Body Mass Index, or BMI, is a single number that relates your weight to your height. Doctors, insurers and public health researchers use it because it is quick, cheap and needs nothing more than a scale and a tape measure. It is often the first number you see on a health check, and it shapes a lot of advice about weight.
But BMI is a screening tool, not a diagnosis. This guide explains how the number is calculated, what each category means, where the cut-offs come from and why muscle, age and ethnicity can make the result misleading. You can check your own figure at any point with our BMI calculator.
What does BMI measure?
BMI estimates whether your weight is in a healthy range for your height. The idea dates back to the Belgian mathematician Adolphe Quetelet in the 1830s, who noticed that adult weight tends to scale with the square of height. The physiologist Ancel Keys popularized the name "body mass index" in 1972 after finding it tracked body fat reasonably well across large groups of people.
The key phrase is "across large groups". BMI does not measure fat directly. It cannot tell muscle from fat or show where fat is stored, which is why it works better for spotting trends in populations than for judging any one individual.
How is BMI calculated?
In metric units, BMI is weight in kilograms divided by height in meters squared: BMI = kg ÷ m². For someone who weighs 70 kg and is 1.75 m tall, that is 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 ≈ 22.9.
In imperial units the formula needs a conversion factor: BMI = 703 × pounds ÷ inches². A person who weighs 160 lb and is 5 ft 9 in (69 inches) tall has a BMI of 703 × 160 ÷ 4,761 ≈ 23.6. If you have your weight in one system and your height in another, our weight converter will switch kilograms, pounds and stones in a moment.
What do the BMI categories mean?
The World Health Organization uses the same adult categories for men and women. They are based on studies linking BMI ranges with the risk of conditions such as type 2 diabetes, high blood pressure and heart disease.
- Below 18.5: underweight.
- 18.5 to 24.9: healthy weight.
- 25.0 to 29.9: overweight.
- 30.0 to 34.9: obesity class I.
- 35.0 to 39.9: obesity class II.
- 40.0 and above: obesity class III.
What is a healthy weight for my height?
You can turn the healthy BMI range back into a weight range by multiplying 18.5 and 24.9 by your height in meters squared. At 1.75 m tall, that gives roughly 56.7 kg to 76.3 kg. At 5 ft 9 in, the equivalent range is about 125 lb to 169 lb.
That is a wide band, about 20 kg for the same height, and it is meant to be. People differ in frame size, muscle and build, so there is no single ideal weight. Moving from a BMI of 29 to 27 can bring real benefits even if you never reach 24.9.
Why BMI can be misleading
Because BMI only uses height and weight, it treats a kilogram of muscle exactly like a kilogram of fat. A muscular athlete who weighs 90 kg at 1.75 m has a BMI of about 29.4, technically overweight, while having little excess fat. The reverse also happens: an older adult who has lost muscle can have a normal BMI but a high body fat percentage.
If your BMI and your appearance or fitness seem to disagree, a second measure helps. Our body fat calculator uses the U.S. Navy tape-measure method to estimate how much of your weight is fat, which gives useful context alongside BMI.
- Muscle mass: athletes and people who lift weights are often classed as overweight.
- Age: body fat tends to rise and muscle fall with age, even at the same BMI.
- Ethnicity: many people of Asian descent face higher health risks at lower BMIs, and some health bodies use lower action points such as 23 and 27.5.
- Fat distribution: fat around the waist carries more risk than fat on the hips, and BMI cannot tell the difference.
- Pregnancy: BMI does not apply during pregnancy.
Is BMI different for children and teenagers?
Yes. Children's bodies change rapidly as they grow, and boys and girls develop differently, so the adult cut-offs do not apply. For anyone aged 2 to 19, the BMI value is compared with growth charts for the same age and sex and expressed as a percentile. A child at the 85th percentile or above is generally considered to be above a healthy weight. A pediatrician is the right person to interpret those results.
What to do with your BMI result
Treat your BMI as a starting point for a conversation, not a verdict. If it falls outside the healthy range, look at other markers as well: waist measurement (a waist under half your height is a common guideline), blood pressure, blood sugar, activity levels and how you feel day to day.
If you decide to change your weight, the most reliable approach is a modest, sustained shift in energy balance. Our calorie calculator estimates how many calories you burn each day and suggests targets for gradual loss or gain.
Height measurements in feet and inches can be checked using the conversions in our guide to metric and imperial units.
For estimates of body fat, daily calories and water needs alongside your BMI, explore our full set of health calculators.
Key takeaways
- BMI is weight in kilograms divided by height in meters squared, or 703 × pounds ÷ inches².
- For adults, 18.5 to 24.9 is classed as a healthy weight, 25 to 29.9 as overweight and 30 or more as obesity.
- BMI does not distinguish muscle from fat, so it can misclassify athletes and older adults.
- Children and teenagers are assessed with age- and sex-specific percentiles, not adult cut-offs.
- Use BMI alongside waist size, body fat and other health markers, and talk to a doctor about any concerns.