Calculate your Body Mass Index (BMI) using metric or imperial units. Get your BMI category, healthy weight range, and comprehensive health insights.
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Body Mass Index (BMI) is a widely used screening tool that measures body fat based on your height and weight. Developed by Belgian statistician Adolphe Quetelet in the 1830s, BMI provides a simple numerical measure to classify individuals as underweight, normal weight, overweight, or obese.
BMI is expressed in units of kg/m² (kilograms per square meter) and is calculated by dividing your weight in kilograms by the square of your height in meters. For imperial measurements, the formula includes a conversion factor of 703 to account for pounds and inches.
While BMI is not a direct measure of body fat percentage, it correlates strongly with body fat levels in most individuals and serves as an effective screening tool for weight-related health risks. Healthcare professionals worldwide use BMI as part of comprehensive health assessments to identify potential weight problems in adults and children.
Example: Weight = 70 kg, Height = 1.75 m
BMI = 70 ÷ (1.75)² = 70 ÷ 3.0625 = 22.86 kg/m²
Example: Weight = 154 lbs, Height = 69 inches
BMI = 703 × 154 ÷ (69)² = 108,262 ÷ 4,761 = 22.74 kg/m²
The number 703 is a conversion factor used in the imperial BMI formula to ensure the result matches the metric calculation. It comes from converting pounds to kilograms (÷ 2.20462) and inches to meters (÷ 39.3701), resulting in approximately 703.
The World Health Organization (WHO) has established standard BMI ranges for adults aged 20 and over:
| BMI Range (kg/m²) | Category | Health Risk |
|---|---|---|
| < 16.0 | Severe Thinness | High risk of malnutrition |
| 16.0 - 17.0 | Moderate Thinness | Moderate health risks |
| 17.0 - 18.5 | Mild Thinness | Low to moderate risks |
| 18.5 - 25.0 | Normal (Healthy Weight) | Minimal health risks |
| 25.0 - 30.0 | Overweight (Pre-obese) | Increased risk of diseases |
| 30.0 - 35.0 | Obese Class I | Moderate to high risk |
| 35.0 - 40.0 | Obese Class II | High risk of diseases |
| ≥ 40.0 | Obese Class III | Very high risk |
While BMI is a useful screening tool, it has several important limitations:
BMI cannot distinguish between muscle mass and fat mass. Athletes and bodybuilders may have high BMIs due to muscle, not fat, while elderly individuals may have normal BMIs despite high body fat percentages.
BMI doesn't indicate where body fat is located. Visceral fat (around organs) poses greater health risks than subcutaneous fat (under skin), but BMI treats all fat equally.
BMI standards are based primarily on Caucasian populations. Asian populations may have higher health risks at lower BMIs, while the same BMI may represent different body fat percentages for men and women.
BMI may not be accurate for pregnant women, competitive athletes, elderly individuals, people with amputations, or those with certain medical conditions affecting weight or height.
For children and teenagers aged 2-20 years, BMI is interpreted differently using BMI-for-age percentiles based on CDC growth charts. The same BMI can mean different things at different ages for children.
| Percentile Range | Weight Status |
|---|---|
| < 5th percentile | Underweight |
| 5th to < 85th percentile | Healthy Weight |
| 85th to < 95th percentile | Overweight |
| ≥ 95th percentile | Obese |
Note: Children's BMI must be compared to others of the same age and sex because body composition changes as children grow. Consult a pediatrician for proper interpretation of children's BMI values.
While BMI is convenient, several other measurements provide more detailed information about body composition:
Measures abdominal fat. Men with waist circumference > 40 inches or women > 35 inches have increased health risks regardless of BMI.
Divides waist circumference by hip circumference. Ratios > 0.90 (men) or > 0.85 (women) indicate increased cardiovascular risk.
Measured via bioelectrical impedance, DEXA scans, or skinfold calipers. Provides direct measurement of fat vs. lean mass.
Waist circumference divided by height. A ratio > 0.5 suggests increased health risks. Simple rule: keep waist less than half your height.
Focus on whole foods including fruits, vegetables, lean proteins, whole grains, and healthy fats. Limit processed foods, sugary drinks, and excessive sodium.
Aim for at least 150 minutes of moderate aerobic activity or 75 minutes of vigorous activity per week, plus muscle-strengthening exercises twice weekly.
Use smaller plates, read nutrition labels, and be mindful of serving sizes. Avoid eating directly from packages or while distracted.
Get 7-9 hours of quality sleep nightly. Poor sleep disrupts hormones that regulate hunger and metabolism, leading to weight gain.
Practice stress-reduction techniques like meditation, yoga, or deep breathing. Chronic stress increases cortisol, which promotes fat storage.
Track your weight and measurements regularly, but don't obsess over daily fluctuations. Focus on long-term trends and overall health markers.
For adults 20 years and older, a healthy BMI range is 18.5 to 24.9 kg/m². This range is associated with the lowest health risks according to WHO guidelines. However, optimal BMI may vary slightly based on ethnicity, age, and individual health factors.
Yes, BMI has limitations. It doesn't distinguish muscle from fat, so athletes may be classified as overweight despite being healthy. It also doesn't account for age, sex, bone density, or fat distribution. BMI should be used alongside other health indicators like waist circumference, blood pressure, and cholesterol levels.
For most adults, checking BMI quarterly (every 3 months) is sufficient to monitor weight trends. If you're actively working on weight loss or gain, monthly checks can help track progress. Focus on long-term trends rather than daily or weekly fluctuations, which are normal.
The BMI formula is the same for both sexes, but women typically have more body fat than men at the same BMI due to biological differences. The standard BMI categories apply to both, though some health organizations suggest women may have slightly different optimal ranges. Always consult healthcare providers for personalized assessment.
BMI Prime is your BMI divided by 25 (the upper limit of normal BMI). A BMI Prime of 1.0 means you're at the boundary between normal and overweight. Values between 0.74-1.00 indicate normal weight, < 0.74 is underweight, and > 1.0 is overweight. It's a simpler way to understand your BMI relative to the healthy range.
For metric: Divide your weight in kilograms by your height in meters squared (BMI = kg ÷ m²). For imperial: Multiply your weight in pounds by 703, then divide by your height in inches squared (BMI = 703 × lbs ÷ in²). Example: 70 kg ÷ (1.75 m)² = 70 ÷ 3.0625 = 22.86 kg/m².
A BMI of 30 or higher is classified as obese. Obesity is further divided into: Class I (30-34.9), Class II (35-39.9), and Class III (40+), with health risks increasing at higher classes. Class III obesity, also called severe or morbid obesity, carries the highest health risks and may require medical intervention.
No. Children and teens (ages 2-20) should use BMI-for-age percentile charts from the CDC, which account for normal growth patterns. The same BMI means different things at different ages for kids. Always consult a pediatrician for children's BMI interpretation and never put children on restrictive diets without medical supervision.
Yes, many life insurance companies use BMI as part of their underwriting process. Very high or very low BMIs may result in higher premiums or policy restrictions because they're associated with increased health risks. However, insurers typically consider BMI alongside other health factors like blood pressure, cholesterol, and medical history.
BMI estimates body fat based on height and weight, while body fat percentage directly measures the proportion of fat in your body. Two people with the same BMI can have very different body fat percentages. Body fat percentage is more accurate but requires special equipment (DEXA scan, bioelectrical impedance) while BMI only needs a scale and measuring tape.
Body Mass Index is a valuable tool for assessing weight status and identifying potential health risks, but it's just one piece of the health puzzle. Our BMI calculator provides instant, accurate results along with your healthy weight range and detailed explanations to help you understand your results.
Remember that BMI is a screening tool, not a diagnostic measure. A high or low BMI doesn't automatically mean you're unhealthy, just as a normal BMI doesn't guarantee perfect health. Consider BMI alongside other health indicators like waist circumference, blood pressure, blood sugar levels, cholesterol, and overall fitness.
If your BMI falls outside the healthy range, don't panic. Instead, consult with healthcare professionals to develop a personalized plan that addresses your specific health needs and goals. Sustainable lifestyle changes in nutrition, physical activity, sleep, and stress management are far more important than any single number.