How to Calculate BMI: Formula, Chart & What It Means
Learn how to calculate your Body Mass Index (BMI) with the simple formula. Understand what your BMI means, its limitations, and when to use other health metrics.
The BMI Formula
BMI is calculated by dividing your weight in kilograms by your height in meters squared: BMI = weight (kg) / height (m)². For imperial units: BMI = (weight in pounds × 703) / (height in inches)². For example, a person weighing 70 kg who is 1.75 m tall has a BMI of 70 / (1.75²) = 22.9.
BMI Categories
The World Health Organization defines BMI categories as: Underweight (below 18.5), Normal weight (18.5-24.9), Overweight (25.0-29.9), Obesity Class I (30.0-34.9), Obesity Class II (35.0-39.9), and Obesity Class III (40.0 and above). These ranges apply to adults aged 20 and older. Different ranges apply to children and teens.
Limitations of BMI
BMI doesn't distinguish between muscle and fat. Athletes and muscular individuals may have a 'overweight' BMI despite being healthy. It also doesn't account for fat distribution — belly fat (visceral fat) is more dangerous than hip/thigh fat. For a more complete picture, combine BMI with waist circumference, body fat percentage, and waist-to-hip ratio measurements.
BMI for Different Demographics
BMI interpretations vary by ethnicity and age. Asian populations may have higher health risks at lower BMI thresholds (WHO suggests overweight starts at 23 for Asians). Older adults may benefit from slightly higher BMIs (25-27) as some research suggests this is protective. For children, BMI is compared to age- and sex-specific percentiles rather than fixed cutoffs.
Better Alternatives to BMI
While BMI is a useful screening tool, more accurate health assessments include: body fat percentage (via DEXA scan or calipers), waist circumference (over 40 inches for men or 35 inches for women indicates risk), waist-to-hip ratio, and metabolic health markers (blood pressure, blood sugar, cholesterol). The best approach combines multiple metrics.
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Frequently Asked Questions
What is a healthy BMI?
For most adults, a BMI between 18.5 and 24.9 is considered healthy. However, this is just a screening tool — your doctor can assess your overall health considering muscle mass, age, ethnicity, and other factors.
Can BMI be too low?
Yes, a BMI below 18.5 is classified as underweight, which can indicate malnutrition, eating disorders, or other health conditions. Being underweight is associated with weakened immunity, bone loss, and fertility issues.
Does BMI apply to athletes?
BMI is less accurate for athletes and highly muscular individuals because muscle weighs more than fat. A bodybuilder with 10% body fat could have a BMI over 30. Body fat percentage is a better metric for athletes.