time lands on an FBMI of 24.38.
That 24.38 sits comfortably inside the paper's 'ideal' band, which runs from 15 to 30 — well clear of both the underweight line at 15 and the overweight line at 30. It illustrates why the formula subtracts leg length twice: a cat with a genuinely larger frame (longer legs) needs a proportionally larger rib cage just to land in the same 'ideal' range as a smaller-framed cat, since both subtractions scale with leg length.
Questions
Is FBMI the Ohio State University cat BMI formula?
No — that is a mix-up that has circulated in some secondary sources, but the formula this calculator uses was developed by Hawthorne and Butterwick and published in 2000, not by Ohio State University. Ohio State's veterinary program has published its own separate feline weight-management materials, and it's easy to see how the two got conflated online, but they are different resources with different origins. This calculator uses the original Hawthorne and Butterwick formula and cites it accordingly.
How do I measure a cat's leg length for this formula?
Leg length here means the hind leg, measured from the middle of the kneecap (the patella) down to the point of the hock — the backward-angled joint partway down the hind leg that corresponds to a cat's 'ankle.' Use a flexible tape measure with the cat standing normally, and take the reading in centimeters. A leg measured while the cat is sitting or with the joint flexed unnaturally can read shorter than its true standing length.
Why use rib cage and leg length instead of just weighing the cat?
Because weight alone can't distinguish a large-framed lean cat from a small-framed overweight one — two cats of identical weight can have very different body fat percentages depending on frame size. FBMI uses rib cage circumference as a proxy for body fat and mass, and leg length as a proxy for skeletal frame size, so the formula effectively normalizes fat-related bulk against how big the cat's frame actually is, similar in spirit to how human BMI normalizes weight against height.
What do the FBMI score bands mean?
As commonly reported alongside the Hawthorne and Butterwick formula, an FBMI under 15 is considered underweight, 15 to 30 is the ideal range, 30 to 42 is overweight, and above 42 is obese. These bands are a useful reference point for interpreting the number this calculator produces, but a veterinarian's hands-on body condition assessment remains the more reliable way to judge an individual cat's actual weight status, especially near a band boundary.
Does this formula work the same way for kittens or very large-breed cats?
The formula was developed and validated using adult domestic cats, so its constants reflect typical adult proportions. A growing kitten's leg-to-frame ratios change rapidly as it develops, and an unusually large-framed breed can sit outside the proportions the original study measured — in both cases, treat the FBMI result as a rougher estimate and lean more on a veterinarian's in-person assessment rather than the number alone.