BMI Calculator

What is BMI?

Body Mass Index (BMI) is a simple number calculated from your height and weight. It was created by Belgian mathematician Adolphe Quetelet in the 1800s and is now used worldwide by doctors and researchers as a quick screening tool to assess whether someone may be underweight, a healthy weight, overweight, or obese.

Think of it as a rough "health snapshot." It isn't perfect - a highly muscular athlete may score as "overweight" even though they're very fit, but it remains one of the most widely used and well-researched health indicators available.

How is BMI calculated?

BMI uses a simple formula based on your height and weight, regardless of which unit system you enter. In metric units, BMI equals your weight in kilograms divided by your height in meters, squared (BMI = kg / m²). In imperial units, BMI equals your weight in pounds, divided by your height in inches squared, multiplied by 703 (BMI = (lbs / in²) x 703). This calculator performs that conversion and math for you instantly, so you don't need to remember the formula or do any unit conversions yourself.

Why do doctors use BMI?

BMI became popular because it is fast, free, and requires no special equipment beyond a scale and a tape measure. For large groups of people, it correlates reasonably well with more direct measures of body fat, which makes it useful for spotting population-level health trends and flagging individuals who may benefit from a closer look by a healthcare provider. Many insurance assessments, fitness programs, and public health studies still rely on BMI as a starting point for this reason.

What are the limitations of BMI?

BMI is a screening measurement, not a diagnosis. It cannot tell the difference between weight from muscle, bone, water, or fat, so it can be misleading for certain groups: athletes and very muscular people may show a higher BMI despite having low body fat, while older adults may have a "normal" BMI while carrying a higher percentage of body fat due to muscle loss. BMI also does not account for where fat is stored on the body, which research suggests can matter for health risk. Pregnant women, growing children, and teenagers should use age- and stage-specific growth charts rather than the standard adult BMI categories. If your BMI result seems unusual for your body type, a healthcare professional can offer more context using additional measurements such as waist circumference, body fat percentage, or blood work.

What can I do if my BMI is outside the "Normal" range?

A BMI outside the 18.5 to 24.9 range is not an emergency and does not mean something is wrong, but it can be a useful prompt to talk with a doctor or registered dietitian about your overall health, habits, and goals. Small, sustainable changes such as gradual adjustments to diet, regular physical activity, sufficient sleep, and stress management tend to have a bigger long-term impact than rapid weight changes. If you are considering a significant change to your diet or exercise routine, especially if you have an existing medical condition, it's best to check with a healthcare provider first.