How to Calculate BMI: Formula, Interpretation & Limitations

September 5, 2026 · 4 min read

Body Mass Index is the most widely used screening tool for weight categories. It is simple to calculate, universally understood, and useful as a starting point for health conversations. It is also frequently misinterpreted and has significant limitations that are important to understand.

The BMI Formula

BMI = weight (kg) / height (m)². For imperial units: BMI = weight (lbs) × 703 / height (inches)². A person who weighs 70 kg and is 1.75 m tall has a BMI of 70 / (1.75²) = 22.9. Convert your measurements to metric first for the simplest calculation, or use our Unit Converter.

BMI Categories

Underweight: below 18.5. Normal weight: 18.5 to 24.9. Overweight: 25.0 to 29.9. Obesity Class I: 30.0 to 34.9. Obesity Class II: 35.0 to 39.9. Obesity Class III: 40.0 and above. These categories were established by the WHO and are used globally for population-level health assessment.

What BMI Does Not Tell You

BMI does not distinguish between muscle and fat. A muscular athlete may have a BMI of 28 (overweight category) while having very low body fat. BMI does not account for body fat distribution — abdominal fat is more dangerous than fat elsewhere. It does not vary by age, sex, or ethnicity, despite different healthy ranges for these groups.

Better Used as a Starting Point

BMI is most useful as one data point among many. It works well for population studies and general screening but should not be the sole measure of individual health. Waist circumference, body fat percentage, blood markers, fitness levels, and overall well-being all matter. Consult a healthcare provider for personalized assessment.

Try It Now

Our free Percentage Calculator handles this instantly — no signup, no limits.

Also useful: our Unit Converter for related calculations.