BMI Calculator
Calculate your Body Mass Index and understand what it means for your health
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Understanding Body Mass Index
Body Mass Index (BMI) is a simple numerical measure that uses your height and weight to estimate whether you fall into an underweight, normal, overweight, or obese category. The formula divides your weight in kilograms by the square of your height in meters (kg/m²). Developed by Belgian mathematician Adolphe Quetelet in the 1830s, it remains the most widely used screening tool for weight classification by healthcare professionals, public health organizations, and insurance companies worldwide.
While BMI does not directly measure body fat, research consistently shows a strong correlation between BMI and more direct measures of adiposity. The World Health Organization (WHO) and the National Institutes of Health (NIH) recognize four primary BMI categories: underweight (below 18.5), normal weight (18.5 to 24.9), overweight (25 to 29.9), and obese (30 and above). Obesity is further divided into Class I (30-34.9), Class II (35-39.9), and Class III (40 and above). These thresholds are linked to statistical increases in the risk of conditions such as type 2 diabetes, cardiovascular disease, and certain cancers.
It is important to understand BMI's limitations. Because it relies solely on total body weight, it cannot distinguish between muscle mass and fat mass. Athletes and highly muscular individuals may have a BMI in the overweight range despite having a low body fat percentage. Similarly, older adults who have lost muscle mass might register a normal BMI while carrying excess fat. Factors such as age, sex, ethnicity, and bone density all influence the relationship between BMI and health risk, which is why clinicians use BMI alongside other assessments like waist circumference, blood pressure, and blood lipid levels.
For most adults, BMI provides a useful starting point for conversations about weight and health. If your BMI falls outside the normal range, consider speaking with a healthcare provider who can evaluate your overall health picture. Maintaining a healthy BMI through a balanced diet rich in whole foods, regular physical activity (at least 150 minutes of moderate exercise per week), adequate sleep, and stress management can significantly reduce your risk of chronic disease and improve your quality of life.
How to Use This BMI Calculator
- Select your preferred unit system by clicking "Metric (kg, cm)" or "Imperial (lbs, in)" at the top of the form. The calculator will automatically adjust input fields and labels.
- Enter your body weight in the Weight field. Use kilograms if you selected metric, or pounds if you selected imperial.
- Enter your height in the Height field. Use centimeters for metric or total inches for imperial (e.g., 70 inches for 5'10").
- Click the "Calculate BMI" button. Your results will appear below, including a visual gauge, your BMI number, weight category, and healthy weight range for your height.
- Review your results and use the category information and healthy weight range to set informed health goals. Consult a healthcare professional for personalized guidance.
Frequently Asked Questions
What is a healthy BMI range?
A healthy BMI for most adults falls between 18.5 and 24.9. This range is associated with the lowest statistical risk of weight-related health problems such as heart disease, type 2 diabetes, and hypertension. However, health is multifactorial, and a BMI within this range does not guarantee good health, just as a BMI outside this range does not necessarily indicate poor health.
How is BMI calculated?
BMI is calculated by dividing your weight in kilograms by the square of your height in meters: BMI = weight (kg) / height (m)². For imperial units, the formula is: BMI = (weight in pounds x 703) / (height in inches)². For example, a person who weighs 70 kg and is 1.75 m tall has a BMI of 70 / (1.75 x 1.75) = 22.9.
Is BMI accurate for athletes and muscular people?
BMI can be misleading for athletes, bodybuilders, and people with above-average muscle mass. Muscle is denser than fat, so highly muscular individuals may be classified as overweight or obese by BMI despite having very low body fat. For these individuals, body fat percentage measurements (via DEXA scan, skinfold calipers, or bioelectrical impedance) provide a more accurate assessment of body composition.
Does BMI apply differently to men and women?
The standard BMI formula and categories are the same for adult men and women. However, women typically have a higher percentage of body fat than men at the same BMI. Some researchers have suggested sex-specific thresholds would be more accurate, but the current WHO categories remain the international standard. For children and teenagers, BMI is interpreted using age- and sex-specific percentile charts.
What should I do if my BMI is too high or too low?
If your BMI is outside the normal range, consult a healthcare provider for a comprehensive evaluation. For elevated BMI, gradual weight loss of 0.5-1 kg per week through a moderate calorie deficit and regular exercise is generally recommended. For low BMI, increasing calorie intake with nutrient-dense foods and strength training can help. In both cases, focus on sustainable lifestyle changes rather than extreme diets.
Does BMI change with age?
The BMI formula itself does not account for age, but body composition changes as you age. After about age 30, most people gradually lose muscle mass and gain fat mass, even if their weight stays the same. This means an older adult with a "normal" BMI may carry more body fat than a younger person with the same BMI. Some health organizations suggest slightly higher BMI thresholds (e.g., 22-27) may be optimal for adults over 65.
How often should I check my BMI?
For most adults, checking BMI every few months or at annual health checkups is sufficient. If you are actively working on weight management, monthly check-ins can help track progress. However, daily or weekly BMI checks are unnecessary and can be counterproductive, as normal weight fluctuations from water retention, meals, and exercise can cause misleading short-term variations. Focus on long-term trends rather than individual readings.
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