Body Mass Index (BMI)

Body Mass Index (BMI) is a screening tool used to estimate a person's body fat based on their height and weight. It is calculated by dividing a person's weight by the square of their height. The BMI score is intended to be a simple, inexpensive screening category rather than a diagnostic tool for defining a person's absolute body fat or overall health. Healthcare providers often use BMI to identify potential weight-related risks in adults, such as being underweight, overweight, or obese. Because it relies entirely on a mathematical formula using two inputs, it does not distinguish between muscle mass and fat mass, meaning athletes or highly muscular individuals may sometimes fall into an "overweight" or "obese" category despite being in excellent physical condition.

BMI Calculator Components

Calculating your BMI requires only two primary data points. An effective BMI calculator will process these inputs to return a numeric score and corresponding weight category.

• Weight: The user's body weight, which can be entered in pounds (lbs) or kilograms (kg).
• Height: The user's height, which is typically entered in feet and inches (ft/in) or meters/centimeters (m/cm).

How BMI is Calculated

To provide context on how the calculator processes your numbers, different mathematical formulas are used depending on the measurement system you choose.

Standard / Imperial Formula

When using pounds (lbs) and inches (in), the calculator multiplies the weight in pounds by 703, divides that by the height in inches, and divides by the height in inches a second time.

BMI = (Weight in pounds * 703) / (Height in inches * Height in inches)

Metric Formula

When using kilograms (kg) and meters (m), the calculation is simply the weight in kilograms divided by the square of the height in meters.

BMI = Weight in kilograms / (Height in meters * Height in meters)

Adult BMI Categories

The BMI score generated by the calculator is compared against standardized ranges established by health organizations to determine weight classifications.

• Underweight: Below 18.5
• Healthy Weight: 18.5 to 24.9
• Overweight: 25.0 to 29.9
• Obese: 30.0 or higher

Frequently Asked Questions (FAQ)

What is a good or healthy BMI?

For most adults, a healthy BMI is considered to be between 18.5 and 24.9. This range is statistically associated with a lower risk of chronic diseases. However, these categories are generalized and may not apply to all populations, such as children, the elderly, or pregnant individuals.

Is BMI an accurate measure of overall health?

Not entirely. While BMI is a reliable indicator of body fatness for a large portion of the general population, it does not account for muscle mass, bone density, age, or sex. For example, a bodybuilder with a low percentage of body fat may have a high BMI and be classified as overweight. Conversely, an individual with a normal BMI could still carry an unhealthy amount of visceral fat.

Does BMI calculate the same way for children?

No. For children and teens, BMI calculations must be age- and sex-specific. This is because the amount of body fat changes as children grow, and the differences in body fat between boys and girls are distinct. A child's BMI score is plotted on a growth chart to determine their BMI-for-age percentile.

Can I use this calculator if I am pregnant?

BMI calculators are generally not accurate or recommended for use during pregnancy. Healthcare providers use specialized tracking methods and weight gain recommendations based on pre-pregnancy weight rather than standard BMI formulas to assess fetal development and maternal health.