Body Mass Index (BMI) - What it is and how to calculate
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What is BMI?
How to calculate your BMI
To compute BMI manually: measure your weight in kilograms and your height in metres, square your height, then divide weight by the squared height. For example, if you weigh 70 kg and are 1.75 m tall: 70 ÷ (1.75 × 1.75) = 22.86. That value falls in the “healthy” range.
BMI Categories (adult)
The World Health Organization (WHO) categories commonly adopted are:
- Underweight: BMI < 18.5
- Normal weight: BMI 18.5 – 24.9
- Overweight: BMI 25 – 29.9
- Obesity: BMI > 30
These bands are useful for population screening — but individual health depends on many factors including muscle mass, bone structure, and fat distribution.
Benefits and limitations of BMI
Benefits: BMI is fast, inexpensive, and widely used for population-level risk assessment. It helps guide public health policy and raises awareness around weight-related health risks.
Limitations: BMI does not distinguish muscle from fat. Athletes with high muscle mass may have an elevated BMI but low body fat. Older adults may have normal BMI but high body fat with low muscle mass. For nuanced assessment, use our Body Fat Calculator or Lean Body Mass Calculator.
What to do with your BMI result
If your BMI is outside the healthy range, consider these steps:
- Consult a healthcare professional for personalized assessment.
- Track diet and physical activity — try our Calorie Calculator and TDEE Calculator.
- For performance or aesthetic goals, pair BMI with body composition tools such as body fat percentage and waist circumference.
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