Cat BMI Calculator (feline body fat % from two measurements)

Understanding the cat body fat calculator — concept and principle

This calculator estimates a cat's body fat percentage from two tape measurements, the feline body mass index (FBMI) method. Owners and vet staff use it to tell whether a heavy cat is truly overweight or just large-framed, and to track a weight-loss plan more closely than with the scale alone.

Two tape measurements: with the default inputs the result is 24.3% body fat (ideal range 15–30%).

How it is calculated

Body fat % = ((ribcage ÷ 0.7067) − leg index) ÷ 0.9156 − leg index

Ribcage: circumference around the chest at the level of the 9th rib, in cm.
Leg index measurement (LIM): from the middle of the kneecap to the point of the hock on the left hind leg, in cm.
Inches are converted with 1 in = 2.54 cm before the formula is applied.
If you enter weight: fat mass = weight × body fat % ÷ 100, lean mass = weight − fat mass.
If body fat is above 30%, the calculator also shows the weight at 30% fat with the same lean mass: lean mass ÷ 0.7.

The principle

The ribcage grows when fat builds up around the chest, while the leg length reflects the skeleton and barely changes with weight. Comparing the two separates "big" from "fat". WALTHAM researchers fitted the two constants by comparing tape measurements with body fat measured in the laboratory, which is why the formula looks unusual. In their work, cats judged ideal on a body condition score had between 15% and 30% body fat, and that range is used here as the reference.

Worked example

Default inputs: centimetres, ribcage 36 cm, leg index 15 cm, weight 4.5 kg.

36 ÷ 0.7067 = 50.94.
50.94 − 15 = 35.94; 35.94 ÷ 0.9156 = 39.25.
Body fat = 39.25 − 15 = 24.3%, within the ideal range of 15–30%.
Fat mass = 4.5 × 0.2425 = 1.09 kg; lean mass = 4.5 − 1.09 = 3.41 kg.
Because body fat is not above 30%, no target weight is shown (—).

How to read the result

Where does your result fall?

Between 15% and 30% matches cats judged ideal. Below 15% suggests a lean cat; above 30% suggests excess fat. The target weight at 30% fat assumes the lean mass stays the same, so it is the most weight a cat could lose as fat alone; a vet may suggest a different target. Tracking the percentage every few weeks shows whether a diet is reducing fat rather than muscle.

Common mistakes

Measuring the ribcage behind the wrong rib or with the tape too loose.
Measuring the leg from the toe or the hip instead of kneecap to hock.
Measuring a cat that is lying down or curled up.
Relying on a single reading; small errors change the result a lot, so average two or three.

Keep in mind: This is an estimate, not a diagnosis. Weight loss in cats must be slow and supervised, because fast loss can cause serious liver disease. Ask your vet before changing your cat's diet.

Frequently asked questions

What is a healthy body fat percentage for a cat?

In the FBMI research, cats of ideal body condition had about 15–30% body fat.

Why not use weight alone?

A heavy cat can simply be large. Comparing chest size with leg length tells you more about fat.

Can I use inches?

Yes. Choose inches and the calculator converts them to centimetres before applying the formula.

Sources: Hawthorne A. J., Bradley R., Butterwick R. F., US Patent 6,969,350 B1 "Body fat measurement system" (2005) · Butterwick R. F., Hawthorne A. J., J Vet Intern Med 2000;14:365 (abstract)

Checked 9 Nov 2026. Hawthorne A. J., Bradley R., Butterwick R. F. (Mars UK), US Patent 6,969,350 B1 “Body fat measurement system” (2005) — equation and constants 0.7067 and 0.9156, and the 15–30% body fat seen in cats of ideal body condition; method first reported by Butterwick & Hawthorne, J Vet Intern Med 2000;14:365 (abstract).

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