BMI Calculator

How BMI is calculated

BMI is calculated by dividing your weight in kilograms by the square of your height in meters: BMI = kg / m². In imperial units, the formula is: BMI = (weight in lbs × 703) / (height in inches)². The result is a unitless number used to broadly classify weight status.

BMI ranges at a glance

• Underweight: < 18.5. May indicate undernutrition; worth a provider conversation.
• Normal weight: 18.5–24.9. Healthy range. Maintain with regular movement and balanced nutrition.
• Overweight: 25–29.9. Consider small, sustainable changes to support long-term health.
• Obese: 30–34.9. Increased health risks; focus on habits and speak with a professional if needed.
• Extremely obese: 35+. Higher health risks; consult a healthcare professional.

Limitations of BMI

• Doesn't measure body fat directly. Two people with the same BMI can have very different body-fat percentages. BMI is a screening signal, not a diagnosis.
• Misclassifies muscular bodies. Heavily muscled individuals (athletes, bodybuilders) often register as overweight or obese despite low body fat. Body-fat percentage via DEXA or calipers is more meaningful.
• Age and sex matter. The same BMI carries different health implications for older adults vs. younger adults, and for men vs. women, but the cutoffs don't change with either.
• Ethnic variation matters too. Some populations have higher health risk at lower BMI values; Asian populations, for example, are often assessed at different cutoffs (overweight at BMI 23+).
• Track trends, not daily numbers. Hydration alone can swing weight 2–5 lbs. Monthly or quarterly checks are more useful than daily ones.

FAQ

What is a healthy BMI?

For most adults, a BMI between 18.5 and 24.9 is considered a healthy weight range. Values below 18.5 are classified as underweight; 25–29.9 as overweight; and 30 and above as obese. These are population-level guidelines, not individual diagnoses.

Can BMI be inaccurate for athletes?

Yes, BMI frequently misclassifies muscular athletes as overweight or obese because muscle tissue is denser and heavier than fat. For athletes and very active individuals, body composition measurements (like DEXA scans or body fat calipers) are more meaningful than BMI.

Is BMI different for children?

For children and teens (ages 2–19), BMI is interpreted using age- and sex-specific percentile charts rather than fixed adult cutoffs. A pediatrician can provide a proper assessment using growth charts from the CDC or WHO.

What other metrics are better than BMI?

Waist circumference, waist-to-hip ratio, and waist-to-height ratio are considered better predictors of metabolic health and cardiovascular risk than BMI alone. Body fat percentage (measured via DEXA, hydrostatic weighing, or bioelectrical impedance) is the most direct measure of adiposity.

How often should I check my BMI?

For most people, checking BMI monthly or quarterly is sufficient for tracking general trends. More frequent checks aren't necessary and can cause unnecessary anxiety over normal daily weight fluctuations (which can be 2–5 lbs from hydration alone).