BMI Calculator
Calculate your Body Mass Index based on height and weight. See where you fall on the WHO classification scale.
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How This BMI Calculator Works
This calculator uses the standard BMI formula adopted by the World Health Organization. For imperial units, it multiplies weight in pounds by 703 and divides by the square of height in inches. For metric units, it divides weight in kilograms by the square of height in meters.
Understanding BMI Categories
Underweight (below 18.5): May indicate insufficient body weight. Can be associated with nutritional deficiencies and weakened immune function.
Normal weight (18.5 to 24.9): Generally associated with the lowest risk of weight-related health problems. Most adults should aim for this range.
Overweight (25 to 29.9): Weight above the normal range. May increase risk for type 2 diabetes, high blood pressure, and heart disease.
Obese (30 and above): Associated with significantly increased health risks. Further divided into Class I (30-34.9), Class II (35-39.9), and Class III (40+).
Limitations of BMI
BMI does not distinguish between muscle and fat mass. Highly muscular individuals may have a high BMI despite low body fat. BMI also does not account for fat distribution. For a more complete picture, use our Body Fat Calculator or Waist-to-Hip Ratio Calculator.
BMI Classification Table (WHO)
| Classification | BMI Range (kg/m²) | Health Risk |
|---|---|---|
| Severe Thinness | < 16.0 | High |
| Moderate Thinness | 16.0 – 16.9 | Moderate |
| Mild Thinness | 17.0 – 18.4 | Low |
| Normal Weight | 18.5 – 24.9 | Low |
| Overweight | 25.0 – 29.9 | Increased |
| Obese Class I | 30.0 – 34.9 | High |
| Obese Class II | 35.0 – 39.9 | Very High |
| Obese Class III | ≥ 40.0 | Extremely High |
BMI Across Different Populations
The standard BMI categories were developed primarily from studies of European populations. Research published in The Lancet suggests that some populations, particularly South and East Asian groups, may experience elevated health risks at lower BMI thresholds. The WHO recommends additional cut-off points of 23.0 and 27.5 for Asian populations when assessing overweight and obesity risk.
For children and teens aged 2 to 20, BMI is calculated the same way but interpreted differently. The Centers for Disease Control and Prevention (CDC) uses age- and sex-specific percentile charts because body composition changes significantly during growth. A child at the 85th percentile is classified as overweight; at the 95th percentile, obese.
BMI and Mortality Risk
Large-scale epidemiological studies consistently show a J-shaped or U-shaped relationship between BMI and mortality. The Global BMI Mortality Collaboration, analyzing data from 10.6 million participants across 239 studies, found that all-cause mortality was lowest at BMI 20-25, with sharp increases below 18.5 and above 30. However, for people aged 65 and older, slightly higher BMIs (25-27) may be protective — a phenomenon called the "obesity paradox."
Practical Steps Based on Your BMI
If your BMI is outside the healthy range, consider the following evidence-based approaches. For weight loss: create a moderate calorie deficit of 500 calories below your TDEE using our Calorie Calculator. Prioritize protein intake (1.2-1.6g/kg) using the Protein Intake Calculator to preserve lean muscle during weight loss. Track your progress with the Body Fat Calculator rather than relying solely on BMI, since body composition matters more than scale weight.
For underweight individuals: gradually increase calorie intake to 300-500 above your TDEE. Focus on nutrient-dense foods and adequate protein. Consider resistance training to build lean mass — track your progress with the One Rep Max Calculator. If underweight is accompanied by other symptoms, consult a healthcare provider.
Regardless of BMI, regular physical activity dramatically improves health outcomes. Even 150 minutes of moderate activity per week (like brisk walking) reduces cardiovascular risk by 30-40%. Track your exercise with our Calories Burned Calculator and optimize your training intensity with the Target Heart Rate Calculator.
Frequently Asked Questions
Explore More Tools
Use our Body Fat Calculator for body composition. Find target weight with Ideal Weight Calculator. For calorie needs, try the TDEE Calculator.
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