- BMI is calculated by dividing weight in kilograms by height in meters squared
- The WHO 2000 thresholds classify results ranging from underweight to class III obesity
- BMI does not differentiate between fat and muscle mass: it should be used alongside other measurements
- Waist circumference is a key complementary indicator for metabolic risk
What is BMI?
BMI stands for Body Mass Index. It is a value derived from the mass and height of an individual, and it is used by doctors, nutritionists, and researchers worldwide to classify body weight in relation to health outcomes.
The formula is simple:
Example: A person who weighs 70 kg and is 1.75 m tall has a BMI of 70 ÷ (1,75 × 1,75) = 70 ÷ 3,06 = 22,9 — which falls within the healthy range.
WHO 2000 Weight Categories
The World Health Organization (WHO) established BMI classification thresholds for adults in 2000. These are the same categories used by our calculator:
| BMI | Category | Health Risk |
|---|---|---|
| < 18.5 | Underweight | Increased (malnutrition, osteoporosis) |
| 18.5 – 24.9 | Healthy Weight | Normal |
| 25.0 – 29.9 | Overweight | Increased |
| 30.0 – 34.9 | Obesity Class I | Moderately High |
| 35.0 – 39.9 | Obesity Class II | High |
| ≥ 40.0 | Obesity Class III | Very High |
Why BMI Doesn't Tell the Whole Story
While BMI is a quick and useful tool, it has significant limitations that are important to keep in mind:
It doesn't distinguish between fat and muscle
An athlete with high muscle mass might have a BMI of 27 (classified as overweight) despite having a low body fat percentage and excellent health. Conversely, someone with a "normal" BMI might carry excess visceral fat and have low muscle mass — a condition known as sarcopenic obesity, which carries similar health risks to standard obesity.
It doesn't show where fat is distributed
Abdominal (visceral) fat is much more dangerous than fat stored around the hips and thighs. Two people with the exact same BMI can face entirely different health risks depending on where their body weight is concentrated.
A normal BMI does not guarantee the absence of metabolic risk. Waist circumference and other health markers should always be evaluated together.
It varies by age, sex, and ethnicity
WHO thresholds were primarily developed using data from European populations. Asian populations often exhibit metabolic risks at lower BMI levels (some Asian guidelines use 23 instead of 25 as the threshold for overweight). Additionally, in older adults, a slightly higher BMI (up to 27) is actually associated with better survival rates.
Waist Circumference: The Essential Companion
For a more accurate health profile, you should also measure your waist circumference (around the abdomen at the level of the belly button, after exhaling naturally). The thresholds for increased risk are:
- Men: increased risk > 94 cm, very high risk > 102 cm
- Women: increased risk > 80 cm, very high risk > 88 cm
If your BMI is within the healthy range but your waist circumference exceeds these thresholds, consult your doctor, as your metabolic and cardiovascular risk may still be elevated.