Body mass index, or BMI, is a number calculated from your weight and height that estimates whether you’re at a healthy weight. It’s used as a screening tool, not a diagnosis. A BMI between 18.5 and 25 is considered healthy for adults, while numbers above or below that range signal potential health concerns worth looking into.
How BMI Is Calculated
The math behind BMI is straightforward. You divide your weight by the square of your height. If you’re using metric units, that’s your weight in kilograms divided by your height in meters squared. In pounds and inches, you do the same division and then multiply by 703 to convert the result.
For example, someone who weighs 150 pounds and stands 5 feet 5 inches (65 inches) would calculate: 150 ÷ (65 × 65) × 703 = 24.96. That lands right at the upper edge of the healthy weight range. You don’t need to do this by hand. Free calculators from the CDC and other health organizations will do it instantly.
What the Numbers Mean for Adults
For anyone 20 and older, BMI falls into these categories:
- Under 18.5: Underweight
- 18.5 to 24.9: Healthy weight
- 25 to 29.9: Overweight
- 30 to 34.9: Class 1 obesity
- 35 to 39.9: Class 2 obesity
- 40 or higher: Class 3 (severe) obesity
These categories are the same for men and women. They were established based on large population studies linking BMI ranges to health outcomes, particularly the risk of chronic diseases like type 2 diabetes and heart disease.
BMI Works Differently for Children
A BMI of 22 means something very different for a 10-year-old than it does for a 35-year-old. Children’s bodies change rapidly, and what counts as a healthy weight shifts with age and sex. Instead of fixed cutoffs, pediatric BMI is expressed as a percentile, comparing a child’s number to other kids of the same age and sex.
For children and teens aged 2 to 20, a BMI between the 5th and 85th percentile is considered healthy weight. Below the 5th percentile is underweight. Between the 85th and 95th is overweight. At the 95th percentile or above, the classification is obesity. Pediatricians track these percentiles over time on growth charts, so a single reading matters less than the overall trend.
Why BMI Is Useful, and Where It Falls Short
BMI’s greatest strength is its simplicity. It requires nothing more than a scale and a tape measure, which makes it practical for routine checkups and population-level health tracking. For most people, it correlates reasonably well with body fat.
But it has real blind spots. BMI cannot tell the difference between weight that comes from fat and weight that comes from muscle or bone. Muscle and bone are denser than fat, so a muscular athlete can easily land in the “overweight” or even “obese” range while carrying very little body fat. On the other end, older adults who have lost muscle and bone density over the years may show a “normal” BMI while actually carrying a high percentage of body fat. BMI also doesn’t reveal where your fat is stored, which turns out to matter a great deal for health risk.
Where Fat Sits Matters Too
Two people with the same BMI can have very different health profiles depending on where they carry their weight. Fat that accumulates around the waist and internal organs (sometimes called visceral fat) is more strongly linked to heart disease and type 2 diabetes than fat stored in the hips and thighs.
A simple waist measurement can fill this gap. For women, a waist circumference over 35 inches raises risk. For men, the threshold is 40 inches. To measure accurately, stand up, wrap a tape measure around your middle just above your hipbones, and read it right after breathing out. Many healthcare providers use waist circumference alongside BMI to get a fuller picture of metabolic risk.
Health Risks Linked to High BMI
The reason BMI gets so much attention is its strong connection to chronic disease. Nearly 9 in 10 people with type 2 diabetes have overweight or obesity. Excess weight also raises blood pressure, which is the leading cause of strokes, and increases blood cholesterol and blood sugar levels that contribute to heart disease.
The list extends well beyond the cardiovascular system. Higher BMI is associated with increased risk for:
- Sleep apnea and asthma: Excess weight can compress airways and worsen lung function.
- Osteoarthritis: Obesity is a leading risk factor for joint breakdown in the knees, hips, and ankles.
- Fatty liver disease: The liver can accumulate fat even in people who drink little or no alcohol.
- Certain cancers: In men, colon, rectal, and prostate cancers occur more often at higher BMIs. In women, breast, uterine, and gallbladder cancers are more common.
- Kidney disease: Obesity independently raises kidney disease risk, even without diabetes or high blood pressure.
- Pregnancy complications: Higher BMI increases the chance of gestational diabetes, preeclampsia, and cesarean delivery.
Being underweight carries its own set of risks, including nutritional deficiencies, weakened immunity, and bone loss, which is why BMI categories flag both ends of the spectrum.
What BMI Can and Cannot Tell You
Think of BMI as a starting point, not a verdict. It’s a screening tool that flags potential concerns. It cannot diagnose obesity on its own, measure your actual body fat percentage, or account for your fitness level, age, sex, or ethnic background, all of which influence what a “healthy” weight looks like for you individually.
If your BMI falls outside the healthy range, it’s a signal to look deeper. That might mean measuring waist circumference, checking blood pressure and blood sugar, or simply having a conversation about your overall health picture. A number that looks concerning on paper may be perfectly fine for someone with a muscular build, just as a “normal” BMI doesn’t guarantee good metabolic health. The number is one piece of information, not the whole story.