What is a BMI Calculator?
A BMI calculator works out your body mass index — a single number that relates your weight to your height. BMI was developed in the 19th century by the Belgian statistician Adolphe Quetelet and is now used worldwide as a quick, inexpensive screening measure. It does not measure body fat directly; instead it gives a standardised figure that places you in a weight category, which clinicians and individuals use as a starting point for a conversation about health.
The maths is simple. In metric units, BMI is your weight in kilograms divided by the square of your height in metres. So a person who is 1.75 m tall and weighs 70 kg has a BMI of 70 ÷ (1.75 × 1.75) ≈ 22.9. For imperial units the calculator converts the values automatically using the standard 703 multiplier, so you can enter feet, inches, and pounds without doing any conversion yourself.
How to Use the BMI Calculator
- Choose your unit system — metric (kg, cm) or imperial (lb, ft/in).
- Enter your height in the selected units.
- Enter your current weight.
- Read the BMI value, calculated and displayed instantly.
- Check the category label to see which standard range your result falls into.
Understanding the BMI Categories
The World Health Organization uses four standard adult categories: a BMI under 18.5 is underweight, 18.5 to 24.9 is normal weight, 25.0 to 29.9 is overweight, and 30.0 or above is obesity. These ranges apply to adults aged roughly 20 and over. They are population-level guidelines, not a verdict on any individual's health.
BMI has real limitations and it is important to read the number honestly. Because it uses only height and weight, it cannot tell muscle from fat — a trained athlete may register as “overweight” despite low body fat, while an inactive person can sit in the “normal” range with little muscle. BMI also does not account for fat distribution, age, sex, ethnicity, or bone density, and it is not validated for children, pregnant women, or the elderly in the same way. Treat BMI as one screening signal among many. It is not a medical diagnosis, and a result outside the “normal” band is a reason to talk to a healthcare professional, not a reason to panic.
Frequently Asked Questions
Is BMI accurate for athletes?
Not reliably. Muscle is denser than fat, so muscular people often score in the overweight range despite being lean. For them, measures like body-fat percentage or waist circumference are more informative than BMI.
Can I use this calculator for my child?
No. Child and teen BMI is interpreted against age- and sex-specific percentile charts, not the fixed adult categories used here. Ask a paediatrician for a proper assessment.
Does a “normal” BMI mean I am healthy?
Not on its own. BMI ignores muscle mass, fat distribution, blood pressure, and many other factors. It is a single screening number — a useful starting point, not a complete picture of health.
Is my height and weight data sent anywhere?
No. The calculation runs entirely in your browser. The values you enter are not transmitted to any server or stored after you leave the page.