1. What is BMI?
Body Mass Index (BMI) is a simple numerical measure derived from your height and weight. First developed by Belgian mathematician Adolphe Quetelet in the 1830s, BMI became a standard medical screening tool in the 1970s and remains the most widely used proxy for body fat percentage in population-level health assessments.
BMI is not a direct measure of body fat, but it correlates well with more direct measures like DEXA scans in most adults. It is used globally by physicians, insurance companies, and public health agencies as a quick, cost-free screening indicator.
2. BMI Formula Explained
The BMI formula is straightforward:
- Metric: BMI = weight (kg) ÷ height² (m²)
- Imperial: BMI = (weight (lbs) × 703) ÷ height² (inches²)
Example: A person who is 175 cm tall and weighs 75 kg has a BMI of 75 ÷ (1.75 × 1.75) = 24.5 — in the "Normal weight" category.
⚖️ Calculate Your BMI Free
Enter your height and weight — get your BMI, category, and healthy weight range instantly.
Open BMI Calculator →3. BMI Categories and What They Mean
- Below 18.5 — Underweight: Potential risk of nutritional deficiencies and bone density loss
- 18.5 – 24.9 — Normal weight: Associated with lowest disease risk in most studies
- 25.0 – 29.9 — Overweight: Moderate elevated risk of Type 2 diabetes and cardiovascular disease
- 30.0 – 34.9 — Obese Class I: Significant health risks; lifestyle intervention recommended
- 35.0 – 39.9 — Obese Class II: High risk; medical supervision typically indicated
- 40.0 and above — Obese Class III (Severe): Very high risk; often qualifies for bariatric surgery evaluation
4. Limitations of BMI
BMI is a useful population-level tool but has known limitations for individuals:
- Muscle vs. fat: Athletes and bodybuilders often have high BMI due to muscle mass, despite very low body fat percentage
- Age: BMI underestimates body fat in older adults who have lost muscle mass
- Ethnicity: The WHO has separate cut-off recommendations for Asian populations, where health risks begin at lower BMI thresholds
- Sex: Women naturally carry more body fat than men at the same BMI
💡 Better Together: Combine BMI with waist circumference (risk increases above 88cm for women, 102cm for men) for a more complete picture than BMI alone.
5. How to Use a Free BMI Calculator
- Select your preferred unit system: metric (cm/kg) or imperial (ft/lbs)
- Enter your current height
- Enter your current weight
- Click Calculate — results appear instantly
- Review your BMI score, category, and the healthy weight range for your height
6. Beyond BMI — Other Health Metrics
- Waist-to-Height Ratio — Your waist should be less than half your height. Simpler and arguably more predictive than BMI for cardiovascular risk.
- Body Fat Percentage — The most accurate body composition metric. Measured by DEXA, hydrostatic weighing, or BIA scales.
- Resting Heart Rate — A strong predictor of cardiovascular fitness. Normal range is 60–100 bpm; athletes often measure 40–60.