BMI v. Body Fat: What’s the difference and how can you measure it?

There are a lot of terms and numbers thrown at you when you are trying to understand your body composition.  Here is a short and easy guide on BMI, Body fat, and the many ways you can measure it.

What is BMI?  
BMI refers to your Body Mass Index.  You can calculate BMI and find your general fitness category here.  It is a ratio of your height to your weight, and in a way can be completely unrelated to your body fat, based on your muscle composition.  Here is how:  Imagine a 5’7″ woman who weighs 160 lbs.  That puts her in the “overweight” category.  However, you find out she is actually a competitive weightlifter, with a large amount of muscle mass, and little actually body fat.  BMI does not differentiate between fat and muscle.

What is Body Fat?   
When fitness professionals refer to body fat, they are referring to subcutaneous fat, that is the fat that sits directly under the skin.  Having excess subcutaneous fat can become problematic for your health, especially if most of that fat is located around the midsection, around your vital organs.  Body fat can be measured many different ways, and they all have their pro’s and con’s.  Further, every method isn’t 100 percent accurate, so it is best to look at your body fat measurement as more of an estimate.

Here is a list of all of the different ways body fat can be measured.  I have included my own measures from each method, just to show you how numbers can vary even in the same person across methods.

1) Skin fold measurement:  This test estimates subcutaneous body fat by measuring folds of skin at various anatomical locations.
fat1

Pros: inexpensive, quick, easy.
Cons: You are literally having your fat pinched.
Accuracy: +/- 3.5%
My Measurement: 16.8%  for the 7-site measurement

2) Bioelectrical Impedance Analysis:  This test runs a low electrical current through either your feet or your hands (depending on which machine you use) to estimate body fat.  The faster the current is able to go, the lower it assumes your body fat to be.   

fat2
Pros: Portable, inexpensive, easy, quick.
Cons: Highly variable based on hydration, time of day, women’s menstrual cycle, etc.
Accuracy: +/- 8%
My Measurement: 17% through hands, 19.5% through feet.

3) Underwater weighing/Hydrodensitometry:  This measure is considered the gold standard for exercise professionals.  The idea is that fat weighs less in water (it floats!), so the difference between your land weight and underwater weight can give you an estimate of your body fat percentage.fat3

Pros: Most accurate
Cons: requires special equipment, estimates lung volumes, difficult to perform (you have to completely submerge yourself and blow all of the air out of your lungs – then hold it for a few seconds)
Accuracy: +/- 3%
My Measurement: 14.6%

4) Bod Pod/air displacement plethysmography: This method uses the same idea as underwater displacement, but with air in a an oval “pod” that you sit in.
fat4
Pros: Easy, fairly accurate
Cons: expensive, special equipment, some people may not fit/will become claustrophobic.
Accuracy: +/- 3.4%

5) DEXA (Dual-energy X-ray absorptiometry): Dexa uses low dose x-rays to measure body fat as well as bone density.  This method is accurate, but you only get a 2D representation, so some professionals question its accuracy.

fat5
Pros: Accurate, easy, you also get bone density information.
Cons: Expensive, special equipment, radiation, and some people may not fit on the table.
Accuracy: +/- 1.6%
My Measurement: 21%

Take Home Message:
Every method has its pro’s and con’s, and not one method is 100% accurate (notice, my numbers ranged from 14-21%, thats a pretty big difference!).  If you have the opportunity to have your body fat measured, go for it, and look at is as an estimate.  Try and do it first thing in the morning in a fasted state, and stay consistent in that every time you get re-measured; that way will give you the best idea of how much fat you have lost, rather than water weight or daily fluctuation.  Otherwise, know its just another number, and try to go off of how you feel and how your clothes fit.

Happy Thursday!

-Juliane, ACSM CEP

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3 thoughts on “BMI v. Body Fat: What’s the difference and how can you measure it?

    1. Thank you! I had some friends get an assessment at their local gym and they got really hung up on the numbers, and didn’t even realize that BMI and body fat were different!

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      1. Alot of people don’t understand the difference! I’m glad that you let them know! I have alot of friends who get SOOOO discouraged about what the scale says!

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