The BMI Calculator computes your Body Mass Index — a screening metric that estimates whether your weight is in a healthy range for your height. It accepts both metric (kg/cm) and imperial (lb/in) inputs and instantly shows which World Health Organization category you fall into.

The formula, and a worked example

BMI is weight in kilograms divided by height in metres squared:

BMI = weight (kg) ÷ height (m)²

Take someone who is 1.75 m tall and weighs 72 kg. Height squared is 1.75 × 1.75 = 3.0625. Then 72 ÷ 3.0625 = 23.5, which sits comfortably in the “normal weight” band. Using imperial units the formula becomes weight(lb) ÷ height(in)² × 703 — the calculator applies whichever version matches your inputs, so you never have to convert by hand.

WHO weight categories

What BMI does and does not tell you

BMI is a fast, free population-level screen — that is its strength and its limit. Because it only uses height and weight, it cannot distinguish muscle from fat. A lean, heavily muscled athlete can register as “overweight” despite low body fat, while an older adult who has lost muscle may sit in the “normal” range yet carry excess fat. BMI also says nothing about where fat is stored, and abdominal fat carries more health risk than fat on the hips and thighs.

For those reasons, treat BMI as a starting point rather than a diagnosis. Waist circumference, waist-to-height ratio, and a body-fat measurement give a fuller picture, and a clinician can interpret them together with your blood pressure, blood sugar, and family history.

Special populations

Standard adult BMI categories do not apply to everyone. For children and teens (ages 2–19), BMI is interpreted against age- and sex-specific percentile charts, not the fixed bands above. Pregnant people, competitive athletes, and some ethnic groups (for whom health risks can begin at lower BMI thresholds) also need adjusted interpretation. If you fall into one of these groups, use the number as context and confirm with a healthcare provider.

Frequently asked questions

Is a lower BMI always better?

No. Below 18.5 is classed as underweight and carries its own risks, including reduced bone density and weakened immunity. The healthiest range for most adults is 18.5–24.9.

How much weight changes my BMI by one point?

For the 1.75 m person above, each whole BMI point is about 3 kg (roughly 6.7 lb). Shorter people move a full point with less weight change, taller people with more.

Is my data private?

Yes. Your height and weight are used only in your browser to compute the result and are never sent to or stored on our servers.

This tool is for general information and is not medical advice. Consult a qualified healthcare professional before making decisions about your health.

Sources and further reading

• U.S. Centers for Disease Control and Prevention – About Adult BMI
• World Health Organization – Obesity and overweight