How to Use BMI Calculator
Follow these three steps to process, validate, and export your data locally inside your browser with zero server latency:
Choose Metric (kilograms and centimeters) or Imperial (pounds, feet, and inches).
Enter your height and weight values.
View the calculated BMI value, standard WHO adult reference category, and healthy weight range for your height.
How Body Mass Index (BMI) Is Calculated in Metric and Imperial Units
Body Mass Index (BMI), originally formulated by Belgian mathematician Adolphe Quetelet in the 1830s and adopted by the World Health Organization (WHO), is a standardized screening index that relates adult body mass to height squared. In the SI metric system, BMI is defined as weight in kilograms divided by the square of height in meters: BMI = kg / m².
When using Imperial measurements (pounds and feet/inches), height is first converted to total inches (feet × 12 + inches), and the formula applies the 703 conversion constant: BMI = 703 × weight(lb) / height(in)². In addition to your exact BMI score and WHO category, our calculator reverse-solves the formula for your height to display the exact weight interval corresponding to a Normal BMI of 18.5 to 24.9.
Clinical Limitations of BMI as a Standalone Metric
BMI measures total body mass relative to height, not body fat distribution or lean muscle mass. Strength athletes with high skeletal muscle mass may register in the Overweight BMI bracket despite low body fat, whereas older adults with sarcopenia may have a Normal BMI alongside elevated visceral adiposity. Pair BMI with waist-to-height ratio and clinical guidance.
World Health Organization (WHO) Adult BMI Classification Table
Standard international BMI thresholds for adults aged 20 and older:
| WHO Classification | BMI Range (kg/m²) | Prime Ratio (BMI / 25) | General Screening Interpretation |
|---|---|---|---|
| Underweight | < 18.5 | < 0.74 | May indicate nutritional deficit or low lean mass |
| Normal Weight | 18.5 – 24.9 | 0.74 – 1.00 | Standard healthy reference weight range |
| Overweight | 25.0 – 29.9 | 1.00 – 1.20 | Moderate elevation; evaluate waist circumference |
| Obesity Class I | 30.0 – 34.9 | 1.20 – 1.40 | Increased cardiometabolic risk profile |
| Obesity Class II | 35.0 – 39.9 | 1.40 – 1.60 | High clinical risk profile |
| Obesity Class III | ≥ 40.0 | ≥ 1.60 | Severe clinical risk profile |
How to Interpret & Use Your BMI Results Responsibly
Context and complementary metrics to consider alongside your Body Mass Index:
1. Check Your Target Healthy Weight Range
Goal SettingLook at the "Healthy Weight Range (18.5–24.9 BMI)" card to see the exact kilogram or pound span recommended for your specific height.
2. Complement BMI with Waist-to-Height Ratio
Body CompositionKeeping your waist circumference under half your height (Waist-to-Height ratio < 0.5) is a strong indicator of healthy visceral fat levels.
3. Pediatric & Teen BMI Uses Age Percentiles
Age ScopeFixed adult thresholds (18.5 / 25 / 30) apply to adults 20+. Children and adolescents (ages 2–19) are evaluated using sex- and age-specific CDC/WHO growth percentiles.
4. 100% Private Health Calculation
Medical PrivacyYour height and weight inputs are never transmitted to any server, analytics tracker, or third-party health database.
Practical Examples & Input/Output Reference
Metric BMI calculation
Calculating BMI for 70 kg and 175 cm.
Sample Input
Weight: 70 kg, Height: 175 cm (1.75 m)Expected Output
BMI: 22.9 kg/m² (Standard adult reference: Normal weight, 18.5–24.9)Frequently Asked Questions
Is BMI a medical diagnosis?
No. BMI is a simple screening ratio of weight to height squared (`kg / m²`). It does not distinguish between muscle mass and fat mass or account for age, sex, or bone density. Always consult a qualified healthcare professional for medical advice.