See whether your weight is in a healthy range for your height. Enter your numbers and this BMI calculator (body mass index) shows your score and what it means — in metric or imperial units. Nothing leaves your browser.
About this BMI calculator
BMI, short for body mass index, is a quick way to check whether your weight is roughly healthy for your height. You enter your weight and height, and the calculator gives one number plus a category — underweight, healthy weight, overweight or obese. It is a screening guide used worldwide, not a diagnosis: it does not account for muscle, bone or where fat sits on your body. Athletes, for example, can have a high BMI while being very lean. Use it as a starting point and speak to a health professional for personal advice. Everything is worked out in your browser, so your numbers stay private.
Frequently asked questions
- What is BMI?
- A single number comparing your weight to your height — a quick guide to whether your weight is in a healthy range. It does not measure body fat directly.
- How is BMI calculated?
- Metric: weight (kg) ÷ height (m)². Imperial: 703 × weight (lb) ÷ height (in)². This tool does it for you.
- What counts as a healthy BMI?
- For most adults, 18.5–24.9 is the healthy range. Under 18.5 is underweight, 25–29.9 overweight, 30+ obese. It is a guide, not medical advice.
The Origin and Limitations of the BMI Formula
The Body Mass Index was originally developed in the 19th century by Adolphe Quetelet, a Belgian mathematician who sought a simple way to categorize population-wide weight distribution. The formula relies on the square-cube law, which dictates that as an object grows in size, its volume and weight increase faster than its height. By dividing weight by the square of height, the index attempts to normalize body proportions across different individuals. While it was never intended for clinical diagnosis of individual health, it remains the standard screening tool for large-scale public health assessments.
The calculation is mathematically consistent but relies on a rigid assumption that human body mass scales perfectly with height squared. In the metric system, this involves dividing kilograms by the square of height in meters, while the imperial version incorporates a constant multiplier to adjust for the difference in units. Because the formula treats all mass as equal, it cannot distinguish between dense muscle tissue and adipose fat. This creates a significant mathematical bias when applied to individuals with high physical fitness or specific body compositions.
The most common mistake users make is treating the resulting score as a definitive health verdict rather than a statistical screening tool. People often ignore that the index fails to consider age, biological sex, ethnicity, or the distribution of visceral fat around internal organs. For instance, an elderly individual may have a healthy BMI while suffering from significant muscle loss, or an athlete may be classified as overweight due to high bone density. Always view this number as a simple ratio rather than a comprehensive assessment of your physical well-being.