What Is Body Mass Index (BMI)?
Body Mass Index (BMI) is one of the most widely used screening tools in global healthcare for estimating body composition based on height and weight measurements. Developed by the World Health Organization (WHO), BMI provides a quick numerical value that categorizes adults into recognized health weight ranges: underweight, normal weight, overweight, and obese. While BMI is not a diagnostic tool and does not account for muscle mass or distribution of body fat, it serves as a useful starting point for health conversations with medical professionals. Our free online BMI Calculator applies the standard WHO formula instantly to give you an accurate, no-cost reading of your current weight status from any device.
The formula is older than the term. Belgian mathematician Adolphe Quetelet derived it in the 1830s to describe populations, not to assess individuals, and the World Health Organization later adopted it as a screening threshold — which is why the same cut-offs of 18.5, 25 and 30 appear on charts everywhere. That origin also explains the well-known blind spot: a ratio built to characterise averages will misclassify a muscular athlete as overweight, because it counts kilograms without asking what they are made of.
How to Calculate
- Enter your current weight in kilograms in the first input field. If you know your weight in pounds, divide by 2.205 to convert it to kilograms before entering.
- Enter your height in centimeters in the second field. To convert from feet and inches, multiply your height in feet by 30.48 to get the equivalent in centimeters.
- Click the Calculate BMI button to see your result. Your BMI score and the corresponding health classification — underweight, normal, overweight, or obese — are displayed instantly.
Who Uses BMI
- Individuals tracking their health or fitness progress and wanting a quick reference point over time.
- Clinicians and nurses using BMI as a fast screening step before more detailed assessment.
- Personal trainers and nutritionists setting baselines and goals with clients.
- Anyone curious where their height-and-weight combination falls on the standard scale.
The Formula Behind BMI
BMI is your weight in kilograms divided by the square of your height in metres: BMI = kg ÷ m². With imperial units the equivalent is 703 × pounds ÷ inches². The result is read against standard categories: below 18.5 is underweight, 18.5 to 24.9 is the normal range, 25 to 29.9 is overweight, and 30 or above is classified as obese.
Because the formula only uses height and weight, BMI cannot tell muscle from fat. A muscular athlete may score as "overweight" while carrying little fat, and it is calibrated for adults rather than children or the elderly. Treat it as a useful screening number, not a diagnosis, and pair it with professional advice for real decisions.
Key Benefits
- Provides an immediate, no-cost assessment of your weight status using the globally recognized WHO formula — the same standard used by healthcare professionals worldwide.
- A useful first step for people who want to understand how their weight relates to their overall health and to identify whether lifestyle changes may be beneficial.
- Completely private and runs entirely in your browser — no personal data such as your height or weight is ever stored, transmitted, or shared with any server.
Your Measurements Stay Private
Height and weight are calculated in your browser using the BMI formula as JavaScript code. Your personal health data is never sent to our servers — the calculation happens entirely on your device.