To calculate BMI, divide your weight by your height squared. The exact formula depends on whether you’re using metric or imperial units, but the math takes about 30 seconds with a calculator. A BMI between 18.5 and 24.9 is considered normal weight for most adults.
The BMI Formulas
BMI stands for body mass index, and it’s a ratio of your weight to your height. There are two versions of the formula depending on your units.
Using pounds and inches: BMI = (weight in pounds ÷ height in inches ÷ height in inches) × 703. The 703 is a conversion factor that adjusts for the difference between imperial and metric units.
Using kilograms and meters: BMI = weight in kilograms ÷ (height in meters × height in meters). If you know your height in centimeters instead, divide your weight by your height in centimeters, divide by your height in centimeters again, then multiply by 10,000.
A Step-by-Step Example
Say you weigh 160 pounds and stand 5 feet 7 inches tall. First, convert your height entirely to inches: 5 × 12 + 7 = 67 inches. Square that number: 67 × 67 = 4,489. Divide your weight by that result: 160 ÷ 4,489 = 0.03563. Multiply by 703: 0.03563 × 703 = 25.1. Your BMI is 25.1.
For a metric example: someone who weighs 72 kilograms and is 1.75 meters tall would calculate 1.75 × 1.75 = 3.0625, then 72 ÷ 3.0625 = 23.5. Their BMI is 23.5.
What Your Number Means
The World Health Organization classifies adult BMI into these ranges:
- Below 18.5: underweight
- 18.5 to 24.9: normal weight
- 25.0 to 29.9: overweight
- 30.0 and above: obese
A BMI below 17.0 is classified as moderate to severe thinness, which carries its own health risks. Diabetes, high blood pressure, and elevated inflammatory markers all show a graded association with higher BMI, meaning risk climbs steadily as the number goes up. But very low BMI (under 20) is also linked to significantly higher risk of heart failure and cardiovascular death, so the relationship isn’t a simple “lower is better” equation.
BMI Works Differently for Children
For anyone aged 2 through 19, a raw BMI number on its own doesn’t tell you much. Children’s body fat changes as they grow, and boys and girls develop at different rates. Instead of fixed categories, pediatric BMI is plotted on age-and-sex-specific growth charts and expressed as a percentile. A 10-year-old boy and a 15-year-old girl could have the same BMI but fall into completely different categories.
You calculate the number using the same formula as adults. The interpretation is what changes. The CDC offers a free Child and Teen BMI Calculator that does the percentile math and plots the result on a growth chart for you.
Adjusted Thresholds for Asian Populations
Standard BMI cutoffs were developed using data from predominantly white populations. People of Asian descent tend to carry more body fat and face higher metabolic risk at lower BMI values. The World Health Organization has proposed lowering the overweight threshold for Asian populations from 25 to 23. If you’re of Asian descent, a BMI of 23 or higher may already signal increased health risk, even though it falls in the “normal” range on the standard scale.
Why BMI Isn’t the Full Picture
BMI correlates with body fat across large populations, but it loses accuracy at the individual level. It can’t distinguish between muscle and fat, so a muscular person may register as overweight despite having low body fat. It also doesn’t tell you where your fat is stored, which matters because fat around the organs (visceral fat) poses more risk than fat under the skin.
In 2023, the American Medical Association adopted a policy recognizing these limitations. The AMA now recommends that BMI be used alongside other measures, such as waist circumference, body composition, and metabolic markers, rather than as a standalone assessment. The policy also acknowledged that BMI’s reliance on data from older, predominantly non-Hispanic white populations makes it an imperfect tool across different racial and ethnic groups, sexes, and age ranges.
BMI remains useful as a quick screening number, a starting point that takes seconds to calculate. It just shouldn’t be the only number you pay attention to. Waist circumference (measured at the navel) is one of the simplest additional checks: above 40 inches for men or 35 inches for women signals higher metabolic risk regardless of what your BMI says.