Last updated March 2026
BMI Calculator
Calculate your Body Mass Index to understand your weight relative to your height.
What Is BMI?
Body Mass Index (BMI) is a numerical value calculated from your weight and height. It provides a simple screening measure to categorize individuals as underweight, normal weight, overweight, or obese. The concept was developed by Belgian mathematician Adolphe Quetelet between 1830 and 1850 as part of his "social physics" research to define the characteristics of an average person.
Although Quetelet never intended his index to measure individual health, the medical community adopted it in the mid-20th century. In 1972, researcher Ancel Keys popularized the term "Body Mass Index" in a landmark study. The World Health Organization (WHO) formally adopted BMI categories in the 1990s, and the National Institutes of Health (NIH) established the standard cutoff points still used today. Its simplicity and low cost make it the most widely used method for classifying weight status in clinical and public health settings worldwide.
BMI Formula
BMI = weight (kg) / height (m)2 Or in imperial units:
BMI = 703 × weight (lbs) / height (inches)2 Metric example: A person weighing 73 kg at 1.78 m tall: BMI = 73 / (1.78 × 1.78) = 73 / 3.168 = 23.0 (normal weight).
Imperial example: A person weighing 160 lbs at 5'10" (70 inches): BMI = 703 × 160 / (70 × 70) = 112,480 / 4,900 = 23.0 (normal weight). For a step-by-step walkthrough, see our guide on how to calculate your BMI.
BMI Categories
| BMI Range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 - 24.9 | Normal weight |
| 25.0 - 29.9 | Overweight |
| 30.0 - 34.9 | Obese (Class I) |
| 35.0 - 39.9 | Obese (Class II) |
| 40.0 and above | Obese (Class III) |
Each category carries different health implications. Underweight individuals may face nutrient deficiencies, weakened immunity, and reduced bone density. Normal weight is associated with the lowest risk for chronic diseases. Overweight increases the likelihood of high blood pressure and elevated cholesterol. Obese categories are linked to significantly higher rates of cardiovascular disease, type 2 diabetes, sleep apnea, and reduced life expectancy.
BMI for Different Age Groups
For children and teenagers (ages 2-19), BMI is plotted on age- and sex-specific growth charts maintained by the CDC. Results are expressed as percentiles: below the 5th percentile is underweight, 5th to 84th is healthy, 85th to 94th is overweight, and 95th or above is obese.
For adults aged 20-65, the standard categories apply as described above. For older adults over 65, research suggests a slightly higher BMI (25-27) may actually be protective against illness—sometimes called the "obesity paradox." Natural muscle loss and bone density decline with aging mean a low BMI in elderly populations can signal frailty rather than healthy weight.
BMI by Ethnicity
Standard BMI cutoffs were developed primarily from European-descent populations. The WHO has noted that Asian populations develop obesity-related health complications at lower BMI values. For South Asian, Chinese, and Japanese individuals, overweight is often defined at BMI 23 (rather than 25) and obesity at 27.5 (rather than 30).
These differences exist because body composition varies between ethnic groups. Asian populations tend to carry more visceral fat around organs at any given BMI, increasing metabolic risks like type 2 diabetes. Conversely, people of African or Polynesian descent may carry more lean muscle mass relative to their BMI. Understanding these variations is essential for accurate global health screening.
BMI vs Body Fat Percentage
Body fat percentage measures the actual proportion of your body that is fat tissue, making it generally more accurate than BMI for assessing health risk. A healthy range is typically 10-20% for men and 18-28% for women. To understand the key differences between these two metrics, read our article on BMI vs body fat percentage.
Body fat percentage is especially useful when BMI gives misleading results, such as for muscular athletes or older adults who have lost muscle but gained fat. Common measurement methods include:
- DEXA scans: The gold standard, providing precise readings of fat, muscle, and bone composition.
- Skinfold calipers: Affordable and portable, measuring fat thickness at specific body sites. Accuracy depends on the person taking the measurement.
- Bioelectrical impedance (BIA): Found in many smart scales, it estimates fat via electrical current. Convenient but influenced by hydration levels.
Limitations of BMI
While BMI is widely used, it has important limitations:
- Muscle vs. fat: Athletes and bodybuilders often have a high BMI due to muscle mass, not excess fat. Many professional athletes register as "obese" despite having low body fat.
- Age: Older adults tend to have more body fat than younger adults at the same BMI.
- Sex: Women generally have more body fat than men at the same BMI. BMI does not adjust for this difference.
- Pregnancy: BMI is not appropriate during pregnancy. Pre-pregnancy BMI is used instead to guide weight gain recommendations.
- Body fat distribution: BMI does not account for where fat is stored. Abdominal (visceral) fat is far more dangerous than subcutaneous fat in the hips and thighs.
BMI should be used as a starting point, not a definitive health measure. Waist circumference, blood pressure, blood sugar, and cholesterol levels all matter. Consult a healthcare provider for a comprehensive assessment.
Tips for Achieving a Healthy BMI
- Balanced diet: Focus on whole foods—fruits, vegetables, lean proteins, whole grains, and healthy fats. Limit processed foods and added sugars. Portion control matters as much as food quality.
- Regular exercise: Aim for at least 150 minutes of moderate aerobic activity per week plus strength training at least two days per week to build muscle and support metabolism.
- Adequate sleep: Adults need 7-9 hours per night. Poor sleep disrupts hunger hormones (ghrelin and leptin), increasing appetite and cravings.
- Stress management: Chronic stress raises cortisol, promoting abdominal fat storage. Mindfulness, meditation, and regular exercise help reduce stress.
- Medical consultation: If lifestyle changes are not working, conditions like hypothyroidism or PCOS may be a factor. A healthcare provider can identify barriers and create a personalized plan.
Frequently Asked Questions
What is a healthy BMI range?
A healthy BMI is generally between 18.5 and 24.9. This range may vary by ethnicity and age. Some health organizations suggest lower cutoffs for Asian populations (23 for overweight, 27.5 for obese).
How accurate is BMI?
BMI is useful for population screening but has limitations for individuals. It does not measure body fat directly and cannot distinguish between muscle and fat. For more accurate assessment, consider waist circumference, body fat percentage, or DEXA scans.
What BMI is considered obese?
A BMI of 30.0 or higher is classified as obese. It is further divided into Class I (30-34.9), Class II (35-39.9), and Class III (40+, sometimes called severe obesity). Higher classes carry greater health risks including heart disease, diabetes, and joint problems.
Can you have a high BMI and be healthy?
Yes. People with significant muscle mass—such as athletes and regular weight lifters—can have a high BMI while maintaining low body fat and excellent cardiovascular health. This is why BMI should not be used in isolation. Body fat percentage, blood pressure, blood glucose, and cholesterol provide a more complete picture.
How often should I check my BMI?
For most adults, once or twice a year during routine checkups is sufficient. If you are actively working on weight changes, monthly check-ins can track progress. Focus on long-term trends rather than daily fluctuations caused by water retention, meals, and exercise.
Related Calculators
- Body Fat Calculator - Estimate your body fat percentage using the Navy method.
- Ideal Weight Calculator - Find your ideal weight range using four formulas.
- TDEE Calculator - Calculate your total daily energy expenditure.