To calculate your BMI, divide your weight by your height squared. If you’re using metric units, that’s your weight in kilograms divided by your height in meters squared. If you’re using pounds and inches, you divide your weight by your height in inches squared, then multiply by 703. The math takes about 30 seconds once you know the formulas.
The BMI Formulas
BMI stands for body mass index, and there are two versions of the formula depending on your units.
Metric formula: BMI = weight (kg) ÷ height (m) × height (m)
For example, if you weigh 70 kg and stand 1.75 m tall: 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9.
Imperial formula: BMI = [weight (lbs) ÷ height (in) ÷ height (in)] × 703
For example, if you weigh 160 pounds and stand 5’9″ (69 inches): 160 ÷ (69 × 69) = 160 ÷ 4,761 = 0.0336 × 703 = 23.6.
If you’re measuring height in centimeters instead of meters, the CDC provides a slightly adjusted metric formula: divide your weight in kilograms by your height in centimeters, divide by your height in centimeters again, then multiply by 10,000. The result is identical.
What Your BMI Number Means
Once you have your number, it falls into one of several standard 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 cutpoints are used as screening thresholds, not diagnoses. The higher the BMI above 25, the greater the statistical risk of heart disease, type 2 diabetes, and mortality from all causes. For type 2 diabetes specifically, risk rises continuously with BMI across the entire range, with no clear safe threshold.
Different Cutpoints for Asian Populations
The standard categories were developed from data on primarily white European populations. A WHO expert panel found that people of Asian descent tend to develop metabolic problems at lower BMI values, so adjusted cutpoints are recommended: 23 to 27.5 is considered overweight, and 27.5 or above is considered obese. If you’re of East Asian, South Asian, or Southeast Asian descent, these lower thresholds give a more accurate picture of your risk.
Getting Accurate Measurements at Home
Your BMI is only as good as the numbers you put into it. Small errors in height or weight can shift the result by a full point or more.
For weight, use a digital scale placed on a hard, flat surface like tile or wood, not carpet. Remove shoes and heavy clothing. Stand with both feet centered on the scale and record the number to the nearest decimal, like 155.5 pounds.
For height, stand against a flat wall (no baseboard molding) with your feet flat and together. Your head, shoulders, buttocks, and heels should touch the wall. Have someone place a flat, rigid object like a hardcover book on top of your head so it forms a right angle with the wall, then mark the wall where the bottom of the book sits. Measure from the floor to the mark with a metal tape measure. Record to the nearest eighth of an inch or 0.1 centimeter.
BMI Works Differently for Kids and Teens
The formula itself is the same for children, but the result is interpreted completely differently. Instead of fixed cutpoints, a child’s BMI is compared to other kids of the same age and sex using percentile charts. A 12-year-old girl with a BMI of 26.6, for instance, might land at the 96th percentile, meaning her BMI is higher than 96% of girls her age in the reference population. That would place her in the obesity category.
The categories for ages 2 through 19:
- Below 5th percentile: Underweight
- 5th to 84th percentile: Healthy weight
- 85th to 94th percentile: Overweight
- 95th percentile or above: Obesity
Because children’s body composition changes rapidly as they grow, a raw BMI number that’s perfectly normal for a 16-year-old could be concerning for a 10-year-old. The CDC offers an online BMI-for-age calculator that does the percentile math for you.
Where BMI Falls Short
BMI measures weight relative to height. It does not measure body fat directly, and it cannot tell the difference between muscle, bone, and fat tissue. A lean, muscular person can easily register as overweight or obese by BMI despite having low body fat and excellent cardiovascular fitness. People with larger bone structures face the same misclassification.
The relationship between BMI and actual body fat also varies significantly by sex, age, race, and ethnicity. Older adults tend to carry more fat at the same BMI than younger adults. Women generally have higher body fat percentages than men at the same BMI. These variations mean a single number can tell very different stories for different people.
BMI also has a blind spot for where fat is stored. Two people with the same BMI can have very different health risks if one carries fat primarily around the abdomen and the other carries it in the hips and thighs. Abdominal fat is far more closely linked to heart disease and metabolic problems. Someone with a “normal” BMI but significant belly fat, a pattern called normal-weight central obesity, may face higher risks than their BMI suggests.
Measurements That Complement BMI
Waist circumference is a simple add-on. The American Heart Association flags increased cardiovascular and metabolic risk at waist measurements above 35 inches for women and 40 inches for men. To measure, wrap a tape measure around your bare abdomen just above your hip bones, keeping it level.
Waist-to-height ratio is gaining traction as an even better predictor of heart disease risk than BMI, particularly for people with diabetes and those of South or Southeast Asian descent. You calculate it by dividing your waist circumference by your height (both in the same units). A ratio above 0.5 generally signals increased risk. Unlike BMI, this metric directly captures abdominal fat and adjusts naturally for body size, making it more consistent across different populations.
Neither of these replaces BMI entirely. They each capture something different. Using BMI alongside a waist measurement gives you a more complete picture than either number alone.