What is BMI?
Body mass index, or BMI, is a value calculated from a person's weight and height. It is commonly used as a starting point for population-level screening in adults, not as a complete assessment of individual health.
How to calculate BMI
For metric measurements, divide weight in kilograms by height in metres squared:
For imperial measurements, multiply weight in pounds by 703 and divide by height in inches squared:
Example calculations
A person who weighs 70 kg and is 1.75 m tall has a BMI of about 22.9. A person who weighs 154 lb and is 69 in tall has a BMI of about 22.7. Rounding can make displayed results differ slightly from hand calculations.
Adult BMI categories
| Adult screening category | BMI (kg/m²) |
|---|---|
| Underweight | Below 18.5 |
| Healthy weight | 18.5 to below 25 |
| Overweight | 25 to below 30 |
| Obesity, class 1 | 30 to below 35 |
| Obesity, class 2 | 35 to below 40 |
| Obesity, class 3 | 40 or above |
Healthy weight range
For a given height, the adult healthy-weight BMI range is 18.5 to below 25. This is a reference range, not a target or a diagnosis. The calculator reports BMI itself so you can discuss the result with a healthcare professional in the context of your age, body composition, and health history.
Approximate adult weight range by height
The table applies BMI 18.5 to 24.9 to several heights. Values are rounded to the nearest kilogram and are illustrative; they are not personalized weight goals.
| Height | Approximate weight range |
|---|---|
| 150 cm | 42–56 kg |
| 155 cm | 45–60 kg |
| 160 cm | 47–64 kg |
| 165 cm | 50–68 kg |
| 170 cm | 54–72 kg |
| 175 cm | 57–76 kg |
| 180 cm | 60–81 kg |
| 185 cm | 63–85 kg |
| 190 cm | 67–90 kg |
Understanding the BMI result
Adult categories summarize ranges used for screening. A result below 18.5 is classified as underweight; 18.5 to below 25 as healthy weight; 25 to below 30 as overweight; and 30 or above as obesity. The obesity range is commonly divided into class 1 (30 to below 35), class 2 (35 to below 40), and class 3 (40 or above). A category by itself does not establish a health condition.
BMI for men and women
The standard adult BMI formula and category cutoffs are the same for adult men and women. BMI does not include sex as a variable. It also cannot describe the differences in body composition, fat distribution, or individual health that may matter when interpreting a result.
BMI compared with body fat percentage and BMR
BMI is a height-to-weight index. Body fat percentage estimates what share of body weight is fat and requires a separate measurement method; the methods vary in precision. BMR, or basal metabolic rate, estimates energy used at rest and is not a weight-status category. These measures answer different questions and are not interchangeable.
BMI limitations
BMI does not distinguish muscle from fat, show where fat is distributed, or describe a person's complete health. A muscular person can have a higher BMI without the index identifying how much of their weight is muscle. Pregnancy changes weight in ways for which these categories are not intended. Age, medical history, ethnicity and population-level differences can also affect interpretation; the same BMI may not correspond to the same amount or distribution of body fat in every population. A clinician may consider waist measurement, blood pressure, laboratory measures, history and other context.
BMI for children and teenagers
Children and teenagers from age 2 through 19 are assessed using BMI-for-age percentiles that account for age and sex. Adult cutoffs such as 18.5 and 25 should not be applied to them. Use a pediatric growth-chart tool or ask a clinician for an age-appropriate interpretation.
Frequently asked questions
For adults, the commonly used healthy-weight screening range is 18.5 to below 25. It is one indicator and does not define an individual's health or ideal weight.
Convert height to metres, square it, then divide weight in kilograms by that number. With pounds and inches, use (pounds × 703) ÷ inches squared.
The standard formula and adult category cutoffs are the same. The index does not account for individual differences in body composition.
BMI cannot distinguish muscle from fat, so it may not represent body fatness well for some muscular people. Other clinical measures may be useful.
Children and teens need BMI-for-age percentiles that account for age and sex. Adult cutoffs should not be used for them.
The adult formula does not use age. There is no single BMI that defines an ideal result for everyone; interpretation depends on broader health context.
Related calculators
See the calculator directory for available tools, including the Age Calculator and other everyday references.
Sources: CDC adult BMI categories, CDC child and teen BMI categories, and World Health Organization information on obesity and overweight.