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BMI calculator

Work out your body mass index (BMI). Enter your height and weight and get your result and category on the World Health Organization (WHO) scale right away.

The calculation runs in your browser. We don't send or store what you enter.

Your result

Your BMI (kg/m²)
Underweight Normal weight Overweight Obesity class 1 Obesity class 2 Obesity class 3

Your BMI is under 18.5, which the WHO classes as underweight. If your weight swings around without a clear reason, talk to a doctor about it.

Your BMI is between 18.5 and 24.9, which the WHO classes as normal weight.

Your BMI is between 25 and 29.9, which the WHO classes as overweight. BMI doesn't tell muscle apart from fat, so it often reads higher for athletes.

Your BMI is between 30 and 34.9, which the WHO classes as obesity class 1. For what that means for you, a doctor is the right person to ask.

Your BMI is between 35 and 39.9, which the WHO classes as obesity class 2. For what that means for you, a doctor is the right person to ask.

Your BMI is 40 or above, which the WHO classes as obesity class 3. For what that means for you, a doctor is the right person to ask.

The WHO BMI scale (adults)

Underweight 0,0 – 18,4
Normal weight 18,5 – 24,9
Overweight 25,0 – 29,9
Obesity class 1 30,0 – 34,9
Obesity class 2 35,0 – 39,9
Obesity class 3 ≥ 40,0

Your next step

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For information only, not a substitute for medical advice.

What is body mass index (BMI)?

Body mass index (BMI) is a simple ratio of weight to height. You work it out by dividing weight in kilograms by height in meters squared: BMI = kg / m². Example: someone 170 cm tall weighing 68 kg has a BMI of 68 / (1.70 × 1.70) = 23.5.

The World Health Organization (WHO) uses BMI as a quick screening measure at population level. For adults, the same scale applies regardless of sex and age: under 18.5 is underweight, 18.5 to 24.9 is normal weight, 25 to 29.9 is overweight, and 30 or above is obesity, split into three classes.

BMI calculator with age: does the same scale apply?

For adults (from age 18 onward), the scale doesn't change with age – a BMI of 23 means the same thing at 25 as it does at 55. It's different for children and teenagers: they're still growing, so their BMI is compared with growth curves for the same sex and age (percentiles). This calculator isn't suited to children; a pediatrician is the right person to assess BMI for them.

For older adults, some clinical guidelines allow slightly higher values, since muscle mass tends to decline with age. That's another reason BMI is a starting point for a conversation, not a final verdict.

What BMI doesn't tell you

BMI doesn't tell muscle apart from fat, doesn't say where fat sits on the body, and doesn't account for body frame. A muscular athlete can land in the overweight category despite carrying little fat. That's why BMI is often used alongside waist circumference and other measurements.

BMI isn't suited to pregnant women or children, and for athletes and older adults it needs to be read with caution. If a result worries you, your own doctor is the best person to ask.

Frequently asked questions

How do I calculate BMI?

Divide weight in kilograms by height in meters squared. At 180 cm and 80 kg: 80 / (1.8 × 1.8) = 24.7. The calculator above does this for you.

What BMI counts as normal?

On the WHO scale, a normal BMI for adults falls between 18.5 and 24.9.

Does a BMI calculator with age give a different result?

Not for adults – the WHO scale is the same at every age from 18 onward. For children and teenagers, BMI is compared against percentile curves for sex and age instead.

What's the ideal weight for my height?

There's no single ideal weight. The calculator shows you the weight range in which your BMI would fall within the WHO normal range (18.5–24.9).

Does BMI apply to athletes too?

Only partly. Because BMI can't tell muscle from fat, muscular people often get a higher BMI than their body composition would suggest.

Sources

  1. WHO – A healthy lifestyle: WHO recommendations (body mass index)
  2. WHO – Obesity and overweight (facts)
  3. CDC – About Body Mass Index (BMI)

Updated: October 2026

For information only, not a substitute for medical advice.

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