Body Mass Index, or BMI, is measured using a simple formula: your weight divided by the square of your height. The result is a single number that places you into a weight category. While the math is straightforward, how BMI gets interpreted depends on your age, sex, ethnicity, and what other measurements your doctor considers alongside it.
The Formula
BMI uses only two inputs: your weight and your height. If you’re working in metric units, the formula is your weight in kilograms divided by your height in meters squared. A person who weighs 70 kg and stands 1.75 m tall would calculate 70 ÷ (1.75 × 1.75), giving a BMI of 22.9.
In pounds and inches, you divide your weight by your height in inches squared, then multiply by 703. So a person weighing 155 pounds at 5’9″ (69 inches) would calculate 155 ÷ (69 × 69) × 703, which also lands around 22.9. The 703 is just a conversion factor that bridges the difference between metric and imperial units.
Standard BMI Categories for Adults
The National Heart, Lung, and Blood Institute groups adult BMI results into four categories:
- Underweight: below 18.5
- Healthy weight: 18.5 to 24.9
- Overweight: 25.0 to 29.9
- Obesity: 30.0 or above
These thresholds apply to adults of all ages, though as you’ll see below, they don’t fit every population equally well.
Getting an Accurate Measurement
Because BMI depends entirely on two numbers, even small errors in height or weight can shift your result. Clinical measurements taken by trained staff using calibrated equipment (a stadiometer for height, a medical-grade scale for weight) are the most reliable. Self-reported height and weight tend to skew optimistically: people typically overestimate their height and underestimate their weight, which pushes BMI lower than it actually is.
For the most consistent reading at home, weigh yourself in the morning before eating, wearing minimal clothing, and use a flat hard surface under your scale. Measure height without shoes, standing against a wall with your heels, back, and head touching it.
How BMI Works Differently for Children
Children and teens are still growing, so a raw BMI number doesn’t mean the same thing it does for an adult. Instead, a child’s BMI is plotted on sex-specific growth charts from the CDC and expressed as a percentile, showing how the child compares to others of the same age and sex. A 10-year-old girl at the 70th percentile has a BMI higher than 70% of girls her age in the reference population.
The categories shift accordingly:
- Underweight: below the 5th percentile
- Healthy weight: 5th to less than the 85th percentile
- Overweight: 85th to less than the 95th percentile
- Obesity: 95th percentile or above
Why BMI Can Be Misleading
BMI treats all weight the same. It cannot tell the difference between fat, muscle, and bone. A muscular athlete and a sedentary person of the same height and weight will have identical BMIs despite very different levels of body fat. BMI also says nothing about where fat is stored, which matters because fat around the organs (visceral fat) carries more health risk than fat under the skin.
When researchers compared BMI classifications to body composition scans using dual-energy X-ray absorptiometry (DEXA), which directly measures fat and lean tissue, BMI misclassified the weight status of roughly one in three people. Some were labeled a healthy weight despite having excess body fat, while others were flagged as overweight despite having high muscle mass.
Different Thresholds for Different Populations
The standard cutoffs were developed largely from data on European populations, and they don’t translate neatly across ethnicities. A WHO expert panel found that Asian populations develop weight-related health problems like type 2 diabetes and heart disease at lower BMI levels. The panel recommended an obesity threshold of 27.5 for Asian populations generally, rather than 30. Some national guidelines go further: consensus recommendations for Asian Indian populations set the obesity cutoff at 25.
To illustrate the gap, a UK study found that South Asian individuals developed the same diabetes risk at a BMI of 25.2 that White individuals developed at 30. That five-point difference can mean the difference between a doctor flagging a health concern or missing it entirely.
Age matters too. Research on older adults suggests the standard “healthy” range of 18.5 to 24.9 may be too low for people over 65. One study found that older adults with a BMI below 25 had higher rates of falls, weaker muscles, and lower functional capacity. The data pointed to an optimal range of 27 to 28 for older men and 31 to 32 for older women, well above what standard charts call healthy weight. Carrying slightly more weight in later life appears to provide a protective buffer against frailty and muscle loss.
What Doctors Use Alongside BMI
In 2023, the American Medical Association adopted a policy recommending that BMI not be used as a standalone measure. The AMA now advises doctors to pair it with other assessments, including waist circumference, body composition estimates, and metabolic markers.
Waist-to-hip ratio is one increasingly favored alternative. A study of nearly 388,000 people published in JAMA Network Open found that waist-to-hip ratio predicted future heart disease, high blood pressure, and type 2 diabetes more accurately than BMI did. The likely reason is that it captures visceral fat more directly. You can measure your own waist-to-hip ratio with a tape measure: divide your waist circumference (at the narrowest point) by your hip circumference (at the widest point).
DEXA scans provide the most detailed picture, directly measuring fat mass, lean tissue, and bone density throughout the body. They’re typically used in clinical or research settings rather than routine checkups, but they offer a useful reality check when BMI and other signs point in different directions.
What Your BMI Actually Tells You
BMI is a screening tool, not a diagnosis. It’s fast, free, and requires no special equipment, which is why it remains widely used in medicine and public health. For most people, it provides a reasonable starting estimate of whether weight might be affecting health. But it works best as one data point among several. If your BMI falls outside the healthy range and you’re curious about what that means for you specifically, waist circumference and basic metabolic bloodwork will give a much fuller picture than BMI alone.