Body Mass Index Calculator
Assess your body weight and health risks instantly with our professional-grade BMI calculator. Calculate Body Mass Index using WHO classification guidelines with step-by-step analysis and visual charts.
Interactive BMI Calculator
Calculate Body Mass Index instantly with WHO-based classification. Professional tool for assessing body weight status and associated health risks. For clinical guidelines, see the CDC BMI assessment page.
BMI Formula & WHO Classification
BMI Definition
Body Mass Index is calculated by dividing weight in kilograms by the square of height in meters, providing a standardized measure for body weight status across populations:
WHO Classification System
| Category | BMI (kg/m²) | Health Risk Level | Population Prevalence (US) |
|---|---|---|---|
| Underweight | < 18.5 | Moderate (nutritional deficiency) | ~1.5% of adults |
| Normal Weight | 18.5 – 24.9 | Low (optimal health range) | ~30% of adults |
| Overweight | 25.0 – 29.9 | Moderate (increased risk) | ~34% of adults |
| Obesity Class I | 30.0 – 34.9 | High (significant health risks) | ~20% of adults |
| Obesity Class II | 35.0 – 39.9 | Very High (severe health risks) | ~8% of adults |
| Obesity Class III | ≥ 40.0 | Extremely High (critical health risks) | ~6% of adults |
Key Facts: According to WHO data, over 1.9 billion adults worldwide are overweight, with 650 million classified as obese. BMI remains the most widely used screening tool despite limitations, correlating strongly with body fat percentage and health outcomes.
How to Use the BMI Calculator
Simple steps for accurate body weight assessment
Enter Weight in kg
Input your body weight in kilograms (convert from pounds: kg = lbs ÷ 2.205)
Enter Height in meters
Input your height in meters (convert from inches: m = inches × 0.0254)
Get Instant Analysis
View your BMI with WHO classification, health risk assessment, and visual chart
Frequently Asked Questions
BMI is a screening tool that measures the ratio of weight to height squared to estimate body fat and assess health risks related to body weight. Developed by Adolphe Quetelet in 1832, it remains the global standard for population-level weight assessment, correlating strongly with body fat percentage and health outcomes.
No, the BMI formula and WHO categories are identical for males and females. However, body composition differs—women typically have 10-13% essential fat vs. 2-5% for men—so interpretation should consider individual factors. Athletes may have high BMI due to muscle mass rather than fat.
A BMI of 25.0–29.9 indicates overweight, and 30.0+ indicates obesity, significantly increasing risks for type 2 diabetes (7x higher), cardiovascular disease (3x higher), certain cancers, and mortality. Obesity costs the US healthcare system $173 billion annually. Even modest 5-10% weight loss reduces risk substantially.
To lower BMI, create a 500-750 calorie daily deficit through a balanced diet (increase vegetables, lean proteins, whole grains) and 150-300 minutes of moderate exercise weekly. Aim for 0.5-1 kg loss per week. Sustainable changes work better than crash diets. Consult healthcare providers for personalized plans, especially if BMI > 30.
BMI is the most widely used screening tool because it’s simple, inexpensive, and correlates strongly (r = 0.7-0.8) with body fat percentage and health outcomes at population levels. Limitations include not distinguishing muscle from fat, racial/ethnic variations, and inaccuracy in pregnancy or extreme ages. It’s a screening tool, not diagnostic.
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