BMI Calculator
BMI Calculator
Calculate your Body Mass Index instantly with our free, accurate BMI calculator. Trusted by users across the USA, UK, Canada, Germany, and Denmark for health assessments based on WHO and CDC guidelines.
BMI Calculator
Your BMI Result
Fill in your height and weight to see your BMI result
Why Trust This BMI Calculator?
This calculator follows internationally recognized standards from the World Health Organization (WHO), Centers for Disease Control and Prevention (CDC), and national health authorities across the United States, United Kingdom, Canada, Germany, and Denmark.
Evidence-Based
All calculations and classifications follow WHO BMI standards and CDC guidelines.
Privacy Protected
All calculations happen in your browser. No data is sent to any server.
Medical Disclaimer
Results are for informational purposes. Always consult a healthcare professional.
Regularly Reviewed
Content is reviewed and updated to reflect the latest medical guidelines.
What Is BMI?
Body Mass Index (BMI) is a simple screening tool that estimates whether your weight is healthy for your height. Developed by Belgian mathematician Adolphe Quetelet in the 1830s, it remains the most widely used metric for population-level weight assessment worldwide.
Doctors, nutritionists, and public health officials use BMI because it is quick, inexpensive, and requires no specialized equipment. A single number allows healthcare providers to flag potential weight-related health risks during routine checkups.
Health authorities including the WHO, CDC (USA), NHS (UK), Health Canada, Germany's Robert Koch Institute, and Denmark's Sundhedsstyrelsen all rely on BMI as a primary screening metric for clinical evaluations and public health monitoring.
Key Point
BMI is a screening tool, not a diagnostic measure. It does not directly measure body fat percentage, muscle mass, or fat distribution. A healthcare provider should perform additional assessments to evaluate your individual health risks.
How BMI Works
BMI divides your weight by the square of your height, producing a standardized number that can be compared across different body sizes. This squaring of height accounts for the fact that taller people naturally weigh more than shorter people, even at the same body composition.
The result is a dimensionless number that falls into one of several categories. Because the formula uses only weight and height, it can be calculated anywhere without special tools — which is precisely why it has become so universal.
Metric Formula
Example: 70 kg at 1.75 m → BMI = 70 / 3.0625 = 22.9
Imperial Formula
Example: 154 lbs at 69 inches → BMI = (154 / 4761) × 703 = 22.7
Both formulas produce nearly identical results. The 703 constant in the imperial version simply converts the units to match the metric scale.
BMI Categories
The following table uses the WHO BMI Classification, the international standard adopted by the CDC, NHS, Health Canada, and European health authorities.
| BMI Range | Classification | Health Risk |
|---|---|---|
| Below 18.5 | Underweight | Malnutrition risk, osteoporosis, anemia |
| 18.5 — 24.9 | Healthy Weight | Low risk (optimal range) |
| 25.0 — 29.9 | Overweight | Moderate risk of cardiovascular disease, type 2 diabetes |
| 30.0 — 34.9 | Obesity Class I | High risk — increased likelihood of hypertension, heart disease |
| 35.0 — 39.9 | Obesity Class II | Very high risk — significant health complications likely |
| 40.0 and above | Obesity Class III | Extremely high risk — severe obesity with major health implications |
Important Note on Asian Populations
The WHO Western Pacific Region recommends lower BMI cut-offs for many Asian populations: BMI 23 for overweight and BMI 27.5 for obesity. Metabolic risks appear at lower body mass levels in these groups.
Country-Specific BMI Guidance
While WHO standards serve as the global benchmark, each country adapts communication and resources to fit national health priorities.
🇺🇸 United States (CDC)
The CDC uses standard WHO classification for adults and provides BMI-for-age percentile charts for children and teens aged 2-19. The CDC emphasizes that BMI is a screening tool requiring additional health assessments.
🇬🇧 United Kingdom (NHS)
The NHS recommends maintaining a BMI between 18.5 and 24.9. Their healthy weight calculator also emphasizes waist circumference as an important companion indicator alongside BMI.
🇨🇦 Canada (Health Canada)
Health Canada aligns with WHO standards and stresses evaluating BMI alongside waist circumference, physical activity, and eating habits for a complete health picture.
🇩🇪 Germany (RKI)
The Robert Koch Institute integrates WHO BMI standards into national health surveys and clinical guidelines, with focused attention on Germany's rising obesity prevalence.
🇩🇰 Denmark (Sundhedsstyrelsen)
The Danish Health Authority follows WHO standards while incorporating Danish population data into comprehensive public health strategies for obesity prevention.
BMI Limitations
BMI is valuable but has clear limitations. It cannot distinguish between muscle and fat, nor does it account for where fat is stored on the body. Understanding these limitations helps you interpret your result appropriately.
Common Limitations
- Muscle Mass: Athletes and bodybuilders may register as overweight despite low body fat. A muscular person with 10% body fat could be classified as overweight by BMI.
- Age: Older adults lose muscle and gain fat, so BMI may underestimate body fat in seniors, leading to false reassurance about health status.
- Sex: Women typically carry more body fat than men at the same BMI due to biological differences in body composition.
- Ethnicity: Asian populations face health risks at lower BMI thresholds. Some studies suggest African American populations may have lower risks at higher BMI levels.
- Fat Distribution: BMI does not measure visceral fat (around organs), which carries greater health risk than subcutaneous fat (under the skin).
When BMI Should Not Be Used
Inappropriate Situations
- Pregnancy: Expected weight gain makes BMI invalid. Use gestational weight gain guidelines instead.
- Children under 2: Use weight-for-length rather than BMI.
- Professional athletes: High muscle mass renders BMI meaningless.
- People with eating disorders: BMI can be triggering and is not the treatment focus.
- Amputees: Standard formulas do not account for missing limbs.
Older Adults
For adults over 65, research suggests a slightly higher BMI (25-27) may correlate with better outcomes — the "obesity paradox" in geriatric populations. The Danish Health Authority and other European bodies acknowledge that BMI interpretation should be adjusted for elderly patients.
Children and Teens
For ages 2-19, BMI uses the same formula but is interpreted via BMI-for-age percentile charts rather than adult categories. The 85th-95th percentile indicates overweight; above the 95th percentile indicates obesity.
Understanding Your Healthy Weight Range
Your healthy weight range is calculated from BMI 18.5 to 24.9 applied to your height. This gives you a target zone rather than a single number, acknowledging that healthy bodies come in various sizes and shapes.
For someone 175 cm tall:
- BMI 18.5 → 56.7 kg
- BMI 24.9 → 76.3 kg
- Healthy range: 56.7 — 76.3 kg
This 19.6 kg window means you do not need to hit an exact target. Your weight will naturally shift with hydration, meals, hormones, and activity levels. What matters is staying within the healthy zone over time.
Healthy Weight Is a Range
Your body functions optimally within a range of weights. Small fluctuations are normal. Focus on sustainable habits rather than a specific number.
Additional Health Metrics Explained
Our calculator provides supplementary metrics for a more complete picture of your body composition and metabolic health. While BMI is the headline number, these additional values give context that can be valuable when discussing your health with a provider.
BMI Prime
Your BMI divided by 24.9 (the upper healthy limit). A value of 1.0 means you are at the top of the healthy range. Below 1.0 is within or below healthy; above 1.0 exceeds it. A BMI of 27.5 gives a BMI Prime of 1.10 — 10% above the healthy upper limit. This metric is particularly useful for tracking progress during weight management, as it shows your relative distance from the healthy threshold.
Ponderal Index
Calculated as weight divided by height cubed (kg/m³). Unlike BMI, which uses height squared, PI is considered more appropriate for individuals at the extremes of height. The normal adult range is approximately 11-14 kg/m³. PI is particularly useful in assessing body composition in very tall or very short individuals where BMI may be less accurate.
Body Surface Area (BSA)
Calculated via the Mosteller formula: BSA (m²) = √([height(cm) × weight(kg)] / 3600). BSA is used primarily for medical drug dosing calculations, particularly for chemotherapy and certain medications where dosing by body surface area is more accurate than dosing by weight alone.
Basal Metabolic Rate (BMR)
BMR represents the number of calories your body needs to maintain basic physiological functions at complete rest — breathing, circulation, cell production, and temperature regulation. We use the Mifflin-St Jeor equation, which is considered the most accurate formula for estimating BMR in modern populations. BMR decreases with age and increases with muscle mass. Understanding your BMR helps in planning caloric intake for weight management goals.
Common BMI Calculation Mistakes
- Wrong units: Entering pounds when kilograms are expected, or centimeters when feet are required.
- Incorrect height conversion: Converting 5 feet 9 inches to 5.9 feet instead of 5.75 feet.
- Time of day: Weight fluctuates 1-3 kg daily. Measure consistently, preferably in the morning.
- Clothing: Heavy clothing can add 1-2 kg. Weigh in minimal clothing for accuracy.
- Rounding errors: Small rounding errors in height significantly affect BMI. Enter as precisely as possible.
Frequently Asked Questions
For adults aged 18-65, the WHO recommends a BMI between 18.5 and 24.9, regardless of age. For adults over 65, some research suggests a slightly higher BMI (25-27) may be associated with better health outcomes. For children and teens, BMI is interpreted using age and sex-specific percentile charts. Always consult your healthcare provider for personalized guidance.
BMI correctly identifies obesity in about 80-90% of the general population. However, it can misclassify muscular individuals as overweight and may underestimate body fat in elderly people. For individual assessment, use BMI alongside waist circumference, body fat percentage, and clinical evaluation.
Yes. For metric: divide weight in kilograms by height in meters squared. For imperial: divide weight in pounds by height in inches squared, then multiply by 703. However, using a calculator eliminates errors and provides additional health insights instantly.
According to WHO and CDC guidelines, a BMI of 30.0 or higher is classified as obese. This is further divided into three classes: Class I (BMI 30.0-34.9), Class II (BMI 35.0-39.9), and Class III (BMI 40.0 and above). These classifications help healthcare providers assess health risk and determine appropriate interventions.
The standard WHO BMI classification uses the same thresholds for both men and women. However, women typically have a higher percentage of body fat than men at the same BMI due to biological differences in body composition. Our calculator includes optional gender input for BMR estimation, which does account for sex differences.
For most adults, checking BMI once every 3-6 months is sufficient. If you are actively working toward weight management goals, monthly checks can help track progress. Daily weighing is not recommended as normal fluctuations in water weight can be discouraging. Focus on long-term trends rather than single measurements.
Large-scale studies suggest the lowest mortality risk occurs at a BMI around 22-23 for most adult populations. However, "ideal" BMI varies by individual factors including age, ethnicity, muscle mass, and overall health status. The WHO healthy weight range of 18.5-24.9 is considered optimal for the general adult population.
Yes, our calculator shows the exact weight you would need to reach specific BMI targets, including BMI 18.5, BMI 22 (near-optimal), and BMI 24.9. However, any weight loss plan should be developed with a healthcare provider. The CDC recommends gradual weight loss of 1-2 pounds (0.5-1 kg) per week for sustainable results.
Methodology & Editorial Policy
Our BMI calculator and content are developed using rigorous evidence-based methodology. All formulas, classifications, and health recommendations are derived from peer-reviewed medical literature and official health organization guidelines.
Calculation Methodology
- BMI: Standard WHO formula — weight (kg) / height (m)²
- BMI Prime: BMI / 24.9 (upper healthy limit)
- Ponderal Index: weight (kg) / height (m)³
- Body Surface Area: Mosteller formula — √([height(cm) × weight(kg)] / 3600)
- BMR: Mifflin-St Jeor equation with optional gender/age adjustment
- Healthy Weight Range: BMI 18.5 and 24.9 applied to user height
Editorial Policy
All content is reviewed against current medical literature. We update our calculator and articles when major health organizations revise their guidelines. Our last comprehensive review was conducted in August 2026.
Sources & References
All information is based on official publications from recognized health authorities. We provide direct links to primary sources for your verification.
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1World Health Organization — BMI Classificationwho.int/news-room/fact-sheets/detail/obesity-and-overweight
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2Centers for Disease Control and Prevention — Adult BMI Calculatorcdc.gov/healthyweight/assessing/bmi/adult_bmi/index.html
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3NHS — BMI Healthy Weight Calculatornhs.uk/live-well/healthy-weight/bmi-calculator/
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4Health Canada — Healthy Livingcanada.ca/en/health-canada/services/food-nutrition.html
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5Robert Koch Institute — Health Monitoringrki.de/EN/Content/Health_Monitoring
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6Danish Health Authority — National Health Guidelinessst.dk/en/english
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7WHO Western Pacific Region — Appropriate BMI for Asian Populationswho.int/westernpacific/health-topics/obesity
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8Mifflin MD, et al. — A new predictive equation for resting energy expenditure in healthy individualspubmed.ncbi.nlm.nih.gov/2305711/
Medical Disclaimer
This BMI calculator is provided for informational and educational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding your health or a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read on this website. The results from this calculator should not be used to make health decisions without consulting a healthcare professional.