BMI Calculator
Modify the values and click the calculate button to use
The Body Mass Index (BMI) Calculator helps determine an individual’s BMI and associated weight status while factoring in age. It supports both metric and US units and includes an additional measurement known as the Ponderal Index.
Use the “Metric Units” tab for International System calculations, or the “Other Units” tab to convert between US and metric systems.
What is BMI?
BMI is a widely used measure that assesses a person’s body weight relative to their height, providing a general indication of whether their weight falls within a healthy range. While it doesn’t measure body fat directly, it is a useful screening tool.
BMI Classifications
BMI values are categorized to identify potential health risks:
Adults (20 years and older) – WHO Guidelines
| Classification | BMI (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 |
Children and Teens (2–20 years) – CDC Guidelines
| Classification | Percentile Range |
|---|---|
| Underweight | < 5% |
| Healthy Weight | 5% – 85% |
| At Risk of Overweight | 85% – 95% |
| Overweight | > 95% |
Health Risks of Abnormal BMI
Risks of Being Overweight
According to the CDC, being overweight can lead to:
- High blood pressure
- Unhealthy cholesterol levels
- Type II diabetes
- Heart disease and stroke
- Sleep apnea
- Certain cancers
- Joint problems (e.g., osteoarthritis)
- Mental health issues (e.g., depression, anxiety)
- Decreased quality of life and increased mortality
Risks of Being Underweight
- Malnutrition and anemia
- Osteoporosis
- Weakened immunity
- Developmental delays in youth
- Reproductive issues in women
- Surgical complications
- Higher mortality rates
Being underweight can also signal conditions like anorexia nervosa or other underlying diseases. Medical consultation is recommended.
Limitations of BMI
BMI is a useful screening tool but not a diagnostic measure. It doesn’t account for:
- Muscle mass vs. fat mass
- Body composition
- Ethnicity, sex, or age differences
- Activity level or fitness
Examples:
- Muscular athletes may have high BMI but low body fat.
- Older adults may have normal BMI but high fat percentage.
- BMI may be more indicative for obese children than for overweight ones.
Despite its limitations, BMI is still useful for 90–95% of the population when combined with other assessments.
How to Calculate BMI
US Customary Units:
BMI=703×weight (lbs)height (in)2\text{BMI} = \frac{703 \times \text{weight (lbs)}}{\text{height (in)}^2}BMI=height (in)2703×weight (lbs)
Example:
A person who is 5’10” (70 in) and weighs 160 lbs: BMI=703×160702=23.0\text{BMI} = \frac{703 \times 160}{70^2} = 23.0BMI=702703×160=23.0
Metric (SI) Units:
BMI=weight (kg)height (m)2\text{BMI} = \frac{\text{weight (kg)}}{\text{height (m)}^2}BMI=height (m)2weight (kg)
Example:
72.57 kg / 1.778² m = 23.0
BMI Prime
BMI Prime compares a person’s BMI to the upper limit of the “normal” BMI range (25 kg/m²): BMI Prime=BMI25\text{BMI Prime} = \frac{\text{BMI}}{25}BMI Prime=25BMI
Interpretation:
| Classification | BMI Range | BMI Prime Range |
|---|---|---|
| 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 – 1.0 |
| Overweight | 25 – 30 | 1.0 – 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 |
Ponderal Index (PI)
The Ponderal Index is another measure of leanness or corpulence, better suited for individuals at height extremes. Unlike BMI, it uses height cubed in its formula.
Formula:
US Customary Units:
PI=height (in)weight (lbs)3\text{PI} = \frac{\text{height (in)}}{\sqrt[3]{\text{weight (lbs)}}}PI=3weight (lbs)height (in)
Example:
PI = 70 / ∛160 ≈ 12.9
Metric Units:
PI=weight (kg)height (m)3\text{PI} = \frac{\text{weight (kg)}}{\text{height (m)}^3}PI=height (m)3weight (kg)
Example:
72.57 / (1.778)³ ≈ 12.9
Final Thoughts
BMI, BMI Prime, and the Ponderal Index are valuable tools to estimate body weight status, but they should not be the sole measure for determining health. Always consult a medical professional for a more comprehensive assessment of your health.