To determine your BMI, you divide your weight by your height squared. The result is a single number that places you in a weight category ranging from underweight (below 18.5) to severe obesity (40 or higher). You can calculate it by hand, but most people use a free online calculator from the CDC or similar source.
The BMI Formula
BMI stands for body mass index, and the math is straightforward. If you’re working in metric units, divide your weight in kilograms by your height in meters squared. So a person who weighs 70 kg and stands 1.75 m tall would calculate 70 ÷ (1.75 × 1.75) = 22.9.
If you’re using pounds and inches, the formula adds one extra step: divide your weight in pounds by your height in inches squared, then multiply by 703. For example, someone who weighs 160 pounds and is 67 inches tall would calculate 160 ÷ (67 × 67) × 703 = 25.1.
If your height is in centimeters rather than meters, you can still get there. Divide your weight in kilograms by your height in centimeters, divide by that height again, then multiply by 10,000. All three formulas produce the same number.
What Your Number Means
For adults 20 and older, BMI falls into these standard 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 cutoffs aren’t arbitrary. A large UK study that followed 3.6 million adults found that the lowest risk of death from cardiovascular disease, cancer, and respiratory disease fell in the 21 to 25 range. Risk began climbing above 25 for most health outcomes, including overall mortality. Contrary to older research suggesting a few extra pounds might be protective, this study found no meaningful benefit to landing in the overweight range.
How BMI Works Differently for Children
A BMI of 22 means something very different for a 10-year-old than for a 35-year-old. Children’s body composition changes rapidly as they grow, so their BMI is plotted on age- and sex-specific growth charts rather than compared to fixed cutoffs. The result is a percentile that shows where a child falls relative to other children of the same age and sex.
For kids and teens ages 2 through 19, the categories look like this:
- 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 child’s pediatrician typically tracks this at routine checkups. A single reading matters less than the trend over time.
Where BMI Falls Short
BMI is a screening tool, not a body scan. It cannot distinguish between fat, muscle, and bone. A competitive weightlifter and a sedentary person of the same height and weight will produce identical BMI numbers, even though their health profiles are vastly different. BMI also tells you nothing about the type of fat you carry or where it sits on your body, both of which matter for metabolic risk.
This is a practical limitation for several groups. Athletes and people who do heavy resistance training often land in the “overweight” or even “obese” range because muscle is denser than fat. Older adults can lose muscle and bone density with age, so their BMI may stay the same or drop even as their body fat percentage climbs. And BMI cutoffs were developed primarily from data on white European populations, so they may not reflect risk equally across all ethnic backgrounds.
Measurements That Add Context
Because BMI is blind to fat distribution, waist circumference is one of the simplest ways to fill in the picture. Fat stored around the midsection (visceral fat) is more strongly linked to heart disease, type 2 diabetes, and certain cancers than fat stored in the hips and thighs. The World Health Organization sets high-risk thresholds at greater than 88 cm (about 34.6 inches) for women and greater than 102 cm (about 40.2 inches) for men.
To measure, wrap a flexible tape around your bare waist just above the top of your hip bones, usually at the level of your navel. Keep the tape snug but not compressing the skin, breathe out normally, and read the number. If your BMI lands in the healthy or overweight range but your waist circumference exceeds these thresholds, your metabolic risk may be higher than BMI alone suggests.
Other tools your doctor might use include skinfold measurements, body composition scans, and blood markers like cholesterol, blood sugar, and blood pressure. None of these replace BMI entirely. They work best in combination, giving a fuller picture than any single number can.