Body Mass Index is one of the most widely cited health numbers — but what it actually measures, and what it doesn't, is often misunderstood. Use our free BMI Calculator to check your number in metric or imperial units, then read on to understand what it means.
What BMI Measures
Body Mass Index is a simple ratio of weight to height used as a quick, population-level screening tool — not a diagnostic measurement of body fat or overall health. It gives a single number that places you in a weight category relative to your height, which is useful as a starting reference but tells only part of the story for any individual.
The Formula — Metric and Imperial
Metric: weight (kg) ÷ height (m)²
Imperial: weight (lb) × 703 ÷ height (in)²
Our BMI Calculator supports both unit systems — enter whichever you use and get an instant result with category and healthy weight range.
WHO BMI Categories
- Underweight: below 18.5
- Normal weight: 18.5–24.9
- Overweight: 25–29.9
- Obese: 30 and above
These World Health Organization cutoffs are the most widely used global standard. Some regions — for example, parts of Asia — apply slightly adjusted thresholds for overweight and obese categories based on population-specific health-risk research. The categories are a screening framework, not a personal diagnosis.
Important Limitations
BMI doesn't distinguish muscle mass from fat mass — a muscular athlete can register as "overweight" despite low body fat. It doesn't account for where fat is distributed in the body, which affects health risk independently of total BMI. The same BMI can mean different things at different ages or between sexes.
BMI is a useful quick screening number for population studies and a starting conversation point, not a personal diagnosis. For an individual assessment of health or body composition, other measurements — waist circumference, body fat percentage, or a doctor's assessment — give a fuller picture.
Frequently Asked Questions
Is BMI an accurate measure of health?
BMI is a useful quick screening tool at a population level, but it has real limitations for individuals — it doesn't account for muscle mass, fat distribution, age, or sex. It's a starting reference point, not a complete health assessment.
Why does BMI use height squared instead of just height?
Weight tends to scale roughly with the square of height across a population, so dividing by height squared (rather than height alone) produces a more consistent comparison across people of different heights.
Do BMI categories differ by country?
The WHO categories are the most widely used global standard, but some health organizations in certain regions use adjusted cutoffs for overweight/obese categories based on population-specific health-risk research.
Can two people have the same BMI but different health status?
Yes — BMI doesn't distinguish between muscle and fat mass, so a muscular, athletic person and a person with higher body fat can show the identical BMI number despite very different body compositions.