BMI Calculator
| Underweight | Normal | Overweight | Obesity |
|---|---|---|---|
| < 18.5 | 18.5 – 24.9 | 25 – 29.9 | 30+ |
BMI Calculator
Understanding the Body Mass Index (BMI)
The Body Mass Index (BMI) is a widely used screening tool that estimates body fat based on a person’s height and weight. This calculator provides your BMI value and corresponding weight category, and can compute results using both metric and US customary units. It also calculates the Ponderal Index, another measure of leanness or corpulence.
What is BMI?
BMI is a simple numerical measurement that correlates weight with height. It is a general indicator used to categorize individuals into weight status groups such as underweight, normal weight, overweight, and obese. These categories can help identify potential health risks associated with being over or under a healthy weight range. It is important to remember that BMI is a screening tool, not a direct diagnosis of body fatness or health.
BMI Categories for Adults (Age 20+)
The following table, based on World Health Organization (WHO) standards, applies to both men and women aged 20 and older.
| Classification | BMI Range (kg/m²) |
|---|---|
| Underweight | |
| Severe Thinness | < 16 |
| Moderate Thinness | 16 – 17 |
| Mild Thinness | 17 – 18.5 |
| Normal Weight | 18.5 – 25 |
| Overweight | 25 – 30 |
| Obesity | |
| Obese Class I | 30 – 35 |
| Obese Class II | 35 – 40 |
| Obese Class III | > 40 |
BMI for Children and Teens (Ages 2-19)
For children and teenagers, BMI is interpreted differently. It is age and sex-specific and is expressed as a percentile based on growth charts from the Centers for Disease Control and Prevention (CDC).
| Category | Percentile Range |
|---|---|
| Underweight | Less than the 5th percentile |
| Healthy Weight | 5th percentile to less than the 85th percentile |
| At Risk of Overweight | 85th percentile to less than the 95th percentile |
| Overweight | Equal to or greater than the 95th percentile |
Health Risks Associated with Weight
Risks of Being Overweight
Carrying excess weight can increase the risk of numerous serious health conditions, including:
- High blood pressure
- Unhealthy cholesterol levels (high LDL, low HDL, high triglycerides)
- Type 2 diabetes
- Coronary heart disease
- Stroke
- Gallbladder disease
- Osteoarthritis (joint breakdown)
- Sleep apnea and breathing problems
- Certain cancers (endometrial, breast, colon, kidney, gallbladder, liver)
- Reduced quality of life and mental health issues
Risks of Being Underweight
Being underweight also carries significant health risks, such as:
- Malnutrition, vitamin deficiencies, and anemia
- Osteoporosis (weakened bones)
- Weakened immune function
- Growth and development issues in young people
- Potential reproductive issues for women
- Increased surgical complications
Important Limitations of BMI
While useful for population studies, BMI has limitations for individuals because it does not directly measure body fat. Key factors it does not account for include:
- Muscle Mass: Athletes and individuals with high muscle mass may have a high BMI but low body fat.
- Bone Density: Individuals with heavier bone structure may have a higher BMI.
- Age and Sex: Older adults and women tend to have more body fat than younger adults and men at the same BMI.
- Body Fat Distribution: BMI does not indicate where fat is stored on the body (e.g., visceral vs. subcutaneous fat).
Therefore, BMI should be used as a starting point and considered alongside other assessments for a complete health evaluation.
How to Calculate BMI
BMI is calculated by dividing a person’s weight by the square of their height.
Formula using Metric Units (kg/m²):BMI = weight (kg) / [height (m)]²
- *Example: A person who is 1.78m tall and weighs 72.5kg would have a BMI of 72.5 / (1.78 × 1.78) ≈ 22.9*
Formula using US Units (lb/in²):BMI = 703 × weight (lb) / [height (in)]²
- *Example: A person who is 70 inches tall and weighs 160lbs would have a BMI of 703 × 160 / (70 × 70) ≈ 22.9*
BMI Prime
BMI Prime is a simple modification of BMI. It is the ratio of a person’s actual BMI to the upper BMI limit considered “normal” (25 kg/m²).
BMI Prime = BMI / 25
- < 0.74: Underweight
- 0.74 – 1.0: Normal weight
- > 1.0: Overweight
- > 1.2: Obese
It provides a quick way to see how much one’s BMI deviates from the standard normal threshold.
Ponderal Index (PI)
The Ponderal Index (PI) is similar to the BMI, but it divides weight by height cubed instead of height squared. This adjustment may make it a more reliable indicator for individuals who are very tall or very short, though it shares many of the same limitations as BMI.
Formula using Metric Units:PI = weight (kg) / [height (m)]³
Formula using US Units:PI = height (in) / ∛weight (lbs)
