Calculate your body mass index.
A developer-designed body mass index calculator. Supports metric and US standard systems, tracks calculations, and estimates health targets.
Your health analysis.
BMI classifications for adults.
Standard classifications defined by the World Health Organization (WHO) for adults aged 20 and over.
| BMI Range | Classification | Risk of Comorbidities |
|---|---|---|
| Below 18.5 | Underweight | Low (Increased risk of nutritional deficiency) |
| 18.5 – 24.9 | Normal / Healthy | Average / Optimal |
| 25.0 – 29.9 | Overweight | Increased |
| 30.0 – 34.9 | Obese Class I (Moderate) | Moderate |
| 35.0 – 39.9 | Obese Class II (Severe) | Severe |
| 40.0 and Above | Obese Class III (Extremely Severe) | Very Severe |
Understanding the body mass index (BMI).
Body Mass Index (BMI) is a clinical screening tool used to assess a person's relative weight in relation to their height. While it is not a direct measurement of body fat percentage, BMI correlates strongly with metabolic health risks and cardiovascular disease indicators, providing a straightforward, cost-effective screening method.
How to use the calculator.
To begin, select your biological sex and input your age. Next, choose your preferred measurement system. The Metric tab accepts height in centimeters (cm) and weight in kilograms (kg), while the US Standard tab accepts height in feet and inches, and weight in pounds (lbs). As you input your values, the calculator automatically runs the conversion equations, displaying your score, clinical classification, and target weight ranges in real time. Click "Calculate BMI" to log your calculations in your browser's private local history database.
Physiological differences: men vs. women.
Although the clinical BMI formula is identical for men and women, body composition differs significantly between the sexes. Women naturally carry higher essential body fat levels (required for reproductive and hormonal functions) and lower muscle density compared to men at the exact same BMI score. Consequently, a healthy BMI range represents different physical profiles and fat-to-muscle ratios depending on biological sex. Interpreting these results with a healthcare professional can help you understand your specific body composition and metabolic health.
How age impacts your healthy range.
Age is an important factor when interpreting BMI. The standard WHO categories serve as a baseline for adults aged 20 and over. However, as bodies age, muscle mass naturally decreases and fat distribution shifts. For older adults (aged 65 and above), clinical research suggests that a slightly higher BMI (between 23 and 27) can provide protective bone density benefits and reduce overall health risks. Therefore, a rigid normal-weight categorization may not be optimal for seniors.
BMI percentiles for children and adolescents.
For children and teenagers (ages 2 to 19), a standard adult BMI scale is clinically inappropriate. Because children experience rapid growth spurts and body composition changes, their BMI scores are mapped to age- and sex-specific growth percentiles. Underweight is defined as a BMI below the 5th percentile, healthy weight is between the 5th and 85th percentiles, overweight is between the 85th and 95th percentiles, and obese is at or above the 95th percentile. Our calculator automatically switches to child percentile interpretations when a youth profile is entered.
Standard clinical formulas.
For reference, the mathematical equations used to calculate BMI are:
Metric System: BMI = weight (kg) / height² (m²)
Imperial System: BMI = 703 × weight (lbs) / height² (inches²)
Advanced body composition metrics.
- Ponderal Index (PI): Calculated as
weight (kg) / height³ (m³), this scales body mass in three dimensions. It normalizes values for very tall or very short individuals, providing a clearer indication of body frame proportions than standard 2D BMI. - BMI Prime: This represents the ratio of your calculated BMI to the upper limit of the healthy weight boundary (25.0). A BMI Prime of 1.0 is the exact upper limit of the healthy range; values above 1.0 indicate overweight thresholds, while values below 0.74 suggest underweight ranges.
Disclaimer: This tool is for informational screening purposes only. BMI does not measure body fat directly and can overestimate fat in athletes or underestimate it in seniors who have lost muscle density. Consult a healthcare provider for personalized medical advice.
Frequently asked questions.
What is Body Mass Index (BMI)?
Body Mass Index (BMI) is a clinical screening tool used to estimate a person's relative body fat based on their height and weight. By entering your measurements, the calculator provides a score that places you into categories ranging from underweight to obese.
How is BMI calculated?
To calculate BMI manually, divide your weight in kilograms by the square of your height in meters (BMI = kg/m²). For imperial units, multiply your weight in pounds by 703, then divide by the square of your height in inches.
Is BMI a reliable indicator of health?
BMI is a reliable initial screening tool but has key limitations. Because it only factors in weight and height, it does not distinguish between muscle mass, bone density, and body fat. For example, muscular athletes may have a high BMI without having high body fat.
How does BMI differ for men and women?
While the standard mathematical formula is the same for men and women, body compositions differ. Women naturally carry more essential body fat and lower muscle mass than men at the exact same BMI. Therefore, the same BMI score might reflect different fat-to-muscle ratios depending on biological sex.
How is BMI interpreted for children and teens?
For children and teens (ages 2 to 19), BMI is interpreted using age- and sex-specific growth charts and percentiles. Since children grow rapidly and experience non-linear development, their scores are compared against national baseline percentiles rather than using adult thresholds.
Does the target healthy BMI change with age?
Yes. Research suggests that for older adults (aged 65 and over), a slightly higher BMI (between 23 and 27) may offer protective benefits, supporting bone density and overall health. Standard adult categories are most accurate for individuals aged 20 to 65.
How is my ideal weight determined?
Your ideal weight range is calculated using the World Health Organization (WHO) normal BMI boundaries of 18.5 to 24.9. The calculator identifies the weight range at which your BMI would sit within these healthy parameters for your specific height.
What is the difference between BMI and body fat percentage?
BMI is an indirect estimate of body composition based on overall weight. Body fat percentage measures the actual proportion of adipose tissue in your body. Tools like skinfold calipers, bioelectrical impedance, or DEXA scans are required to measure actual body fat directly.