Body Mass Index (BMI) is a numerical value calculated from a person's weight and height. It provides a simple, inexpensive, and non-invasive way to estimate body fat levels. BMI is widely used by healthcare professionals as a screening tool to identify potential weight-related health issues. While not a direct measure of body fat, it correlates well with more direct measures in most individuals.

Metric: BMI = Weight (kg) ÷ Height² (m²)

Imperial: BMI = (Weight (lbs) × 703) ÷ Height² (in²)

For example, a person who is 70 kg and 175 cm tall: BMI = 70 ÷ (1.75)² = 70 ÷ 3.0625 = 22.9 — which falls in the Normal range.

BMI Prime is the ratio of your BMI to the upper boundary of the normal BMI range (25 kg/m²). A BMI Prime of 1.0 means you're exactly at the upper limit of normal weight. Values below 0.74 indicate underweight, 0.74–1.0 is normal, and above 1.0 means overweight or obese. It makes it easy to compare how far someone is from their ideal weight range.

The Ponderal Index (PI) is similar to BMI but uses the cube of height instead of the square: PI = Weight (kg) ÷ Height³ (m³). It is considered more accurate for very tall or very short individuals, where BMI can sometimes give misleading results. A normal Ponderal Index typically ranges from 11 to 15 kg/m³.

BMI has several well-known limitations: It doesn't distinguish between muscle and fat, so athletes and muscular individuals may have a high BMI despite being healthy. It also doesn't account for fat distribution (where fat is stored matters for health risk). Elderly people may have healthy BMI but excess fat. Additionally, BMI thresholds may need to be adjusted for different ethnicities. Always consult a healthcare professional for a comprehensive health assessment.

For children and teens, BMI is interpreted differently using percentile charts based on age and gender (CDC growth charts). The categories are:

• Underweight: Below 5th percentile
• Healthy Weight: 5th to 85th percentile
• At Risk of Overweight: 85th to 95th percentile
• Overweight: Above 95th percentile

This calculator provides an adult BMI result; for children, please consult a pediatrician.