BMI Calculator (metric & imperial, with category)
Enter your height and weight to get your BMI and the WHO category, instantly. Works in your browser — no data is sent to a server.
How it works
What is BMI?
Body Mass Index (BMI) is a single number that summarizes whether your weight falls in a healthy range for your height. It was developed by Adolphe Quetelet in the 19th century and adopted by the World Health Organization as a population-level screening tool.
BMI is the ratio of your weight in kilograms to the square of your height in meters: BMI = kg / m². The imperial form is BMI = 703 × lb / in². Both formulas give the same number; this calculator uses the appropriate one based on the unit you choose.
How to read the result
The WHO classifies BMI into eight bands ranging from severe underweight to obesity class III. Most adults aged 20 and over fall into the 'normal weight' band when BMI is between 18.5 and 25.
BMI is a screening tool, not a diagnosis. It does not distinguish between fat and lean mass, so very muscular people may register a high BMI without excess body fat. It also doesn't account for age, sex, or fat distribution.
When BMI is misleading
Athletes and bodybuilders often score in the overweight or obese range despite low body fat. Older adults may lose muscle mass, making their BMI look 'normal' even when fat percentage is elevated. Children and adolescents should use age- and sex-specific BMI percentile charts rather than the adult bands.
For a more complete picture, combine BMI with waist circumference, waist-to-hip ratio, or a body fat percentage measurement. Talk to a healthcare professional before using BMI to make decisions about your health.
Frequently asked questions
›Is this BMI calculator accurate?
It uses the standard WHO formula and bands, so the number itself is exact. The interpretation is a population-level estimate and may not apply to athletes, pregnant people, or children.
›Does my data leave my browser?
No. All calculations happen on your device. Nothing is uploaded or stored on our servers.
›Why do I get the same BMI in both unit systems?
The formulas BMI = kg/m² and BMI = 703 × lb/in² are mathematically equivalent — the constant 703 converts pounds and inches into the same units used by the metric form.
›What's a healthy BMI?
The WHO defines 18.5–25 as the normal range for adults. Optimal BMI varies by ethnicity, age, and body composition; some Asian populations use a lower upper bound of 23.
›Can BMI be wrong for me?
Yes. Muscular athletes, very tall or very short individuals, older adults with reduced muscle mass, and pregnant people may all need different metrics.
›Should children use this calculator?
Children and teens should use BMI-for-age percentiles from the CDC or WHO growth charts rather than adult thresholds.
›How often should I check my BMI?
For most healthy adults, once or twice a year is enough. Daily fluctuations are dominated by hydration and food intake, not body composition change.
›Where can I learn more?
The WHO BMI page and the CDC's Adult BMI page both provide authoritative references and BMI-for-age tools for children.
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