Body Mass Index (BMI) is a numerical value calculated from your height and weight. It is widely used as a screening tool to categorize adults into weight status categories — underweight, normal weight, overweight, and obese.
BMI is a useful population-level screening tool, but it has significant limitations for individuals. It does not distinguish between muscle and fat — a muscular athlete may have a high BMI without excess body fat. It also does not account for age, sex, ethnicity, or where fat is distributed on the body. BMI should be used alongside other health measurements such as waist circumference and body fat percentage.
Body fat percentage measures the proportion of your body that is fat tissue, making it a more direct measure of health risk than BMI. However, measuring body fat accurately requires special equipment. BMI is a practical alternative for general screening, but for the most complete picture of your health, use it in conjunction with other metrics.
Disclaimer: BMI is a screening tool, not a diagnostic measure. This calculator is for adults 20 years and older. For children and teens, different BMI charts are used. Always consult a qualified healthcare provider for medical advice.
BMI is calculated using one of two formulas depending on your unit system. In metric units, BMI = weight(kg) ÷ height(m)². In imperial units, BMI = (weight(lbs) × 703) ÷ height(inches)². For example, a person who is 175 cm tall and weighs 80 kg would calculate: 80 ÷ (1.75)² = 80 ÷ 3.0625 = 26.1 — placing them in the Overweight category.
| BMI Range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal (Healthy) Weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obese (Class I) |
| 35.0 – 39.9 | Obese (Class II) |
| 40.0 and above | Severely Obese (Class III) |
The World Health Organization (WHO), the US Centers for Disease Control (CDC), and Health Canada all use the same core BMI thresholds for adults: underweight below 18.5, normal 18.5–24.9, overweight 25–29.9, and obese 30+. The main differences arise in how they address specific populations. WHO offers additional cut-off points for Asian populations (overweight at 23, obese at 27.5), since research shows higher metabolic risk at lower BMI values in some Asian ethnic groups.
For children and adolescents aged 2–19, BMI is interpreted using age- and sex-specific percentile charts rather than fixed thresholds. A BMI-for-age at the 85th to 94th percentile is considered overweight, and at or above the 95th percentile is obese. This approach accounts for the natural changes in body composition during growth and development.
Muscle tissue is significantly denser than fat tissue. A highly muscular athlete — such as a weightlifter or rugby player — may register a BMI of 27 or 28, technically classifying them as overweight, even though their body fat percentage is well within the healthy athletic range. In these cases, body fat percentage or waist-to-hip ratio provides a more accurate picture of health.
Health authorities recommend using waist circumference alongside BMI. Abdominal fat is particularly associated with metabolic risk. Health risk increases when waist circumference exceeds 35 inches (88 cm) in women or 40 inches (102 cm) in men. A large waist measurement combined with an elevated BMI is a stronger predictor of health risk than either measure alone.
Research consistently shows a correlation between higher BMI and increased risk of several chronic conditions. People with a BMI above 30 have significantly elevated risks of type 2 diabetes, cardiovascular disease, hypertension, obstructive sleep apnea, certain cancers, and joint problems. However, BMI is a risk indicator, not a diagnosis — many other factors including diet quality, physical fitness, sleep, and genetics also play important roles.
Several metrics can complement or sometimes replace BMI for assessing body composition and health risk. Body fat percentage directly measures the proportion of fat tissue in your body. Waist-to-hip ratio compares waist and hip circumference to assess fat distribution. Visceral fat measurement (via DEXA scan or CT) identifies the most metabolically dangerous fat stored around internal organs. Each of these provides information that BMI alone cannot.
This free BMI calculator lets you calculate your Body Mass Index instantly using either metric (kg/cm) or imperial (lb/in) measurements. BMI is a widely used screening tool that helps identify whether a person is underweight, at a healthy weight, overweight, or in the obese range. Simply enter your height and weight to get your BMI and see your weight classification in seconds.
The BMI calculator uses the standard WHO formula: BMI = weight(kg) / height(m)². While BMI is a useful general indicator, it does not measure body fat directly — for a more detailed analysis, pair this tool with our body fat percentage calculator. Understanding your BMI is an important first step toward setting and achieving your health goals.
For adults aged 20 and older, a BMI between 18.5 and 24.9 is considered healthy. Below 18.5 is underweight, 25.0–29.9 is overweight, and 30.0 or above is obese. These thresholds are established by the World Health Organization and are used by most health authorities globally.
BMI is a useful general screening tool but has known limitations. It does not distinguish between muscle and fat, so muscular athletes may show an elevated BMI without excess fat. It also does not account for age, sex, or ethnicity. Pairing BMI with waist circumference or body fat percentage gives a more accurate picture of health.
For children and teens aged 2–19, BMI is calculated the same way but interpreted using age- and sex-specific percentile charts rather than fixed cut-offs. A BMI at the 85th–94th percentile is overweight, and at or above the 95th percentile is considered obese.
Yes. This is sometimes called "normal weight obesity" — having a normal BMI but a high body fat percentage and low muscle mass. People in this category can have elevated metabolic health risks. Diet quality, physical fitness, and other lifestyle factors also significantly impact health beyond what BMI captures.
A BMI of 30.0 or higher is classified as obese. Obesity is further divided into Class I (30–34.9), Class II (35–39.9), and Class III or severe obesity (40+). Higher classes carry greater health risks including type 2 diabetes, cardiovascular disease, and sleep apnea.
Lowering BMI requires reducing body weight through a calorie deficit. Effective approaches include eating a nutrient-dense diet with fewer ultra-processed foods, getting at least 150 minutes of aerobic exercise per week, and adding strength training to preserve muscle. Small, sustainable changes outperform extreme diets. Always consult a healthcare provider before starting a weight loss program.
BMI itself only changes if your height or weight changes. However, body composition naturally shifts with age — older adults tend to lose muscle mass and gain fat even at the same weight, meaning BMI can underestimate health risks in older individuals. Some guidelines suggest slightly higher BMI ranges may be acceptable for people over 65.
BMI is a simple height-to-weight ratio and does not directly measure fat. Body fat percentage measures what proportion of your total body weight is actually fat tissue, making it a more precise indicator of health risk. While body fat measurement requires special equipment, BMI is a quick and accessible screening tool for general use.