BMI Calculator
Body Mass Index (BMI) Calculator
The Body Mass Index (BMI) Calculator helps determine BMI values and corresponding weight classifications while considering age. Users can select the "Metric Units" tab for the International System of Units or the "Other Units" tab to convert measurements between US and metric units. Additionally, the calculator computes the Ponderal Index, which is discussed further below.
Introduction to BMI
BMI is a measurement of body leanness or corpulence based on height and weight. It serves as a general indicator of whether an individual's weight is healthy relative to their height. The calculated BMI value categorizes individuals into different weight groups: underweight, normal weight, overweight, or obese. These classifications may vary based on age, region, and other factors. While BMI is not a perfect health metric, it serves as a useful screening tool to determine if further evaluation or lifestyle changes are needed.
BMI Classification Table for Adults
The World Health Organization (WHO) provides the following BMI classifications for adults (age 20+), applicable to both men and women:
| Classification | BMI Range (kg/m²) |
|---|---|
| Severe Thinness | < 16 |
| Moderate Thinness | 16 - 17 |
| Mild Thinness | 17 - 18.5 |
| Normal | 18.5 - 25 |
| Overweight | 25 - 30 |
| Obese Class I | 30 - 35 |
| Obese Class II | 35 - 40 |
| Obese Class III | > 40 |
BMI for Children and Teens (Ages 2-20)
The Centers for Disease Control and Prevention (CDC) recommends the following BMI percentiles for children and adolescents:
| Category | Percentile Range |
| Underweight | <5% |
| Healthy weight | 5% - 85% |
| At risk of overweight | 85% - 95% |
| Overweight | >95% |
Health Risks of Being Overweight
Excess weight increases the risk of various serious health conditions, including:
- High blood pressure
- Unhealthy cholesterol levels
- Type 2 diabetes
- Heart disease and stroke
- Gallbladder disease
- Osteoarthritis
- Sleep apnea and breathing problems
- Certain cancers (e.g., breast, colon, kidney)
- Reduced quality of life and increased mental health risks
- Increased mortality risk
Maintaining a BMI below 25 kg/m² is generally advisable, though individuals should consult a healthcare provider for personalized guidance.
Health Risks of Being Underweight
Being underweight also poses health risks, such as:
- Malnutrition, vitamin deficiencies, and anemia
- Osteoporosis and increased risk of fractures
- Weakened immune function
- Growth and development issues (especially in children)
- Reproductive complications in women
- Higher surgical complication risks
- Increased mortality risk
In some cases, being underweight may signal an underlying health condition, such as an eating disorder. Consultation with a healthcare professional is recommended.
Limitations of BMI
While BMI is a useful general indicator of health, it does not account for muscle mass, bone density, or fat distribution. Some limitations include:
- BMI does not distinguish between muscle and fat.
- Athletes with high muscle mass may have a high BMI but low body fat.
- Older adults may have normal BMI but excess body fat due to muscle loss.
- BMI may not be as accurate for children and adolescents due to variations in height and development.
BMI should be used alongside other health assessments for a more accurate evaluation.
BMI Calculation Formula
BMI can be calculated using the following formulas:
USC Units: Example: A 5'10" (70-inch), 160-pound individual:
Metric Units: Example: A 1.778m, 72.57kg individual:
BMI Prime
BMI Prime is the ratio of a person’s BMI to the upper limit of the normal BMI range (25 kg/m²). The formula is:
| Classification | BMI | BMI Prime |
| Severe Thinness | < 16 | < 0.64 |
| Moderate Thinness | 16-17 | 0.64-0.68 |
| Mild Thinness | 17-18.5 | 0.68-0.74 |
| Normal | 18.5-25 | 0.74-1 |
| Overweight | 25-30 | 1-1.2 |
| Obese Class I | 30-35 | 1.2-1.4 |
| Obese Class II | 35-40 | 1.4-1.6 |
| Obese Class III | > 40 | > 1.6 |
BMI Prime allows for quick assessment of whether an individual is within a healthy weight range and enables comparisons across different populations.
Ponderal Index (PI)
The Ponderal Index (PI) is similar to BMI but adjusts for height more effectively, particularly for very tall or short individuals. The formula is:
USC Units: Example: A 5'10", 160-pound individual:
Metric Units: Example: A 1.778m, 72.57kg individual:
While BMI is a useful general measure, PI may provide a more accurate assessment for individuals at height extremes.
