Body Mass Index (BMI) is a simple screening tool that estimates whether a person's weight is proportionate to their height. It's used worldwide by doctors, nutritionists, and health organizations as a quick way to identify potential weight-related health risks. While BMI has important limitations, it remains one of the most widely used health metrics.
The BMI formulas
For a person who weighs 70 kg and is 1.75 m tall: BMI = 70 ÷ (1.75)² = 70 ÷ 3.0625 = 22.86, which falls in the "normal weight" range.
BMI categories (WHO classification)
- Below 18.5: Underweight — may indicate malnutrition, an eating disorder, or other health issues.
- 18.5 – 24.9: Normal weight — generally associated with the lowest health risks.
- 25.0 – 29.9: Overweight — elevated risk for cardiovascular disease, type 2 diabetes, and hypertension.
- 30.0 – 34.9: Obese (Class I) — significantly increased health risks.
- 35.0 – 39.9: Obese (Class II) — high risk of weight-related health conditions.
- 40.0 and above: Obese (Class III / severe) — very high risk. May qualify for surgical intervention.
Limitations of BMI
BMI is a useful screening tool, but it has well-known blind spots:
- Doesn't distinguish muscle from fat. Athletes and bodybuilders often have "overweight" BMIs despite being very lean. Muscle is denser than fat.
- Doesn't account for fat distribution. Abdominal (visceral) fat carries more health risk than fat stored in hips and thighs. Waist circumference is a better predictor of metabolic risk.
- Age and sex differences. Older adults naturally lose muscle mass, and women tend to carry more body fat than men at the same BMI.
- Ethnicity. Health risks associated with BMI vary across ethnic groups. Asian populations, for example, face elevated risk at lower BMI thresholds.
What to do with your BMI result
BMI is a starting point, not a diagnosis. If your BMI falls outside the normal range, consider discussing it with a healthcare provider alongside other measurements like waist circumference, body fat percentage, blood pressure, and bloodwork. For tracking weight management progress, BMI is helpful as one data point among several — combine it with our TDEE calculator to understand your daily calorie needs.