A couple of weeks ago, I came across an article by Dr J (though twitter) on the Feline Body Mass Index. It's the same concept as the human BMI measurement where you look at a person's weight and height and work out the percentage of body mass made up of fat.
Being the math geek in me, the article had a formula for calculating a cat's body mass index so I thought I'd give it a whirl for my two 1 year old indoor cats, Rocket and Rosie (who will forever be known as The Kittens in our household).
Here's your cat’s FBMI calculation:
While your cat is in a standing position, with the legs perpendicular to the ground and the head upright (good luck with that!):
- Measure the circumference of your cat’s rib cage. (The level of the 9th rib is ideal.)
- Now measure the length of the lower back leg from the knee to the ankle and write down both numbers, because with what is coming, you probably won’t remember them.
Calculating the FBMI
- Divide the rib cage measurement by 0.7062 and subtract the length of the leg.
- Divide that answer by 0.9156.
- Subtract the leg length from that number, and that is your cat’s BMI.
And here it is in action for Rocket and Rosie (it was not easy to get them to stand still for the measurements - tape measures look like fun dangly things to feisty young cats):
| BMI calculation | Rosie | Rocket | Plump |
| (1) Rib cage | 15.0 | 15.5 | 32.6 |
| (2) Leg | 4.0 | 4.0 | 4.0 |
| (3) = (1) / 0.7062 -(2) | 17.2 | 17.9 | 42.1 |
| (4) = BMI = (3) / 0.9156 - (2) | 14.8 | 15.6 | 42.0 |
Since a cat is considered overweight, or plump, at a FBMI score of 42 or more, I also worked out what waist my cats would have to have to be considered overweight - the "plump" column in the table above.
Instead of 15-16 inches it is now, they'd have to double their girth to be considered overweight. Quite frankly, I'd be worrying well before that and putting them on a diet much sooner. I might consider the double girth as obese, not just overweight, but that's just me.
So instead of relying on a formula to know if your cat is overweight, follow the following rule of thumb:
If you can't feel your cat's ribs when you run your hand down her chest, she likely needs to lose a pound or two.
Some other tips from Dr J on managing your cat's weight:
- feed high quality food (we feed the kittens Wellness canned twice a day and some Core dried kibble to go with it)
- limit free feedings (we do free feed some Core during the day but only enough for both cats to be done during the day before night time feeding)
- exercise freely (both are indoor cats but we have play time in the evening and they love racing up and down their cat tree and chasing puff balls around the house)
It's hard keeping an indoor cat trim but definitely worth the effort I think. How do you manage with your cats?
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Measuring my cats' body mass index - the FBMI
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