BMI (Body Mass Index) is a simple ratio that shows whether your weight is in a healthy range for your height. It's the screening measure health organizations use most, because it only needs your height and weight. This article explains how BMI is calculated, what the ranges mean and where it can mislead you. To see your own value right away, use our BMI calculator.
How is BMI calculated?
The formula is simple: divide your weight in kilograms by the square of your height in metres.
BMI = weight (kg) ÷ height² (m²)
Example: someone who is 180 cm (1.80 m) tall and weighs 75 kg:
BMI = 75 ÷ (1.80 × 1.80) = 75 ÷ 3.24 = 23.1
That value is in the "normal" range. If you enter height in centimetres, remember to divide by 100 first (180 cm = 1.80 m).
BMI ranges (adults)
The World Health Organization (WHO) classification for adults:
| BMI | Classification |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal (healthy range) |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obese |
The healthy range is 18.5 to 24.9. This applies to adults; children and teens are assessed with age- and sex-specific percentile charts.
What BMI can't measure
BMI is quick and useful, but it can't tell muscle from fat. A muscular person who trains with weights may have a high BMI yet be perfectly healthy; conversely, someone with little muscle can carry excess fat at a "normal" BMI. It also says nothing about where fat sits (waist circumference matters). So treat BMI as a screening tool, not the whole story. For a fuller picture use our body fat calculator, and for a weight range suited to your height, our ideal weight calculator.
What if your BMI is high?
If your BMI is above the healthy range, there's no need to panic — small, sustainable steps work best. Weight management comes down to calorie balance: taking in a little less than you burn. To learn your daily calorie needs, try our calorie calculator. If you have a specific health concern, it's always best to speak with a doctor.
In short
BMI = weight ÷ height²; the healthy range is 18.5–24.9. It's a simple, useful screen but can't separate muscle from fat, so don't rely on it alone. To see your own BMI and healthy weight range, use our BMI calculator. This content is for general information and is not medical advice.