BMI Calculator

Body Mass Index (BMI) is a standardized screening metric used to estimate a person’s relative leanness or corpulence based on their height and weight. Widely adopted by the World Health Organization (WHO) and the Centers for Disease Control and Prevention (CDC), BMI serves as a general indicator of whether an individual falls within a healthy weight classification. While BMI does not measure body fat directly, it is a highly effective, non-invasive starting point to evaluate potential health risks associated with being underweight, overweight, or obese.

Our free BMI Calculator is a comprehensive body composition analyzer. By entering your age, gender, height (feet/inches or centimeters), and weight (pounds or kilograms), the tool instantly computes your BMI, provides your official WHO weight classification, projects your healthy weight range, and calculates your BMI Prime and Ponderal Index.

WHO Adult BMI Classifications (Ages 20 and Older)

For adults ages 20 and older, the World Health Organization recommends the following weight classifications based on BMI values (expressed in kg/m²):

CDC Children & Teen BMI Percentiles (Ages 2 to 20)

Because children and adolescents experience rapid growth and sexual maturation, their BMI calculations cannot be evaluated using adult benchmarks. Instead, the CDC categorizes BMI for children and teens ages 2 through 20 based on age-and-sex-specific percentiles:

• Underweight: BMI falls below the 5th percentile.
• Healthy Weight: BMI falls between the 5th and 85th percentiles.
• At Risk of Overweight: BMI falls between the 85th and 95th percentiles.
• Overweight: BMI is at or above the 95th percentile.

The Formulas: How to Calculate BMI

BMI represents weight divided by height squared, solved using either US Customary (USC) or Metric (SI) units:

1. US Customary System (USC) Formula

BMI = 703 × (Weight in Pounds / Height in Inches²)

Example: For a 5’10” individual (70 inches) weighing 160 pounds:
BMI = 703 × (160 / 70²) = 703 × (160 / 4900) = 23.0 (Normal Classification).

2. Metric System (SI) Formula

BMI = Weight in Kilograms / Height in Meters²

Example: For an individual weighing 72.57 kg standing 1.778 meters tall:
BMI = 72.57 / 1.778² = 72.57 / 3.1613 = 23.0.

Understanding BMI Prime and the Ponderal Index

For a more advanced analysis, our calculator tracks two alternative indicators:

1. BMI Prime

BMI Prime is the ratio of your calculated BMI to the upper limit of normal BMI (25.0 kg/m²). The formula is:

BMI Prime = Measured BMI / 25.0

Because it is a ratio, BMI Prime is a dimensionless value. A BMI Prime of 1.0 represents the exact upper limit of a healthy weight. A value below 0.74 represents underweight, 0.74 to 1.0 is normal, 1.0 to 1.2 is overweight, and above 1.2 indicates obesity.

2. Ponderal Index (Corpulence Index)

While standard BMI squares height, the Ponderal Index cubes height. This makes the Ponderal Index a more reliable leanness indicator for extremely tall or short individuals, as scaling weight against a squared height tends to skew body fat estimates on the extreme ends of the spectrum.
The Metric Formula:

Ponderal Index = Weight in Kilograms / Height in Meters³

Example: For a 5’10” individual weighing 160 lbs, the Ponderal Index is 12.9 kg/m³.

Health Risks and Efficacy Limitations

Maintaining a healthy BMI (18.5 to 25.0) lowers the risk of many chronic health conditions. Being overweight increases the risk of high blood pressure, coronary heart disease, stroke, Type II diabetes, sleep apnea, osteoarthritis, and elevated LDL cholesterol. Conversely, being underweight increases the risk of malnutrition, vitamin deficiencies, anemia, osteoporosis, compromised immune function, and reproductive complications.

The Limitations of BMI

While BMI is indicative of body fat for 90% to 95% of the population, it has notable limitations because it measures excess weight rather than excess fat.

• Muscle vs. Fat: Muscular individuals and highly trained athletes may have high BMIs (classifying them as overweight or obese) due to dense muscle mass, despite having very low body fat.
• Gender and Age: Women typically carry more body fat than men at equivalent BMIs. Similarly, older adults generally possess higher body fat percentages than younger adults with the same BMI.

Track your calories on our Calorie Calculator, calculate macro splits with the Macro Calculator, or audit your body fat percentage using the Body Fat Calculator.

Frequently Asked Questions (FAQ)

Is BMI calculated differently for men and women?

No, the formula for calculating BMI is identical for both adult men and women. However, because women naturally carry higher body fat percentages than men at equivalent BMIs, the index should be interpreted alongside body fat measurements and waist circumference.

Why does height scaling skew standard BMI calculations?

Because humans grow in three dimensions, scaling weight (which relates to volume) against height squared (two dimensions) naturally makes taller people look heavier and shorter people look lighter than they are. The Ponderal Index resolves this by cubing height.

What is a healthy weight for a 5’10” adult?

According to WHO guidelines, a healthy BMI range of 18.5 to 25.0 corresponds to a healthy weight range of 128.9 pounds to 174.2 pounds for an adult standing 5 feet 10 inches tall.

Can a child use the adult BMI table?

No. Children and teens are experiencing active physical growth, so their BMI values must be plotted on CDC growth percentile charts. A child is considered overweight only if their BMI is in the 95th percentile or higher for their specific age and sex.