What is BMI and how is it calculated? Complete guide
The Body Mass Index (BMI) is the most widely used metric in the world to estimate if someone has a healthy weight. But it is also one of the most misunderstood. Here is the formula, the official WHO ranges, and most importantly when BMI is not reliable.
⚖️ Calculate my BMI now →What is BMI?
The Body Mass Index is a numerical value that relates a person's weight to their height. It has been used since the 19th century and the World Health Organization (WHO) adopted it as the reference indicator in the 1990s.
It serves as a first approximation of nutritional status for a population or individual, but it is not a diagnosis.
BMI formula
BMI = weight (kg) / height² (m)Height must be squared and in meters, not centimeters. That is the most common mistake when calculating it by hand.
Example
A person who weighs 75 kg and is 1.80 m tall:
- Height² = 1.80 × 1.80 = 3.24
- BMI = 75 / 3.24 = 23.1
According to WHO ranges, this BMI is "normal weight".
WHO ranges
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obesity class I |
| 35.0 – 39.9 | Obesity class II |
| 40 or more | Obesity class III (morbid) |
BMI limitations (what nobody tells you)
BMI was invented as a statistical tool for populations, not for specific individuals. It has several issues:
1. Does not distinguish muscle from fat
A bodybuilder weighing 100 kg at 1.75 m has a BMI of 32.6 → "obesity". Obviously not. Same for many athletes: muscle weighs more than fat.
2. Does not measure fat distribution
Abdominal (visceral) fat is much more dangerous than subcutaneous fat, but BMI does not differentiate. Two people with the same BMI can have very different cardiovascular risks.
3. Loses accuracy in elderly and children
In people over 65, muscle mass declines and BMI can read "normal" when sarcopenia is present. For children and adolescents, age and sex specific curves are used, not the adult BMI.
4. Does not consider body type or ethnicity
Studies show the risk associated with the same BMI varies between Asian, African and European populations. The WHO recognizes this and proposes regional adjustments.
Alternatives and complements
- Waist circumference: men above 40 inches (102 cm) and women above 35 inches (88 cm) indicates elevated risk.
- Waist-to-hip ratio: ratio between both measurements.
- Body fat percentage: measured with bioelectrical scale, skin-fold calipers or DEXA. More precise but less accessible.
- Body composition analysis: distinguishes muscle, fat, water and bone.
Is it useful at all?
Yes, as an initial screening. If your BMI is clearly out of the normal range (well above or below), that is reason enough to dig deeper with other measurements and consult a professional.
But a "normal" BMI does not guarantee health, nor does a "high" BMI always mean actual obesity.
Calculate your BMI
⚖️ Open the calculator →Conclusion
BMI is a useful and easy first step: two numbers and one division. But it is not a diagnosis and does not directly measure body fat. Use it as a starting point and complement with other measurements if you want a real picture of your fitness. If you are concerned about your weight, the best you can do is talk to a doctor or nutritionist.