BMI (Body Mass Index) is one of the most widely used screening tools in medicine and public health. It provides a quick numerical estimate of whether a person's weight is appropriate for their height. While it has well-known limitations, BMI remains the standard first step in weight-related health assessments in Poland and worldwide. This article explains the formula, the classification ranges, a worked example, and what BMI cannot tell you.
Medical disclaimer: BMI is a screening tool, not a diagnostic measure. Consult a qualified healthcare professional for personal medical advice.
The BMI Formula
The calculation is straightforward:
BMI = weight (kg) / height (m)²
For example, a person weighing 80 kg and standing 1.75 m tall has:
- Height squared: 1.75 × 1.75 = 3.0625
- BMI: 80 / 3.0625 = 26.1
This result places the person in the overweight category according to standard WHO classifications.
BMI Classification Ranges (WHO Standard)
The World Health Organisation divides BMI into the following categories for adults:
- Below 18.5 — Underweight (risk of nutritional deficiencies, weakened immunity)
- 18.5 – 24.9 — Normal weight (healthy range for most adults)
- 25.0 – 29.9 — Overweight (elevated risk of metabolic conditions)
- 30.0 – 34.9 — Obese Class I (moderate health risk)
- 35.0 – 39.9 — Obese Class II (severe health risk)
- 40.0 and above — Obese Class III / Morbidly Obese (very high health risk)
Further Worked Examples
- Person A: 55 kg, 1.68 m → BMI = 55 / (1.68)² = 55 / 2.8224 = 19.5 (Normal weight)
- Person B: 95 kg, 1.80 m → BMI = 95 / (1.80)² = 95 / 3.24 = 29.3 (Overweight)
- Person C: 110 kg, 1.70 m → BMI = 110 / (1.70)² = 110 / 2.89 = 38.1 (Obese Class II)
Limitations of BMI
Despite its widespread use, BMI has several important limitations that every user should understand:
- Muscle vs fat — BMI cannot distinguish between muscle and fat tissue. A professional athlete with high muscle mass may have a BMI in the overweight range despite having very low body fat.
- Age and sex — The same BMI value carries different health implications for men vs women and for older vs younger adults. Older adults naturally lose muscle and gain fat without changes in BMI.
- Ethnicity — The WHO has recognised that people of Asian descent face increased cardiometabolic risk at lower BMI values. Many Asian health authorities use adjusted thresholds: overweight at BMI ≥23, obese at BMI ≥27.5.
- Height extremes — Very tall or very short individuals may receive misleading BMI results.
Better Complementary Metrics
If you want a more complete picture of body composition and health risk, consider these additional measures alongside BMI:
- Waist circumference — high abdominal fat (waist >94 cm for men, >80 cm for women) is an independent risk factor for heart disease and type 2 diabetes
- Waist-to-height ratio — a value above 0.5 indicates elevated risk, regardless of BMI
- Body fat percentage — measured by DEXA scan, bioimpedance scales, or skinfold callipers
Calculate your BMI instantly and see your classification using the BMI calculator on Liczbnik.pl. Enter your weight and height in metric or imperial units and get your result in seconds — no maths required.