BMI (Body Mass Index) is a measure of body fat based on height and weight. It's calculated by dividing your weight in kilograms by your height in meters squared (kg/mยฒ).
BMI is a useful screening tool but has limitations. It doesn't distinguish between muscle and fat mass, so athletes may be classified as overweight despite being healthy. Age, sex, and body composition also affect accuracy.
A healthy BMI range for adults is 18.5 to 24.9. Below 18.5 is underweight, 25-29.9 is overweight, and 30+ is obese.
Alternatives include waist circumference, waist-to-hip ratio, body fat percentage (measured via DEXA, calipers, or BIA), and complete health assessments by healthcare providers.
BMI can be used for children ages 2+, but results are interpreted differently using age- and sex-specific percentile charts rather than the standard adult categories.
Checking monthly is sufficient for most people. If you're actively trying to lose or gain weight, weekly checks can help track progress, but don't obsess over daily fluctuations.