Calculate your Body Mass Index (BMI) instantly using our free online calculator. Supports both metric (kg/cm) and imperial (lbs/inches) measurements.
Calculator
Metric Formula: BMI = weight(kg) / height(m)² | Imperial Formula: BMI = (weight(lbs) / height(in)²) × 703
BMI Categories (WHO Standard)
| BMI Range | Category | Health Risk |
|---|---|---|
| Below 18.5 | Underweight | Possible nutritional deficiency |
| 18.5 – 24.9 | Normal weight | Low risk |
| 25.0 – 29.9 | Overweight | Moderate risk |
| 30.0 – 34.9 | Obese (Class I) | High risk |
| 35.0 – 39.9 | Obese (Class II) | Very high risk |
| 40.0 and above | Obese (Class III) | Extremely high risk |
How It Works: The Math Behind the Calculation
The Body Mass Index (BMI) is a simple mathematical formula used to classify a person's body weight relative to their height. It is widely used by doctors and health professionals as a quick, non-invasive screening tool to identify potential weight categories that may lead to health problems.
To calculate BMI using metric units, divide a person's weight in kilograms by the square of their height in metres. For example, a person who weighs 70 kg and is 1.75 m tall has a BMI of 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9, which falls in the "Normal weight" category. Using imperial units, multiply the weight in pounds by 703, then divide by the square of the height in inches.
Understanding the BMI Categories
Once the BMI number is calculated, it is grouped into standard categories established by the World Health Organization (WHO). For adults 20 years and older, a BMI below 18.5 is classified as Underweight, 18.5 to 24.9 as Normal weight, 25.0 to 29.9 as Overweight, and 30.0 or above as Obese. It is important to note that BMI is a screening tool, not a diagnostic tool. It does not measure body fat directly and cannot account for differences in muscle mass, bone density, age, or sex.
Historical Context: The Origins of BMI
The BMI formula was not originally created by a doctor, but by a Belgian mathematician and statistician named Lambert Adolphe Jacques Quetelet in the 1830s. He developed the formula — originally called the Quetelet Index — as part of his statistical work defining the "average man" based on population data. It was never intended as a clinical tool for individual health assessment.
It was not until 1972 that physiological researcher Ancel Keys coined the term "Body Mass Index" and validated it as a useful proxy for measuring body fat across large populations. The WHO adopted BMI as an international standard for measuring obesity in the 1990s, which is when it became widely used in clinical settings around the world.
Frequently Asked Questions
Is BMI an accurate measure of health?
BMI is a useful general indicator for populations, but it has limitations for individuals. Because it only uses height and weight, it cannot distinguish between muscle mass and fat mass. Highly muscular athletes may have a BMI that categorises them as "overweight" even if their body fat percentage is very low. Always consult a healthcare professional for a complete health assessment.
Does BMI apply to children and teenagers?
The mathematical formula for calculating BMI is the same for children and teens, but the resulting number is interpreted differently. Because children are still growing, their BMI is compared against age- and sex-specific percentile charts rather than the fixed adult categories. A paediatrician should always interpret BMI results for children.
What should I do if my BMI is outside the normal range?
If your BMI falls outside the "Normal weight" range, the best step is to consult with a healthcare provider. They will consider your BMI alongside other factors such as your diet, physical activity level, waist circumference, family history, and blood pressure to get a complete picture of your health.