How to Calculate BMI — Body Mass Index Explained
Blog › Health · 6 min read · Published 2026-02-17
Learn the BMI formula, what your score means, and its limitations as a health metric.
What Is BMI?
Body Mass Index (BMI) is a numerical value calculated from a person's height and weight. It was developed by Belgian mathematician Adolphe Quetelet in the 1830s and is used as a screening tool to categorize adults into weight status categories — underweight, normal weight, overweight, and obese.
BMI is widely used by healthcare providers, public health researchers, and insurance companies because it's simple to calculate and correlates reasonably well with body fat levels at the population level. However, it has significant limitations that are important to understand.
The BMI Formula
The formula is straightforward:
- Metric: BMI = weight (kg) / height² (m²)
- Imperial: BMI = 703 × weight (lbs) / height² (inches²)
Example (Metric): A person weighing 70 kg with a height of 1.75 m has BMI = 70 / (1.75²) = 70 / 3.0625 = 22.9
Example (Imperial): A person weighing 154 lbs at 5'9" (69 inches) has BMI = 703 × 154 / (69²) = 108,262 / 4,761 = 22.7
BMI Categories (WHO Standard)
- Underweight: BMI < 18.5
- Normal weight: BMI 18.5–24.9
- Overweight: BMI 25.0–29.9
- Obese Class I: BMI 30.0–34.9
- Obese Class II: BMI 35.0–39.9
- Obese Class III (Severe): BMI ≥ 40
Limitations of BMI
Despite its widespread use, BMI has important limitations:
- Athletes and muscular individuals: Muscle is denser than fat. A bodybuilder may have a high BMI (classified as "obese") despite having very low body fat.
- Older adults: As we age, muscle mass decreases and fat increases. An older adult may have a "normal" BMI but relatively high body fat.
- Ethnic differences: Asian populations tend to have higher health risks at lower BMI values. Many Asian countries use lower cutoffs (23 for overweight, 27.5 for obese).
- Doesn't measure fat distribution: Abdominal fat (visceral fat) is more dangerous than fat stored elsewhere. Waist circumference is a better predictor of metabolic risk.
- Doesn't apply to children: Children's BMI must be interpreted using age- and sex-specific growth charts (BMI-for-age percentiles).
What To Do With Your BMI
Use BMI as a starting point, not a definitive health assessment. If your BMI falls outside the normal range, consult your healthcare provider. They may recommend additional assessments like body fat percentage measurement, waist-to-hip ratio, blood tests (cholesterol, blood glucose, blood pressure), and DEXA scanning for accurate body composition.
Remember: health is multidimensional. A healthy BMI combined with regular exercise, a balanced diet, adequate sleep, and low stress levels is the goal — not just a number on a chart.