Understanding your BMI
Body mass index (BMI) relates weight to height. It is a quick screening measure, not a direct measurement of body fat or a diagnosis. Use the number above as a starting point for understanding the calculation, rather than a verdict on your health.
How to use this calculator
Use the units shown beside the fields: kilograms and centimeters in metric mode, or pounds and inches in US mode. Change the unit preference with the settings gear. The calculator updates as you change either value. It calculates the index only: it does not collect age, assess pregnancy, measure body composition or calculate a child’s BMI-for-age percentile.
Check the units before entering measurements. In metric mode, enter 180 for a height of 1.80 meters because the height field uses centimeters. In US mode, enter total inches: 6 feet 5 inches is 77 inches. Do not enter a feet-and-inches expression into a single numeric field. Extra decimal places do not compensate for an inaccurate measurement.
The formula and a worked example
BMI = weight in kilograms ÷ (height in meters × height in meters). The unit is kg/m². Convert centimeters to meters by dividing by 100 before squaring the height.
For an illustrative adult weighing 80 kg at 180 cm: 180 ÷ 100 = 1.80 m; 1.80 × 1.80 = 3.24 m²; 80 ÷ 3.24 = 24.691… kg/m². Rounded to one decimal place, the displayed BMI is 24.7.
Height is squared, so a height-entry error can materially change the answer. The calculation is not a percentage: a BMI of 24.7 does not mean 24.7% body fat. Near a category boundary, remember that a rounded display can hide a small difference in the underlying value.
Interpreting an adult result
The CDC uses these screening categories for adults aged 20 and older. The tool does not determine whether these adult categories are appropriate for the person whose measurements were entered.
- Below 18.5: underweight.
- 18.5 to below 25: healthy weight.
- 25 to below 30: overweight.
- 30 or higher: obesity; class 1 is 30 to below 35, class 2 is 35 to below 40, and class 3 is 40 or higher.
These labels describe screening ranges, not a complete assessment of an individual. For ages 2–19, use age- and sex-specific BMI percentiles through an appropriate child and teen calculator; this tool does not provide that interpretation.
Where BMI came from
The height–weight relationship traces to the nineteenth-century work of Belgian statistician Adolphe Quetelet. A 1972 paper by Ancel Keys and colleagues helped establish the name “body mass index” and its use for describing relative weight. This background matters: a compact statistical index can be useful for comparing groups while still leaving out important differences between individuals.
The history explains the formula’s role; it does not establish that one number can describe every person’s health. Modern interpretation needs evidence beyond the arithmetic.
Why it is useful—and what it misses
BMI is inexpensive, noninvasive and easy to calculate, making it practical for screening and population comparisons. However, body weight includes muscle, fat and bone. BMI cannot separate them or show where fat is carried. A clinician considers other information, such as blood pressure, cholesterol and examination findings.
Use this tool to understand the measurement and prepare questions about your result. Do not use it alone to choose a target weight, diagnose a condition or prescribe a diet. A change in the displayed number needs context rather than an automatic assumption that health improved or worsened.
Sources and review
Source review: October 27, 2026. Educational explanation; no claim of individualized clinical assessment.