Your BMI (body mass index) and body fat percentage are two very distinct things. So how do you know which one to measure in order to determine your ideal weight? People often use the two terms interchangeably, adding to the confusion. With an understanding what each is used to gauge, you’ll be able to chart your journey towards a healthier you.
BMI stands for body mass index. The determining factors for BMI are height and weight. It’s a very rough formula that doctors use to gauge your health.
The formula for calculating BMI is: (weight in lbs. x 703) / (height in inches x height in inches). If the resulting number is less than 18.5, it means you are underweight. If it falls between 18.5 and 24.9, your weight is normal, if it falls between 25.0 and 29.9 you are overweight and 30.0 and above lies in the obese category.
Related: Calculate Your BMI
BMI And Muscle Mass
One of the major flaws of BMI is that it doesn’t give you an accurate picture of how much of your weight is fat and how much is muscle. People who strength train or exercise regularly tend to have a higher proportion of muscle weight. Consequently, on a BMI scale, they may fall in the overweight category, even though they are physically fit and look lean.
By BMI’s standards, actor Dwayne “The Rock” Johnson is technically obese, with a BMI of 34.3. His Furious 7 co-star, Vin Diesel, would also come in at overweight, with a BMI of 27.1.
Of course, no one would look at these muscular, fit men and tell them they’re unhealthy or unfit. In fact, it’s pretty normal for a star athlete or body builder to have a relatively high BMI, because most of their weight is made up of lean muscle and not fat.
This is where body fat percentage comes in. Body fat percentage is quite literally the composition of fat on your body. Women naturally have more fat on their bodies than men, so the ideal numbers are different for each gender.
Women ages 18 to 40 should have body fat percentages between 20 per cent and 30 per cent, while women between 40 and 59 should fall between 25 and 35 per cent. Anything higher than 40 per cent body fat is considered obese.
Men between the ages of 20 to 29 should be considered to have normal body fat between 16 per cent to 20 per cent. Men’s body fat tends to increase after age 30 as it’s harder to maintain muscle. From age 30 to 39 the range is 19 to 22 per cent. From age 40 to 49, it’s 21 and 24 per cent. Between age 50 and 59, this jumps a few percentages, to between 22 and 26 percent. Men over 60 should aim for a body fat percentage of 23 to 26 per cent.
Body Fat Percentage = Fat Mass ÷ Body Weight
Generally speaking, the healthier and fitter you are, the less body fat you will have (except in extreme cases, of course).
Let’s take an example of two women, Rochelle and Fiona. They’re both the same height and weight; 5’4” and 140lbs. With these numbers we find they both have a BMI of 24, which falls in the normal range. Except the two girls have very different lifestyles.
Rochelle eats mostly junk food, but she eats smaller portions so as to not gain weight. She never works out. According to her body fat percentage (or BF) she has 42 pounds of fat and 98 pounds of lean mass. This brings her BFP to 30.
Fiona, on the other hand, weight trains regularly, and does cardio three days a week. She also eats a healthy, clean diet. Fiona has 28 pounds of fat and 112 pounds of lean mass, this brings her BFP to 20 per cent.
Even though their BMIs are the same, their body compositions are very different and their BFPs reflect that.
BMI And Skinny Fat
“Skinny fat” is a term for when your BMI technically falls into the normal or fit category, but you have much more body fat than you should.
How Do I Calculate My Body Fat Percentage?
There are many ways through which you can determine your body fat percentage. Unfortunately, as the accuracy of the test increases so does the expense. Here are some options:
- DEXA scan: This is a full body X-ray, the kind used to determine bone density. It’s quite accurate.
- Hydrostatic Weighing: The body fat is measured while weighing you submerged underwater. Also very accurate.
- Skin-fold calipers (“pinch test”): This is fairly simple but it’s not very accurate unless done by someone who’s had lots of training.
- Bioelectrical Impedance (BIA): These plates and scales run a low-level current through your body. For most accurate results it should be done first thing in the morning, when you haven’t had an alcoholic drink in at least the last two days.
- Navy tape measure method: This method is used in the military, specifically the navy. For best results measure in centimeters or to 1/4th of an inch. Take three readings and use the average.
Though not always accurate, BMI can nonetheless be a helpful, crude tool to quickly assess most people’s general, physical health.
However, BMI does not always accurately depict how fit or unfit someone really is. Body fat percentage measurements, especially the more scientific methods, can therefore be more useful in determining how fit you really are.