BMI stands for body mass index, but you’ll almost always find it referred to simply as BMI. It’s an estimate of how much body fat a person has, and it’s calculated by dividing a person’s weight in kilograms by his or her height in square meters.
Don’t be intimidated by the number crunching — there are loads of online calculators that will generate your BMI when you put in your stats. The resulting number can help you determine whether you’re at a healthy weight. Here’s what your number means:
- Less than 18.5 = underweight
- 18.5 to 24.9 = normal weight
- 25 to 29.9 = overweight
- 30 or higher = obese
BMI has long been a popular tool for measuring body fat because it’s easy to use and doesn’t require any fancy equipment to calculate. There’s a downside to that simplicity, though: It sometimes delivers an oversimplified picture of your health (more on that later).
According to this measure, more than one-third of American adults is considered obese, notes the Centers for Disease Control and Prevention.
How Does BMI Differ Among Men, Women, and Other Groups of People?
The BMI formula is universal — it’s the same for both adults and children (though the numbers are interpreted differently for young people because gender and age are factored in). Among adults, BMI is interpreted the same way for both men and women, says Michelle Jaelin, RD, a Hamilton, Ontario–based registered dietitian and blogger at NutritionArtist.com.
But as the Centers for Disease Control and Prevention (CDC) notes, there are a few differences among certain demographics when it comes to body fat. (3)
- Women usually have more body fat than men. According to the AARP, women should aim for 20 to 21 percent body fat, while men should have between 13 and 17 percent.
- Black people usually have less body fat than white people, and Asians typically have more than white people.
- Older people generally have more body fat than younger people.
- Athletes usually have less than nonathletes.
BMI tends to be problematic among older people, Jaelin says. According to one study, BMI isn’t as useful in older adults because it doesn’t account for the fact that many people get shorter as they age, which can lead to underestimated fat levels. BMI also can underestimate fatness among seniors because as people age, fat mass usually replaces fat-free mass (muscle). So while an older adult may clock a normal BMI, he or she could have a high body fat percentage. The researchers call this “normal-weight obesity,” which puts people at an increased risk for metabolic syndrome and a variety of cardiovascular issues.
These discrepancies have led some researchers to suggest that BMI targets should be different for older adults. One meta-analysis explored the relationship between BMI and risk of death among people 65 and older and found that the lowest risk of death was among people with a BMI of around 27.5 — which qualifies as overweight. The study found that in older people a BMI between 22 and 23 actually increased the risk of death, even though it’s in the normal range.