Body mass index (BMI) is your weight divided by the square of your height. In metric units it is kilograms ÷ meters²; in US units, multiply by 703: BMI = 703 × pounds ÷ inches². A person who is 5 feet 9 inches tall and weighs 170 pounds has a BMI of 25.1, just inside the CDC’s overweight category. BMI is a quick, free screening number used by doctors and public health agencies, but it does not measure body fat directly, and it can mislead for muscular people, older adults and some ethnic groups.
The BMI formula
The 703 converts pounds and inches to the metric result: 1 lb = 0.45359237 kg and 1 in = 0.0254 m, and 0.45359237 ÷ 0.0254² ≈ 703.07.
Worked examples
US units: 5 ft 9 in (69 inches), 170 lb.
- Square the height: 69 × 69 = 4,761
- Divide weight by it: 170 ÷ 4,761 = 0.035707
- Multiply by 703: 0.035707 × 703 = 25.1
Metric units: 1.65 m, 68 kg.
- Square the height: 1.65 × 1.65 = 2.7225
- Divide: 68 ÷ 2.7225 = 25.0
The BMI calculator works in either system and shows the healthy weight range for your height.
Adult BMI categories
The CDC and the World Health Organization use the same adult cutoffs:
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to less than 25 | Healthy weight |
| 25 to less than 30 | Overweight |
| 30 to less than 35 | Obesity, class 1 |
| 35 to less than 40 | Obesity, class 2 |
| 40 or higher | Obesity, class 3 (severe obesity) |
These categories apply to adults 20 and older, regardless of sex or age. Children and teens use percentiles instead (below).
Healthy weight range by height
Rearranging the formula gives the weight for any target BMI: weight (lb) = BMI × height (in)² ÷ 703.
| Height | Healthy range (BMI 18.5–24.9) | BMI 25 at | BMI 30 at |
|---|---|---|---|
| 5′0″ | 95–128 lb | 128 lb | 154 lb |
| 5′2″ | 101–136 lb | 137 lb | 164 lb |
| 5′4″ | 108–145 lb | 146 lb | 175 lb |
| 5′6″ | 115–154 lb | 155 lb | 186 lb |
| 5′8″ | 122–164 lb | 164 lb | 197 lb |
| 5′10″ | 129–174 lb | 174 lb | 209 lb |
| 6′0″ | 136–184 lb | 184 lb | 221 lb |
| 6′2″ | 144–194 lb | 195 lb | 234 lb |
| 6′4″ | 152–205 lb | 205 lb | 246 lb |
Why BMI is used
BMI was developed by the Belgian statistician Adolphe Quetelet in the 1830s to describe populations, and researcher Ancel Keys named it the body mass index in 1972. It caught on in medicine because it is:
- Easy: it needs only a scale and a tape measure.
- Consistent: the same cutoffs apply to all adults, which makes it useful for tracking population trends.
- Correlated with risk: at the population level, higher BMIs are associated with higher rates of type 2 diabetes, high blood pressure, heart disease and some cancers, and very low BMIs with other risks.
Where BMI falls short
It cannot tell fat from muscle
BMI uses total weight, so it treats 10 pounds of muscle and 10 pounds of fat the same. A 5′10″, 200-pound athlete has a BMI of 28.7, “overweight,” even with low body fat. The reverse also happens: someone with little muscle and a “healthy” BMI can carry a high percentage of body fat.
It ignores where fat is stored
Fat around the abdomen (visceral fat) is linked to higher metabolic and heart risk than fat on the hips and thighs. Two people with the same BMI can have very different risk. NIH guidance flags a waist circumference above 40 inches for men or 35 inches for women as higher risk, and many clinicians use a waist-to-height ratio under 0.5 as a simple target. The waist-to-hip ratio calculator adds that perspective.
It varies with age, sex and ancestry
- Older adults tend to lose muscle and bone, so the same BMI may mean more body fat. Some research finds the lowest mortality in older adults at BMIs slightly above 25.
- Women generally have more body fat than men at the same BMI; the BMI calculator for women adds waist-based context.
- Asian populations often face higher diabetes and heart risk at lower BMIs, which is why the WHO suggested lower action points and the American Diabetes Association screens Asian Americans for diabetes from a BMI of 23.
- Some Black adults may have more lean mass at a given BMI, so BMI can overestimate body fat.
Height extremes
Because weight does not scale exactly with height squared, BMI tends to overstate fatness in very tall people and understate it in very short people.
Children, pregnancy and certain conditions
For ages 2 to 19, BMI is plotted on CDC growth charts by age and sex: the 85th to below the 95th percentile is overweight, and the 95th percentile or above is obesity. BMI is not used to judge weight status during pregnancy (though pre-pregnancy BMI guides weight-gain targets) and is less meaningful for people with amputations, edema or significant muscle loss.
In 2023 the American Medical Association adopted a policy recognizing these limits and encouraging clinicians to use BMI alongside other measures rather than on its own.
Better together: measures to pair with BMI
| Measure | What it adds |
|---|---|
| Waist circumference | Abdominal fat, linked to metabolic risk |
| Waist-to-height or waist-to-hip ratio | Fat distribution relative to body size |
| Body fat percentage | Separates fat from lean mass; see how to measure body fat percentage |
| Blood pressure, glucose, cholesterol | Direct markers of health risk |
| Fitness and strength | Strongly linked to health at every BMI |
To estimate body fat from tape measurements, try the body fat calculator. If your goal is to change weight, start with how to calculate calories for weight loss.
This guide is for general education and is not medical advice. BMI is a screening tool, not a diagnosis; talk with a healthcare professional about what your numbers mean for you.
Frequently asked questions
What is the BMI formula in pounds and inches?
BMI = 703 × weight in pounds ÷ (height in inches)². For someone 5 feet 9 inches (69 inches) and 170 pounds: 703 × 170 ÷ 4,761 = 25.1.
What is a healthy BMI for adults?
The CDC classifies an adult BMI of 18.5 to less than 25 as the healthy weight range. Below 18.5 is underweight, 25 to less than 30 is overweight, and 30 or higher is obesity.
Is BMI accurate for athletes?
Often not. BMI cannot tell muscle from fat, so muscular people can have a BMI in the overweight or even obesity range with low body fat. Body fat percentage or waist measurements give a better picture for them.
Is BMI calculated differently for children?
The formula is the same, but the result is compared with CDC growth charts for the child's age and sex. A BMI at or above the 85th percentile is classified as overweight and at or above the 95th percentile as obesity.
Should Asian adults use different BMI cutoffs?
Research shows health risks rise at lower BMIs in many Asian populations. The World Health Organization has suggested public health action points of 23 and 27.5, and the American Diabetes Association recommends diabetes screening for Asian American adults starting at a BMI of 23 rather than 25.