BMI = weight (kg) / height (m)². The simplest screening metric: in a second it tells you whether your body mass falls in the healthy range.
The main thing to understand: BMI does not distinguish fat from muscle and knows nothing about where the fat is located. So it is good as a first filter but works poorly for athletes (overstates) and for older adults losing muscle (understates).
Reference ranges
WHO classification for adults:
| Underweight | below 18.5 |
| Normal | 18.5 – 24.9 |
| Overweight | 25.0 – 29.9 |
| Obesity class I | 30.0 – 34.9 |
| Obesity class II | 35.0 – 39.9 |
| Obesity class III | 40.0 and above |
What an elevated level means
A BMI above 25 is overweight, above 30 is obesity. As BMI rises, so does the risk of type 2 diabetes, arterial hypertension, cardiovascular disease, fatty liver disease, sleep apnea and osteoarthritis.
But an important caveat: a high BMI on its own does not equal disease. In a person with a large muscle mass it may formally show "obesity" without any metabolic risk. What matters is not the number but body composition and visceral fat.
What a low level means
A BMI below 18.5 is underweight. It also carries risks: reduced bone mineral density, weakened immunity, loss of muscle mass (sarcopenia), and menstrual cycle disruption.
In older adults a low BMI is associated with higher mortality — the risk-versus-body-mass relationship is U-shaped: both too much and too little are bad.
How the test is done
Weigh yourself in the morning, fasting, after the toilet, on the same scale, in minimal clothing. Measure height without shoes, standing straight.
Convert height to meters for the formula. Example: weight 70 kg, height 175 cm → BMI = 70 / (1.75 × 1.75) = 22.9.
What affects the value
- Calorie balance — intake versus expenditure.
- Muscle mass — in athletes BMI overstates "obesity"; in older adults it masks muscle loss.
- Age — over the years muscle is lost and fat accumulates at the same weight.
- Hormones — hypothyroidism, excess cortisol, menopause.
- Medications — glucocorticoids, some antidepressants and antipsychotics.
- Sleep and stress — lack of sleep raises appetite via ghrelin and leptin.
Calculate using this marker
Calculators where BMI is used directly:
Frequently asked questions
What BMI is considered normal?
By WHO classification — 18.5 to 24.9. But that is a population range, not a personal target: in a person with well-developed muscle a BMI of 26–27 can be perfectly healthy, while in an older adult with sarcopenia a BMI of 23 may mask excess fat.
Why is BMI criticized?
Because it does not tell fat from muscle and does not show where the fat is. And it is visceral fat (around the organs) that drives metabolic risk. So BMI is worth complementing with waist circumference, the waist-to-height ratio or a body-fat estimate.
What should replace BMI?
Not replace but complement. The waist-to-height ratio (WtHR) better reflects visceral obesity, a body-fat estimate reflects composition, and the LAP index detects hidden visceral fat from blood tests.