MasterTool

Cat BMI Calculator

Calculate the feline body mass index (FBMI) and estimated body fat from ribcage and leg measurements.

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What this tool does

Calculate the feline body mass index (FBMI) and estimated body fat from ribcage and leg measurements. Both measurements are in centimetres with the cat standing. The fat-percentage equation was validated against reference body-composition methods in the original Waltham research; long coats and a tense, hunched cat distort the ribcage tape reading.

How to use the Cat BMI Calculator

  1. Enter or select ribcage circumference at the 9th rib (cm).
  2. Enter or select leg index measurement — patella to calcaneus (cm).
  3. Read the calculated result; change any measurement to compare alternatives.

Formula

feline body mass index (Hawthorne & Butterwick, Waltham): FBMI = ((ribcage cm ÷ 0.7067) − leg index cm) ÷ 0.9156 − leg index cm; estimated body fat % = 1.54 × ribcage − 1.58 × leg index − 8.67
ribcage
Ribcage circumference at the 9th rib (cm)
leg
Leg index measurement — patella to calcaneus (cm)

Both measurements are in centimetres with the cat standing. The fat-percentage equation was validated against reference body-composition methods in the original Waltham research; long coats and a tense, hunched cat distort the ribcage tape reading.

Worked example

For cat bmi calculator, the following measurements illustrate the exact method: Ribcage circumference at the 9th rib (cm): 45; Leg index measurement — patella to calcaneus (cm): 15.

Inputs

  • Ribcage circumference at the 9th rib (cm)45
  • Leg index measurement — patella to calcaneus (cm)15

Result

  • Feline BMI (FBMI)38.16
  • Estimated body fat (%)36.93
  • Screening bandObese band

Results explained

Feline BMI (FBMI)
Feline BMI (FBMI) from the formula above. Both measurements are in centimetres with the cat standing. The fat-percentage equation was validated against reference body-composition methods in the original Waltham research; long coats and a tense, hunched cat distort the ribcage tape reading.
Estimated body fat (%)
Estimated body fat (%) from the formula above. Both measurements are in centimetres with the cat standing. The fat-percentage equation was validated against reference body-composition methods in the original Waltham research; long coats and a tense, hunched cat distort the ribcage tape reading.
Screening band
Screening band from the formula above. Both measurements are in centimetres with the cat standing. The fat-percentage equation was validated against reference body-composition methods in the original Waltham research; long coats and a tense, hunched cat distort the ribcage tape reading.

Frequently asked questions

Run a hand along the ribcage and count back — the tape goes around the chest at the level of the 9th rib, snug but not compressing. The leg index (LIM) is the length of the lower hind leg, from the middle of the kneecap (patella) to the heel bump (calcaneus), with the leg extended.

Most healthy indoor cats sit around 15–25% body fat. Above roughly 30% is overweight territory and 40%+ is obese — confirm with your vet's body condition score before starting a diet.

FBMI deliberately replaces the scale with two tape measurements — one that grows with fat (ribcage) and one fixed by frame size (leg length) — so it estimates composition, not just mass. A big-framed cat can weigh more and still be lean.

Thick fur adds centimetres to the ribcage tape. Part the coat and keep the tape against the skin without pulling tight, and repeat the measurement twice for consistency.

No. It is a screening estimate. Cats must lose weight slowly (crash diets risk hepatic lipidosis), so any cat in the overweight or obese band needs a vet-set feeding plan.