The body mass index, or BMI (Body Mass Index), is the most popular tool for a quick assessment of whether your body weight falls within a healthy range relative to your height. Although it has its limitations, it remains a starting point in doctors' offices and health programs around the world. In this article we show the formula, the World Health Organization (WHO) ranges, and a complete step-by-step calculation.
The BMI Formula
BMI is calculated by dividing body weight in kilograms by the square of height expressed in metres:
BMI = weight [kg] / (height [m])²
The result is a dimensionless number that is compared against established ranges. Remember to give your height in metres, not centimetres — this is the most common mistake when calculating by hand.
BMI Ranges According to WHO
For adults, the World Health Organization uses the following ranges:
- below 18.5 — underweight
- 18.5–24.9 — normal weight
- 25.0–29.9 — overweight
- 30.0–34.9 — class I obesity
- 35.0–39.9 — class II obesity
- 40.0 and above — class III obesity (morbid)
Step-by-Step Calculation
Let us assume a person weighs 78 kg and is 175 cm tall. We calculate:
- Convert height to metres: 175 cm = 1.75 m
- Square the height: 1.75 × 1.75 = 3.0625 m²
- Divide weight by the square of height: 78 / 3.0625 = 25.5
The result of 25.5 falls within the 25.0–29.9 range, which means overweight. To reach the upper limit of the normal range (BMI 24.9), this person would need to weigh about 76 kg.
Limitations of BMI
BMI does not distinguish muscle mass from fat mass. An athlete with a large amount of muscle may have a BMI indicating "overweight" despite a low level of body fat. On the other hand, an older person who has lost muscle may have a normal BMI while carrying excess fat. That is why it is worth complementing BMI with a waist circumference measurement or the WHR ratio.
For Whom BMI Can Be Misleading
- Athletes and muscular people — inflated BMI despite a healthy physique
- Pregnant women — the index does not apply
- Children and adolescents — percentile charts are used, not adult thresholds
- Older people — a shift in the muscle-to-fat ratio distorts the result
BMI and Metabolic Health
Research shows that the risk of cardiovascular disease and type 2 diabetes rises with a BMI above 25, but it correlates even more strongly with abdominal obesity. The index itself is a warning signal, not a diagnosis. A low BMI also carries risk — it may be linked to deficiencies and weakened immunity.
Remember
BMI is a screening tool, not a diagnostic one. It does not replace a consultation with a doctor or dietitian. If your result deviates from the norm, treat it as an encouragement to talk to a specialist who will assess your body composition, lifestyle and health history.
Want to find out your result without doing the maths in your head? Use our BMI calculator on liczbnik.pl — enter your weight and height and get the result along with the WHO range interpretation in an instant.