To calculate BMI, divide your weight in kilograms by your height in meters squared. If you use pounds and inches, the formula is: weight (lb) ÷ height (in)² × 703. A 5’6″ person weighing 155 pounds, for example, has a BMI of about 25.
The BMI Formulas
There are two versions of the formula depending on which units you’re working with.
Metric: BMI = weight (kg) ÷ [height (m)]². If you know your height in centimeters rather than meters, divide your weight in kilograms by your height in centimeters, divide by your height in centimeters again, then multiply by 10,000.
Imperial (pounds and inches): BMI = weight (lb) ÷ [height (in)]² × 703. The 703 is a fixed conversion factor that bridges the difference between metric and imperial units.
A Step-by-Step Example
Say you weigh 170 pounds and stand 5 feet 9 inches tall. First, convert your height entirely to inches: 5 × 12 + 9 = 69 inches. Square that number: 69 × 69 = 4,761. Divide your weight by that result: 170 ÷ 4,761 = 0.0357. Multiply by 703: 0.0357 × 703 = 25.1. Your BMI is 25.1.
For metric, imagine someone who is 1.75 meters tall and weighs 72 kilograms. Square the height: 1.75 × 1.75 = 3.0625. Divide weight by that: 72 ÷ 3.0625 = 23.5. Done.
What the Number Means for Adults
The CDC groups adult BMI (ages 20 and older) into these categories:
- Underweight: below 18.5
- Healthy weight: 18.5 to 24.9
- Overweight: 25 to 29.9
- Class 1 obesity: 30 to 34.9
- Class 2 obesity: 35 to 39.9
- Class 3 (severe) obesity: 40 or higher
These thresholds apply the same way regardless of sex for adults, though they come with important caveats covered below.
BMI for Children and Teens
The math is identical for kids, but the result is interpreted differently. Because children are still growing, a raw number like 22 means something very different for a 7-year-old than for a 15-year-old. Instead of fixed cutoffs, pediatric BMI is plotted on sex- and age-specific growth charts and expressed as a percentile. That percentile tells you where a child falls relative to other children of the same age and sex.
The categories for ages 2 through 19:
- Underweight: below the 5th percentile
- Healthy weight: 5th to just under the 85th percentile
- Overweight: 85th to just under the 95th percentile
- Obesity: 95th percentile or above
- Severe obesity: 120% of the 95th percentile or above, or a BMI of 35 or higher
Your pediatrician’s office will calculate the percentile for you, but the CDC also offers an online child and teen BMI calculator that does it automatically when you enter the child’s date of birth, sex, height, and weight.
Getting an Accurate Measurement
Small errors in height or weight can shift your BMI by a full point or more, which is enough to bump you into a different category. A few details help keep the numbers reliable.
Weigh yourself in light clothing, without shoes, ideally at the same time of day each time. Morning is generally best because food, water, and activity throughout the day can add variability. For height, stand barefoot against a wall with your heels, shoulder blades, and the back of your head touching the surface, then look straight ahead. Have someone place a flat object (a book works fine) on top of your head and mark the wall. Your height can actually shrink by up to 2 centimeters between morning and evening as gravity compresses the discs in your spine, so consistency in timing matters if you’re tracking changes over months.
Where BMI Falls Short
BMI treats all weight the same. It cannot distinguish between muscle, fat, bone, and water. In one study of adolescent athletes, 62% of those classified as obese by BMI were false positives. Their actual body fat levels were well within a healthy range. This is a common issue for anyone who carries significant muscle mass, since muscle is denser than fat and adds body weight without the same health risks.
The standard cutoffs also don’t account for differences across ethnic groups. Research published in Diabetes Care found that the diabetes risk a non-Hispanic white person faces at a BMI of 30 kicks in at a BMI of just 24 for South Asians and 25 for Chinese Americans. In other words, someone of South Asian descent with a BMI of 25 may already carry the metabolic risk that the standard chart associates with obesity.
Age and sex also play a role. Older adults tend to have more body fat at the same BMI than younger adults. Women typically carry more body fat than men at equivalent BMI levels. None of these nuances show up in the simple formula.
The AMA’s Updated Position
In 2023 the American Medical Association adopted a policy acknowledging that BMI “loses predictability when applied on the individual level.” The AMA noted that the standard cutoffs were developed primarily from data on non-Hispanic white populations, and recommended that clinicians use BMI alongside other measures: waist circumference, body composition assessments, metabolic markers, and family history. The policy still recognizes that a BMI above 35 remains useful for risk stratification, but it frames BMI as one data point among several rather than a standalone diagnostic tool.
Other Ways to Assess Body Composition
If you want a fuller picture beyond your BMI number, waist circumference is the simplest addition. Wrap a tape measure around your bare midsection at the top of your hip bones, level with your navel, and read the number at the end of a normal exhale. For most adults, health risks climb when waist circumference exceeds 40 inches for men or 35 inches for women.
Waist-to-hip ratio goes a step further. Measure your waist as above, then measure the widest part of your hips, and divide the waist number by the hip number. A 2023 study in JAMA Network Open found that waist-to-hip ratio was more strongly and consistently linked to death from any cause than BMI or fat mass index, and the researchers concluded the relationship is likely causal. This makes sense biologically: fat stored around the organs in your midsection is more metabolically active and more dangerous than fat stored in your hips or thighs, and waist-to-hip ratio captures that distinction in a way BMI never can.
More advanced options include DEXA scans, which use low-dose X-rays to separately measure fat, muscle, and bone, and bioelectrical impedance scales, which estimate body fat percentage by sending a small electrical current through your body. These give more granular information but aren’t necessary for most people. For a quick, free health check, your BMI paired with a tape measure around your waist tells you most of what you need to know.