BMI & Ideal Weight Calculator
Calculate Body Mass Index in metric or imperial units, see where it falls on the World Health Organization category scale, and compare reference weight ranges from standard clinical formulas.
What BMI actually measures
Body mass index is weight in kilograms divided by the square of height in metres. That is the entire calculation — it uses no other information: not age, not sex, not body composition, not medical history. The formula dates to the 1830s, devised by Belgian statistician Adolphe Quetelet, who was studying characteristics of populations. It was never intended as a measure of an individual's health, and Quetelet said as much.
Where the categories come from
The familiar bands published by the World Health Organization for adults are: below 18.5 underweight, 18.5 to 24.9 healthy weight, 25.0 to 29.9 overweight, and 30.0 and above obese. These thresholds were derived from statistical associations across large populations, predominantly of European descent. They describe where health risks tend to rise on average across thousands of people. They do not describe what is happening in any particular body, and the boundaries are conventions rather than sharp biological transitions — nothing meaningful changes between 24.9 and 25.0.
What BMI cannot see
It cannot distinguish muscle from fat. Muscle is denser, so a very muscular person can register as overweight while carrying little body fat — the most frequently cited limitation. It cannot see where fat is stored, although research consistently finds abdominal fat carries different implications than fat on the hips and thighs. And it says nothing about blood pressure, blood sugar, cholesterol or fitness, all of which are measurable and relevant. It is especially unreliable for children and adolescents, who need age- and sex-specific growth charts; for older adults, whose muscle mass and height change; during pregnancy; and for athletes. Several health bodies also recommend lower thresholds for people of South Asian and East Asian descent, where elevated risk appears at lower BMI values.
The ideal weight formulas
The Devine, Robinson and Hamwi formulas calculate a weight from height and sex. It is worth knowing their origin: the Devine formula was published in 1974 to help calculate medication dosages, not to advise people on body weight. They are simple linear formulas, they disagree with each other by design, and none reflects your build or body composition. Read them as rough reference points rather than targets.
Quick tips
- BMI is a population screening measure, not a diagnosis or a measure of health.
- Switching between metric and imperial clears the fields so values are never misread.
- Waist circumference is often more informative than BMI alone.
- For anything concerning your own health, a doctor who knows your history is a far better guide.
How is BMI calculated?
BMI is your weight in kilograms divided by the square of your height in metres. Imperial measurements are converted to metric first, so both unit systems produce the same result.
What do the BMI categories mean?
The World Health Organization classifies adult BMI below 18.5 as underweight, 18.5 to 24.9 as healthy weight, 25.0 to 29.9 as overweight, and 30.0 or above as obese. These are screening bands for populations, not individual diagnoses.
Is BMI accurate for everyone?
No. BMI uses only height and weight, so it cannot separate muscle from fat. It is frequently misleading for athletes and very muscular people, older adults, pregnant people, and children and teenagers, who need age- and sex-specific growth charts instead. Some health bodies also recommend lower thresholds for people of South Asian and East Asian descent.
What are the ideal weight formulas?
The Devine, Robinson, and Hamwi formulas are decades-old clinical estimates originally developed to help calculate medication dosages from height. They deliberately disagree with each other and are best read as rough reference points rather than personal targets.
Should I make health decisions based on this?
No calculator can assess your health. BMI is one crude data point among many, and a doctor who can consider your full medical history, body composition, and circumstances is a far better guide.