To determine your BMI (body mass index), you divide your weight by your height squared. The result is a single number that estimates whether your body weight falls into an underweight, normal, overweight, or obese range. You can calculate it by hand, use an online calculator, or ask your doctor to do it at a checkup.
The BMI Formula
BMI uses the same basic math regardless of your measurement system: weight divided by height squared. If you use metric units, divide your weight in kilograms by your height in meters squared. A person who weighs 70 kg and stands 1.75 m tall would calculate 70 ÷ (1.75 × 1.75) = 22.9.
If you measure in pounds and inches, the formula adds one extra step. Divide your weight in pounds by your height in inches squared, then multiply by 703. So a person who weighs 160 pounds and stands 5’8″ (68 inches) would calculate 160 ÷ (68 × 68) × 703 = 24.3.
If your height was measured in centimeters rather than meters, you can still get the right answer. Divide your weight in kilograms by your height in centimeters squared, then multiply by 10,000.
What Your Number Means
The World Health Organization uses these BMI categories for adults:
- Under 18.5: underweight
- 18.5 to 24.9: normal weight
- 25.0 to 29.9: overweight
- 30.0 or higher: obese
These thresholds were developed decades ago using data primarily from white European populations, which is one reason they don’t fit everyone equally well. They’re a screening tool, not a diagnosis. A BMI of 26 doesn’t automatically mean your health is at risk, and a BMI of 23 doesn’t guarantee everything is fine.
BMI Works Differently for Children
Because children and teens are still growing, a raw BMI number doesn’t tell you much on its own. Instead, that number gets compared to other kids of the same age and sex using growth charts from the CDC. The result is a percentile. A 10-year-old boy at the 75th percentile, for example, has a BMI higher than 75% of boys his age.
The categories for ages 2 through 19 are:
- Underweight: below the 5th percentile
- Healthy weight: 5th to less than 85th percentile
- Overweight: 85th to less than 95th percentile
- Obesity: 95th percentile or above
Your pediatrician’s office typically calculates this automatically and plots it on a growth chart over time, which is more useful than any single measurement.
Why BMI Can Be Misleading
BMI measures total body weight relative to height. It cannot distinguish between fat and muscle. A muscular person who lifts weights regularly could land in the “overweight” or even “obese” range while carrying very little body fat. Skeletal muscle, which makes up the bulk of lean body mass, is actually protective for heart and metabolic health, so classifying someone as unhealthy based on weight alone misses the point.
In 2023, the American Medical Association formally recognized these problems and adopted a policy stating that BMI alone is an imperfect clinical measure. The AMA noted that BMI correlates reasonably well with body fat across large populations but “loses predictability when applied on the individual level.” The policy recommends using BMI alongside other measures like waist circumference, body composition, and metabolic factors rather than relying on it as a sole indicator.
Ethnicity Changes the Risk Thresholds
The standard BMI cutoffs don’t carry the same meaning across all ethnic groups. A large population study published in The Lancet found that South Asian individuals develop type 2 diabetes at the same rate as white individuals at a BMI of just 23.9, compared to 30.0 in white populations. For Chinese individuals, the equivalent cutoff was 26.9. That’s a significant gap. The WHO and the UK’s National Institute for Health and Care Excellence now recommend a lower BMI threshold of 27.5 for South Asian and Chinese populations as a trigger for lifestyle interventions.
If you’re of South Asian, Southeast Asian, or East Asian descent, a BMI in the “normal” range by standard cutoffs could still signal elevated metabolic risk. This is one of the clearest examples of why BMI needs context.
BMI Shifts With Age
For older adults, the “ideal” BMI appears to be higher than the standard guidelines suggest. A study in The Journal of Nutrition, Health & Aging found that among adults over 65, the lowest risk of dying from any cause was at a BMI of 27.1, which falls squarely in the “overweight” category. The BMI range where none of the studied causes of death were elevated by more than 10% was 24.2 to 30.1. Being too thin in older age, with a BMI below 21.1, was associated with significantly higher mortality.
This likely reflects the fact that some extra weight provides a reserve during illness, surgery, or periods of reduced appetite. For people over 65, a BMI in the mid-20s to low-30s is generally not cause for concern on its own.
Measurements That Add Useful Context
If you want a more complete picture than BMI alone, waist circumference is one of the simplest additions. The National Heart, Lung, and Blood Institute flags increased health risk at a waist measurement above 35 inches for women or above 40 inches for men. You can measure this at home with a flexible tape measure placed around your waist at the level of your navel.
Waist-to-hip ratio, which compares your waist measurement to the widest point around your hips, may be even more informative. A study of obese adult women found that waist-to-hip ratio was significantly linked to blood pressure, cholesterol ratios, and triglyceride levels. BMI, by comparison, was only significantly linked to cholesterol ratios in the same group. Where your body stores fat matters more than how much you weigh overall, and waist-based measurements capture that distinction in a way BMI cannot.
Body composition testing, which estimates your actual percentage of fat versus lean tissue, is available through methods like bioelectrical impedance scales (common in gyms and some home scales) or more precise clinical tools. These give you a direct answer to the question BMI is only approximating.