BMI = weight(kg) ÷ height(m)². WHO: <18.5 underweight, 18.5–24.9 normal, 25–29.9 overweight, 30+ obese.
A BMI calculator tells you your Body Mass Index — a single number that relates your weight to your height. Doctors and researchers around the world use it as a quick screening tool to sort adults into weight categories. It does not measure body fat directly, but for most people it tracks closely enough with body fat to be a useful first check.
The World Health Organization (WHO) defines the standard adult categories:
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal (healthy) weight |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obese |
The formula is simple and has been used since the 19th century:
BMI = weight in kilograms ÷ (height in metres)²
That is it — one division. Because height is squared, small differences in height change the result more than you might expect, which is why entering your height accurately matters. This calculator takes your height in centimetres and converts it automatically.
The result updates instantly as you type, so you can also experiment — for example, see what weight would move you into the healthy range.
Someone weighing 70 kg with a height of 175 cm (1.75 m):
BMI = 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9
22.9 sits inside the 18.5–24.9 band, so the category is normal (healthy) weight.
If your BMI lands outside the healthy range, treat it as a nudge rather than an alarm. Small, steady changes beat dramatic overhauls: a daily walk, one less sugary drink, and an extra serving of vegetables move most people in the right direction within weeks. If your BMI is above 30 or below 18.5, or if you have conditions such as diabetes or high blood pressure, a doctor or dietitian can build a plan around your full health picture — not just this one number.
The WHO defines 18.5–24.9 as the healthy range for most adults.
Not always — muscle weighs more than fat, so very muscular people can show a high BMI while being lean.
