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Cat BMI Calculator

Estimate feline body fatness from rib cage girth and lower leg length measurements.

Formula shown2 inputs definedUpdated Nov 2025By CalcuTotal TeamFree, no sign-up

Quick Summary

Use this when you need:
  • Screen indoor cats for overweight status using simple tape measurements
  • Track FBMI over time while adjusting diet or activity
  • Discuss body condition numerically with a veterinarian
You’ll need:Rib cage circumference, Lower hind leg length
You’ll get:Feline BMI

Calculator Tool

How this is calculated

Formula:

FBMI = (((ribcage / 0.7062) - leg_length) / 0.9156) - leg_length

In plain language:

The Hawthorne & Butterwick method rescales rib cage circumference by constants, subtracts hind leg length twice, and outputs a feline body mass index number correlated with body fat.

The formula is always visible so you can verify the math and explain your numbers to anyone who asks.

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What these terms mean

Rib cage circumference

Rib cage circumference used in the calculation.

Lower hind leg length

Lower hind leg length used in the calculation.

Feline BMI

Feline BMI used in the calculation.

When to use this calculator

  • Screen indoor cats for overweight status using simple tape measurements
  • Track FBMI over time while adjusting diet or activity
  • Discuss body condition numerically with a veterinarian
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Medical Disclaimer

This calculator is for informational and educational purposes only and should not be considered medical advice.

The results provided are estimates based on the information you enter and should not be used as a substitute for professional medical advice, diagnosis, or treatment.

Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have calculated here.

If you think you may have a medical emergency, call your doctor or emergency services immediately.