To calculate your BMI, divide your weight by your height squared. If you’re using pounds and inches, multiply the result by 703. If you’re using kilograms and meters, no conversion factor is needed. The number you get places you into a weight category that offers a rough snapshot of health risk, though it tells you less than you might think.
The Formula in Pounds and Inches
Most people in the U.S. know their weight in pounds and height in feet and inches, so start by converting your full height to inches. If you’re 5 feet 6 inches tall, that’s 66 inches (5 × 12 + 6). Then plug your numbers into this formula from the CDC:
BMI = (weight in pounds ÷ height in inches ÷ height in inches) × 703
For example, say you weigh 160 pounds and stand 66 inches tall. Divide 160 by 66 to get about 2.424. Divide that by 66 again to get roughly 0.0367. Multiply by 703, and your BMI is approximately 25.8.
The 703 is just a conversion factor that bridges the gap between imperial and metric units. It’s baked into every online BMI calculator, but knowing where it comes from helps if you want to check the math yourself.
The Formula in Kilograms and Meters
If you’re working in metric units, the calculation is simpler:
BMI = weight in kilograms ÷ (height in meters × height in meters)
Someone who weighs 72 kg and is 1.75 meters tall would calculate 1.75 × 1.75 = 3.0625, then divide 72 by 3.0625 to get a BMI of about 23.5. If you know your height in centimeters instead of meters, you can divide your weight by your height in centimeters twice, then multiply by 10,000 to get the same result.
What Your Number Means
The World Health Organization defines four main categories:
- Below 18.5: Underweight
- 18.5 to 24.9: Normal weight
- 25.0 to 29.9: Overweight
- 30.0 and above: Obese
A large UK study of 3.6 million adults found that the lowest risk of death from cancer, cardiovascular disease, and respiratory disease falls in the 21 to 25 range. Risk rises on both sides of that window. Compared to people in the normal-weight range, those with a BMI of 30 or higher had an estimated life expectancy about 4 years shorter from age 40. Being underweight carried a similar penalty: roughly 4 to 4.5 fewer years of life expectancy, depending on sex.
BMI Works Differently for Children
The formula itself is the same for kids, but the result is interpreted differently. Because children and teens are still growing, a raw BMI number is meaningless without context. Instead, the CDC compares a child’s BMI to other children of the same age and sex using percentile charts. A child at the 60th percentile, for instance, has a BMI higher than 60% of kids their age and sex.
The categories for children and teens are:
- Below the 5th percentile: Underweight
- 5th to below the 85th percentile: Healthy weight
- 85th to below the 95th percentile: Overweight
- 95th percentile or above: Obese
The CDC offers a free online calculator specifically for children aged 2 through 19 that does the percentile lookup for you.
Why BMI Can Be Misleading
BMI correlates well with body fat across large populations, but it loses accuracy when applied to any single person. The reason is simple: the formula can’t tell the difference between fat and muscle. A muscular athlete with low body fat can land in the “overweight” or even “obese” category purely because muscle is denser than fat. In 2023, the American Medical Association formally acknowledged these limitations, recommending that BMI never be used as the sole measure of health and that clinicians pair it with other assessments.
The standard categories were also built primarily on data from non-Hispanic white populations, which means they don’t reflect risk equally across ethnic groups. For Asian populations, health risks begin rising at lower BMI values. A joint WHO and International Obesity Task Force recommendation set the overweight threshold for Asians at 23 rather than 25, and the obesity threshold at 25 rather than 30. Research in the American Diabetes Association’s journal found that screening Asian Americans for type 2 diabetes at a BMI of 23 or above catches far more cases than waiting until 25.
Age matters too. A study in the Annals of Geriatric Medicine and Research found that adults over 65 with BMIs between 25 and 35 often had better functional outcomes than those below 25. Older adults in the “normal” range actually showed higher rates of falls, balance problems, muscle weakness, and malnutrition. The protective value of carrying some extra weight in later life is well documented enough that many geriatric specialists use different thresholds for their patients.
Measurements That Fill the Gaps
If you want a more complete picture than BMI alone provides, waist circumference is the easiest additional check. It estimates visceral fat, the deep abdominal fat packed around your organs that drives metabolic risk more than fat stored under the skin. The National Heart, Lung, and Blood Institute flags increased health risk at a waist measurement above 35 inches for women and above 40 inches for men.
To measure, wrap a flexible tape measure around your bare abdomen at the top of your hip bones, keep it level, and read the number after a normal exhale. Two people with identical BMIs can have very different waist circumferences, and the person with more visceral fat faces higher risks for heart disease and type 2 diabetes regardless of what the BMI chart says. The AMA now recommends pairing BMI with measures like waist circumference, body composition, and metabolic markers to get a meaningful assessment of where you stand.