What your BMI means
BMI sorts adults into four broad bands: underweight (below 18.5), normal weight (18.5 to 24.9), overweight (25 to 29.9), and obese (30 and above). These cutoffs come from large population studies linking body weight to health risk, so they describe groups better than any single person. A BMI of 24, for example, sits inside the healthy range, close to the upper edge of normal.
Where BMI falls short
Because BMI only uses height and weight, it cannot tell muscle from fat. A muscular athlete and a sedentary adult can share the same BMI while having very different body compositions. BMI also reads differently across ages, ethnic backgrounds, and for pregnant people. Treat it as a starting point, then look at waist circumference, body-fat percentage, and how you feel for a fuller picture.
Using BMI to set goals
If your number lands outside the healthy band, small consistent changes usually move it more reliably than crash diets. Losing or adding roughly a pound a week through modest calorie and activity adjustments keeps the change sustainable. Recheck your BMI every few weeks rather than daily, since normal water-weight swings can mask real progress from one morning to the next.
More worked examples
An adult who is 5 ft 4 in and weighs 150 lb has a BMI of 25.7, in the overweight category. The calculator shows a healthy weight range of 108 – 145 lb for that height, and a BMI Prime of 1.03.
Using metric units: an adult who is 183 cm tall and weighs 102 kg has a BMI of 30.5, in the obese category. The healthy weight range for that height is 62 – 83.4 kg, and the BMI Prime is 1.22.
When to use this calculator
Use it as a quick screening check for adults 20 and older, to see which of the four categories a height and weight fall into, and to find the weight range that corresponds to a BMI of 18.5 to 24.9 at your height. Switch Units between US and metric depending on how you measure. For anyone under 20, use the pediatric percentiles described in the FAQ instead.
Common mistakes
Entering height in inches only (for example 64 in the feet field) or mixing metric and US values produces a wrong number, so check the Units setting first. Weighing at different times of day adds noise; compare readings taken under similar conditions. And remember that a single BMI value is a screening result, not a diagnosis.
Reading the result panel
The headline is your BMI to one decimal place, and the category is based on that rounded value. Below it, the panel shows your healthy weight range, which is the span of weights that would put your BMI between 18.5 and 24.9 at your height. Weight to obese line shows how many pounds or kilograms separate you from a BMI of 30. BMI Prime is your BMI divided by 25, the upper limit of the normal range: a value below 1.00 is under that limit and a value above it is over. The panel also repeats your height and weight in the other unit system, which is useful when a doctor's form asks for metric figures.
Age and the adult ranges
The calculator asks for your age because the adult categories only apply from age 20. For younger users it still computes the BMI number, but the four adult bands are not the right yardstick; a pediatric growth chart from a health provider is the appropriate comparison.