BMI Calculator | Free Body Mass Index Calculator
Calculate your Body Mass Index (BMI) instantly with our free online calculator. Determine your weight status, understand health risks, and get personalized recommendations for achieving and maintaining a healthy weight. Supports both metric and imperial units.
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Your Results
Enter your weight and height, then click Calculate BMI to see your results.
What is BMI (Body Mass Index)?
BMI (Body Mass Index) is a numerical measurement that assesses whether your weight is appropriate for your height by calculating the ratio of your body weight to your height squared. Developed in the 1830s by Belgian mathematician Adolphe Quetelet, BMI serves as a screening tool to categorize individuals into weight status groups and identify potential health risks associated with being underweight, normal weight, overweight, or obese, based on the internationally recognized standards of 18.5-24.9 for healthy weight.
Healthcare professionals worldwide use BMI as an initial assessment tool because it's simple, cost-effective, and correlates with body fat percentage in most populations. However, BMI should always be considered alongside other health metrics such as waist circumference, body composition, lifestyle factors, and medical history for a comprehensive health evaluation.
How to Use the BMI Calculator
Using our BMI calculator is straightforward and takes less than 30 seconds. Follow these simple steps to calculate your Body Mass Index accurately:
Step 1: Select Your Unit System
Choose between metric units (kilograms and centimeters) or imperial units (pounds and feet/inches) by clicking the appropriate button at the top of the calculator. Select whichever system you're most comfortable with—both will provide equally accurate BMI calculations.
Step 2: Enter Your Weight
Input your current body weight in the designated field. If using metric, enter your weight in kilograms (e.g., 70 kg). If using imperial, enter your weight in pounds (e.g., 154 lbs). For the most accurate results, weigh yourself first thing in the morning after using the bathroom and before eating or drinking, wearing minimal clothing.
Step 3: Enter Your Height
For metric users, enter your height in centimeters (e.g., 175 cm). For imperial users, enter your height using the separate feet and inches fields (e.g., 5 feet 9 inches). Measure your height while standing straight against a wall without shoes for maximum accuracy. The calculator automatically converts your height to the proper format for BMI calculation.
Step 4: Review Your Results
Your BMI calculates instantly as you enter your measurements. The results panel displays your BMI number, weight category (underweight, normal weight, overweight, or obese), detailed health information about your category, and your ideal weight range for your height. The color-coded result helps you quickly understand your weight status at a glance.
Understanding the BMI Formula
The Mathematical Formula
BMI is calculated using a simple mathematical formula that creates a ratio between your weight and height. The standard international formula uses metric units:
BMI = Weight (kg) ÷ Height² (m²)
For imperial measurements, the formula includes a conversion factor to ensure the result matches the metric calculation:
BMI = (Weight (lbs) × 703) ÷ Height² (in²)
Why This Formula Works
The BMI formula was designed to create a number that correlates with body fat percentage across most populations. By dividing weight by height squared (rather than just height), the formula accounts for the fact that as height increases, body volume increases proportionally more than body surface area. The number 703 in the imperial formula is a conversion factor that ensures imperial measurements produce the same BMI result as metric measurements. This mathematical approach allows BMI to serve as a reasonable proxy for body fat percentage in most adults, though it has important limitations for certain populations including athletes, elderly individuals, and different ethnic groups.
Practical Calculation Example
Metric Example:
Person: 70 kg weight, 1.75 m height (175 cm)
Calculation: 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9
Result: BMI of 22.9 (Normal Weight)
Imperial Example:
Person: 154 lbs weight, 5'9" height (69 inches)
Calculation: (154 × 703) ÷ (69 × 69) = 108,262 ÷ 4,761 = 22.7
Result: BMI of 22.7 (Normal Weight)
BMI Categories and Ranges
The World Health Organization (WHO) and Centers for Disease Control (CDC) have established standardized BMI categories that correlate with health risks. Understanding these categories helps you assess your current weight status and identify potential health concerns:
| Classification | BMI Range | Health Risk | Description |
|---|---|---|---|
| Severe Thinness | < 16 | Extremely High | Severely underweight. High risk of nutritional deficiency, weakened immune system, and bone loss. Urgent medical evaluation recommended. |
| Moderate Thinness | 16 - 16.9 | High | Moderately underweight. Risk of nutritional deficiency and health complications. Consider consulting a healthcare provider. |
| Mild Thinness | 17 - 18.4 | Increased | Mildly underweight. May indicate inadequate nutrition. Consider balanced diet with adequate calories and protein. |
| Normal Weight | 18.5 - 24.9 | Low | Healthy weight range. Associated with lowest health risks. Maintain with balanced nutrition and regular physical activity. |
| Overweight | 25 - 29.9 | Increased | Above normal weight. Increased risk of cardiovascular disease, type 2 diabetes. Consider lifestyle modifications including diet and exercise. |
| Obese Class I | 30 - 34.9 | High | Obesity class I. High risk of metabolic syndrome, heart disease, type 2 diabetes. Medical consultation recommended for weight management. |
| Obese Class II | 35 - 39.9 | Very High | Obesity class II. Very high risk of serious health complications. Comprehensive medical treatment approach recommended. |
| Obese Class III | ≥ 40 | Extremely High | Severe obesity. Extremely high risk of life-threatening conditions. Specialized medical intervention and treatment options should be explored. |
Important Note on Ethnic Variations
These are standard international BMI categories. However, some ethnic groups face increased health risks at lower BMI thresholds. For Asian populations, overweight is often defined as BMI ≥23 and obesity as BMI ≥27.5. Always consult healthcare providers familiar with population-specific health patterns for personalized guidance.
BMI and Associated Health Risks
Research consistently shows strong correlations between BMI categories and various health conditions. Understanding these associations helps motivate healthy lifestyle changes and informs medical screening priorities:
| BMI Category | Health Risk Level | Associated Health Conditions |
|---|---|---|
| Severe Thinness (<16) | Extremely High | Malnutrition, organ failure, anemia, osteoporosis, fertility issues |
| Moderate Thinness (16-16.9) | High | Nutritional deficiency, weakened immunity, low energy |
| Mild Thinness (17-18.4) | Increased | Malnutrition, osteoporosis, weakened immunity, anemia |
| Normal Weight (18.5-24.9) | Low | Lowest risk of weight-related health problems |
| Overweight (25-29.9) | Increased | Hypertension, dyslipidemia, insulin resistance |
| Obese Class I (30-34.9) | High | Type 2 diabetes, heart disease, sleep apnea |
| Obese Class II (35-39.9) | Very High | Severe metabolic disease, joint problems, cancer risk |
| Obese Class III (≥40) | Extremely High | Life-threatening conditions, reduced life expectancy |
Good News About Weight Loss Benefits
Even modest weight loss of 5-10% of body weight can significantly reduce health risks. For someone weighing 200 lbs, losing just 10-20 lbs can improve blood pressure, cholesterol levels, blood sugar control, and reduce strain on joints. Small, sustainable changes matter more than dramatic transformations.
Important BMI Limitations
While BMI is a useful screening tool, it has significant limitations that must be understood to avoid misinterpretation. BMI should never be used as the sole indicator of health status:
Muscle vs. Fat Mass
BMI cannot distinguish between muscle and fat tissue. Athletes, bodybuilders, and individuals with high muscle mass may have elevated BMI readings that classify them as overweight or obese despite having low body fat percentages and excellent health. Conversely, people with low muscle mass may have "normal" BMI but high body fat percentage.
Age and Body Composition Changes
Body composition changes significantly with age. Older adults typically have more body fat and less muscle mass than younger adults at the same BMI. This means a BMI of 23 might represent very different health statuses for a 25-year-old versus a 75-year-old. Age-specific BMI interpretations may be more appropriate for seniors.
Fat Distribution Not Measured
BMI doesn't indicate where body fat is located, yet fat distribution matters enormously for health. Visceral fat (around organs) poses far greater health risks than subcutaneous fat (under skin). Two people with identical BMI can have vastly different health risks based on whether fat is concentrated around the abdomen versus distributed more evenly.
Ethnic and Genetic Variations
Different ethnic groups have different body compositions and health risk patterns at the same BMI. Asian populations typically have higher body fat percentages and increased disease risk at lower BMIs. Pacific Islander and Polynesian populations often have higher muscle mass and bone density, requiring higher BMI thresholds for health risk assessment.
Not Suitable for All Populations
BMI is not appropriate for pregnant women, children under 18 (who require age-specific percentile charts), competitive athletes, people with unusual body proportions, individuals with limb amputations, or those with conditions affecting bone density or muscle mass like osteoporosis or muscular dystrophy.
Better Alternative Metrics Available
More accurate health assessments include waist circumference (men <40", women <35"), waist-to-hip ratio (men <0.90, women <0.85), body fat percentage via DEXA scan or bioelectrical impedance, waist-to-height ratio (<0.5), and comprehensive metabolic panels measuring blood pressure, lipids, and glucose levels.
Tips for Achieving and Maintaining Healthy BMI
Whether you need to lose weight, gain weight, or maintain your current healthy weight, these evidence-based strategies can help you achieve your goals safely and sustainably:
1. Focus on Sustainable Changes, Not Crash Diets
Aim for gradual weight loss of 1-2 pounds per week rather than dramatic rapid weight loss. Crash diets rarely work long-term and can slow metabolism, cause muscle loss, and create nutritional deficiencies. Instead, make small, permanent lifestyle modifications you can maintain for life: reduce portion sizes slightly, add one extra serving of vegetables daily, or replace sugary drinks with water. These small changes compound over time into significant results.
2. Prioritize Whole, Unprocessed Foods
Build your diet around whole foods: vegetables, fruits, whole grains, lean proteins, and healthy fats. These foods are more filling, more nutritious, and naturally lower in calories than processed alternatives. Fill half your plate with vegetables, a quarter with lean protein, and a quarter with whole grains. This simple approach automatically improves nutrition while managing calorie intake without requiring detailed calorie counting.
3. Incorporate Regular Physical Activity
Aim for at least 150 minutes of moderate aerobic activity (brisk walking, swimming, cycling) or 75 minutes of vigorous activity (running, HIIT) weekly, plus strength training twice weekly. Physical activity burns calories, preserves muscle mass during weight loss, improves metabolic health, reduces disease risk, and enhances mental wellbeing. Find activities you genuinely enjoy to ensure long-term adherence—the best exercise is the one you'll actually do consistently.
4. Manage Stress and Prioritize Sleep
Chronic stress elevates cortisol, promoting fat storage particularly around the abdomen. Poor sleep (less than 7 hours nightly) disrupts hormones that regulate hunger and satiety, increasing appetite and cravings for high-calorie foods. Prioritize stress management through meditation, yoga, or other relaxation techniques. Establish consistent sleep schedules, create dark cool sleeping environments, and limit screen time before bed to ensure quality rest.
5. Track Progress Beyond the Scale
Weight can fluctuate 2-5 pounds daily due to water retention, food in digestive system, and hormonal changes. Don't obsess over daily weigh-ins. Instead, track multiple indicators: how clothes fit, body measurements (waist, hips, chest), energy levels, sleep quality, fitness improvements (running farther, lifting heavier), and biomarkers (blood pressure, cholesterol, blood sugar). These provide a more complete picture of health improvements beyond BMI.
6. Seek Professional Guidance When Needed
Consult registered dietitians for personalized nutrition plans, especially if you have medical conditions like diabetes or food allergies. Work with certified personal trainers to develop safe, effective exercise programs. Discuss with physicians if BMI exceeds 40 or 35 with obesity-related conditions—you may benefit from medical weight loss programs, medications, or bariatric surgery. Mental health professionals can address emotional eating, body image issues, or eating disorders.
7. Stay Hydrated and Mindful of Liquid Calories
Drink adequate water throughout the day (aim for 8-10 glasses). Sometimes thirst masquerades as hunger. However, be mindful that liquid calories from sodas, juices, alcohol, and fancy coffee drinks add up quickly without providing satiety. A single large flavored latte can contain 400+ calories. Replacing sugary beverages with water, unsweetened tea, or black coffee is one of the easiest ways to reduce calorie intake without feeling deprived.
8. Practice Mindful Eating Habits
Eat slowly, chewing thoroughly and putting utensils down between bites. This allows satiety signals to reach your brain before overeating occurs (takes 15-20 minutes). Minimize distractions during meals—turn off TV, put away phones, and focus on your food. Recognize true hunger versus boredom, stress, or habit-driven eating. Stop eating when satisfied rather than uncomfortably full. These mindful practices naturally reduce calorie intake without strict dieting.
Frequently Asked Questions About BMI
What is BMI and how is it calculated?
BMI (Body Mass Index) measures body fat based on height and weight using the formula: weight (kg) ÷ height² (m²). For imperial units, multiply by 703. BMI is a screening tool to categorize weight status (underweight, normal, overweight, obese) but doesn't measure body fat directly. Healthcare professionals use it alongside other assessments for complete health evaluation. See our detailed BMI categories section below for ranges.
What is a healthy BMI range for adults?
A healthy BMI for most adults aged 18+ is 18.5-24.9, associated with lowest risk of weight-related health conditions. BMI below 18.5 is underweight, 25-29.9 is overweight, and 30+ indicates obesity. Optimal BMI varies by age, ethnicity, and muscle mass. Asian populations may face increased health risks at lower BMI thresholds. Always consult healthcare professionals for personalized guidance based on your unique circumstances.
Is BMI an accurate measure of health?
BMI has limitations as a sole health indicator. It cannot distinguish muscle from fat—athletes may have high BMIs despite low body fat. Older adults may have normal BMIs but higher body fat due to muscle loss. BMI doesn't account for fat distribution (visceral vs subcutaneous). Healthcare providers use BMI as an initial screening tool alongside waist circumference, body fat percentage, blood pressure, cholesterol, and blood glucose for comprehensive health assessment.
Can a BMI be too low?
Yes, BMI below 18.5 is underweight and can indicate serious health concerns including nutritional deficiencies, weakened immunity, osteoporosis, fertility problems, and anemia. BMI under 16 may risk organ failure and life-threatening complications. Causes include eating disorders, hyperthyroidism, digestive disorders, or inadequate caloric intake. If your BMI is below healthy range, consult a healthcare provider to identify causes and develop a safe weight gain plan with adequate nutrition and strength training.
How often should I check my BMI?
For adults maintaining stable weight, check BMI every 3-6 months for general monitoring. If actively managing weight, weekly or bi-weekly measurements help track progress. Athletes may benefit from monthly BMI checks with body composition analysis. Children should have BMI monitored at regular pediatric check-ups. BMI is just one metric—focus on overall health trends including energy levels, fitness improvements, and how clothing fits rather than daily fluctuations.
Does age affect BMI interpretation?
Age significantly impacts BMI interpretation. As we age, muscle mass decreases while body fat increases, even at stable weight. Older adults may have higher body fat at the same BMI as younger individuals. Some research suggests slightly higher BMI ranges (25-27) may be acceptable for adults over 65. For children and adolescents, BMI is interpreted using age and gender-specific percentile charts since body composition changes during growth. Always discuss age-appropriate BMI targets with your healthcare provider.
Why do athletes have high BMI despite being healthy?
Athletes, particularly in strength sports, often have elevated BMI readings (overweight/obese) despite low body fat and excellent health. BMI uses total body weight without differentiating between fat and muscle mass. Muscle tissue is denser than fat, so muscular individuals weigh more at the same height. Many professional athletes have BMIs over 30 but body fat under 10%. For athletic populations, body composition analysis (DEXA scans, bioelectrical impedance, skinfold measurements) provides more accurate health assessments than BMI alone.
How does ethnicity affect BMI standards?
BMI standards should be adjusted for different ethnic populations due to variations in body composition and disease risk patterns. Asian populations have higher body fat and increased health risks at lower BMI levels—overweight is BMI ≥23, obesity ≥27.5. Pacific Islander and Polynesian populations typically have higher muscle mass and bone density, requiring higher BMI thresholds. African Americans tend to have higher muscle mass at the same BMI. These variations highlight why BMI should be interpreted within cultural and genetic context by healthcare providers.
What are better alternatives to BMI?
Several metrics provide more comprehensive health assessments than BMI. Waist circumference measures abdominal fat (men <40 inches, women <35 inches). Waist-to-hip ratio shows health risks (>0.90 men, >0.85 women). Body fat percentage directly quantifies fat versus lean tissue. Waist-to-height ratio (keep waist under half your height) works across ages and ethnicities. For comprehensive assessment, combine these with blood pressure, lipid panels, blood glucose, and fitness assessments to evaluate true health status beyond simple weight calculations.
Can BMI be used during pregnancy?
BMI should not be used as a health indicator during pregnancy, as weight gain is expected and necessary for healthy fetal development. However, pre-pregnancy BMI determines recommended weight gain: normal BMI (18.5-24.9) gain 25-35 lbs, underweight (BMI <18.5) gain 28-40 lbs, overweight (25-29.9) gain 15-25 lbs, obese (BMI ≥30) gain 11-20 lbs. Pregnant women should focus on prenatal nutrition quality and follow healthcare provider guidance rather than monitoring BMI changes during pregnancy.
How can I improve my BMI if it's too high?
Improving high BMI requires sustainable lifestyle changes focused on gradual weight loss of 1-2 pounds weekly. Dietary modifications: reduce portions, choose whole foods over processed options, increase vegetables/fruits, select lean proteins, limit added sugars and saturated fats. Incorporate 150 minutes of moderate aerobic exercise weekly plus strength training twice weekly. Track food intake and activity. Get adequate sleep (7-9 hours) and manage stress. Consult healthcare providers or registered dietitians for personalized guidance, and consider medical interventions if BMI exceeds 40 or 35 with comorbidities.
What health risks are associated with high BMI?
Elevated BMI increases risk for numerous serious health conditions. Cardiovascular risks include hypertension, coronary artery disease, stroke, and heart failure. Metabolic complications include type 2 diabetes, metabolic syndrome, and non-alcoholic fatty liver disease. Obesity raises cancer risk (breast, colon, endometrial, kidney, esophageal). Musculoskeletal problems like osteoarthritis in weight-bearing joints become more likely. Respiratory issues include sleep apnea. Reproductive health may be affected through infertility and pregnancy complications. Mental health impacts include increased depression and anxiety. Even modest weight loss of 5-10% can significantly reduce these risks.
Is BMI different for men and women?
The same BMI categories apply to both adult men and women (18.5-24.9 is normal weight), though body composition differs significantly between sexes. Women naturally have higher body fat percentages than men at the same BMI due to biological differences (essential fat: 2-5% men, 10-13% women). Despite compositional differences, the same BMI cutoffs predict similar health risks for both sexes. However, healthcare providers increasingly consider body composition analysis alongside BMI for more accurate individual assessment, particularly for athletic or elderly populations.
How does BMI differ for children and teenagers?
BMI for children ages 2-19 is calculated the same way as adults but interpreted using gender-specific BMI-for-age percentile charts (CDC/WHO). Because children's body composition changes dramatically during growth, a single BMI number has different meanings at different ages. Healthy weight is 5th-85th percentile, underweight below 5th, overweight 85th-95th, and obesity ≥95th percentile. These percentiles account for normal developmental changes. Pediatricians use these charts at well-child visits to track growth patterns and identify potential weight issues early for intervention.
Can medications affect my BMI?
Yes, numerous medications significantly impact weight and BMI. Weight gain is common with antidepressants (SSRIs, tricyclics), antipsychotics, mood stabilizers, corticosteroids, diabetes medications (insulin, sulfonylureas), beta-blockers, and some contraceptives. Some medications promote weight loss including stimulant ADHD medications, bupropron, metformin, and weight management medications. If experiencing unexpected weight changes after starting medication, consult your healthcare provider rather than stopping independently. Alternative medications with less weight impact may be available, or additional strategies can manage medication-related weight changes while maintaining necessary treatment.