How BMI is calculated
Body mass index is your weight in kilograms divided by the square of your height in metres (kg/m²). This calculator accepts imperial units too and converts pounds and inches to metric before applying the same formula, so the result is consistent whichever system you use. BMI sorts into standard categories set by the World Health Organization: below 18.5 is underweight, 18.5 to 24.9 is a healthy weight, 25 to 29.9 is overweight, and 30 or above is obese. It is a quick screening number that estimates whether your weight is in a healthy range for your height.
What BMI does and doesn't tell you
BMI is a useful population-level screening tool, but it is a blunt instrument for an individual. Because it uses only height and weight, it cannot tell muscle from fat — a very muscular athlete can register as 'overweight' despite low body fat, while someone with little muscle can sit in the healthy range yet carry excess fat. It also does not account for age, sex, ethnicity or where fat is stored, all of which affect health risk. Treat BMI as a starting point, not a diagnosis, and pair it with measures like waist circumference for a fuller picture.
Your healthy weight range
The healthy weight range this calculator shows is the span of weights that would put your BMI between 18.5 and 24.9 at your current height. It is often more motivating than the index itself because it is expressed in the units you weigh yourself in. If your weight sits above the range, the calculator shows how much over the top of the healthy band you are; if below, how much under. Small, sustainable changes toward the range matter more than hitting a specific number, and the range is wide precisely because healthy bodies vary.
Using BMI sensibly
BMI is most helpful tracked over time and alongside other signals — energy, fitness, blood markers and how your clothes fit — rather than as a single verdict. It is not validated for children and teenagers in the same way (they use age- and sex-specific percentiles), nor for pregnancy. BMI and calorie figures are general estimates, not medical advice. They do not account for muscle mass, body composition, pregnancy or medical conditions — consult a doctor or registered dietitian for guidance specific to you.
Frequently asked questions
How is BMI calculated?
BMI is your weight in kilograms divided by your height in metres squared. This calculator converts imperial units to metric first, then applies the same formula.
What is a healthy BMI?
A BMI between 18.5 and 24.9 is considered healthy. Below 18.5 is underweight, 25 to 29.9 is overweight, and 30 or above is obese, per WHO categories.
Is BMI accurate for athletes?
Not always. BMI cannot distinguish muscle from fat, so muscular people may be classed as overweight despite low body fat. Use it as a screening tool, not a diagnosis.
Does BMI work for children?
Not directly. Children and teens are assessed with age- and sex-specific BMI percentiles rather than the adult categories this calculator uses.