BMI 計算機

Body Mass Index (BMI) is a simple number that relates your weight to your height, used worldwide as a quick screening check for whether an adult is in a healthy weight range. This calculator works it out and places the result in the World Health Organization (WHO) categories. It is a screening signal, not a measure of body fat or health on its own, but it is quick, free and needs only two numbers you already know.

BMI is your weight in kilograms divided by the square of your height in metres (kg/m²). For example, 70 kg at 1.75 m gives a BMI of about 22.9. You can enter your figures in metric or imperial units and the tool converts as needed, so you never have to do the maths by hand.

The WHO categories are: under 18.5 underweight, 18.5 to 24.9 normal weight, 25 to 29.9 overweight, and 30 or above obesity, which is further split into classes I, II and III. These thresholds are the same for men and women. The calculator highlights which band your result falls into so you can see it at a glance.

BMI has real limits. It does not distinguish muscle from fat, and it ignores build and where you carry weight, so it can misjudge people at the same height and weight who have very different body compositions. It is a population-level screening tool: useful for spotting a possible concern, but never a diagnosis on its own. For a fuller picture, pair it with your waist measurement or a body fat estimate.

Because it only sees height and weight, BMI is least reliable at the extremes and for specific groups: very muscular or athletic people (whose muscle reads as excess weight), some older adults (who lose muscle), and anyone with an unusual build. It is not designed for children, teenagers or pregnancy, which need age- and situation-specific charts. If any of these apply to you, treat the number cautiously and ask a doctor for the right reference.

Alongside your BMI, the tool shows the actual weight range that would keep you in the normal category for your height, which is often more useful than the single score because it gives you a concrete target to aim for. If you enter your age, it also shows a commonly used age-adjusted reference range, since the BMI linked to the lowest health risk tends to rise slightly as adults get older. Everything is calculated in your browser, and you can switch between metric and imperial units at any time.