The complete guide
The free online BMI Calculator, explained.
Our BMI Calculator is a free online tool that turns two simple numbers — your height and your weight — into a clear picture of where you stand. Whether you prefer kilograms and centimetres or pounds and feet, you can calculate BMI in seconds, with no sign-up and your inputs never leaving your browser.
What is BMI (Body Mass Index)?
BMI, or Body Mass Index, is a standard measure of body weight relative to height. First developed by Adolphe Quetelet in the 1830s and later adopted by the World Health Organization, the body mass index is used worldwide as a quick screening tool to estimate whether a person is underweight, at a healthy weight, overweight, or obese.
How is BMI calculated? The BMI formula
The BMI formula is straightforward. In metric units, you calculate BMI by dividing weight in kilograms by height in metres squared:
BMI = weight (kg) ÷ height² (m²)
For imperial units, divide your weight in pounds by your height in inches squared, then multiply by 703. Our online BMI calculator handles both automatically, so you never have to do the maths by hand.
How to calculate BMI manually
To calculate BMI manually, square your height in metres, then divide your weight by that number. For example, someone who is 1.75 m tall and weighs 70 kg has a BMI of 70 ÷ (1.75 × 1.75) = 22.9 — comfortably within the healthy range. It's a quick sum, but the BMI calc above removes the guesswork and rounds it precisely for you.
BMI chart and categories
The BMI scale groups results into clear categories. The full BMI chart reads as follows: below 18.5 is underweight, 18.5–24.9 is a healthy weight, 25–29.9 is overweight, and 30 and above is classed as obese. These ranges come straight from WHO standards and apply to most adults aged 20 and over. You can explore every band in detail on our full BMI chart.
BMI calculator for men and women
Because the underlying maths is identical, a BMI calculator for men and a BMI calculator for women use the same thresholds — your score is not adjusted for sex. That said, males and females carry fat and muscle differently, so reading a BMI calculator male or BMI calculator female result alongside measures like waist-to-height ratio gives both men and women a fuller picture than BMI alone.
BMI calculator by age
Age matters too. For adults, the BMI categories stay constant, but a BMI calculator by age is essential for children and teenagers, whose results are read against age- and sex-specific percentile charts rather than fixed numbers. Entering your age and your weight in kg with age in mind helps you interpret the result in its proper context — which is why our calculator asks for both.
Benefits and limitations of BMI
The benefits of BMI are clear: it is fast, free, needs no special equipment, and works as a reliable population-level screen. Its limitations matter just as much. BMI does not distinguish muscle from fat, so athletes may read as overweight, and it can't account for bone density, ethnicity, or body shape. Treat your number as a useful starting point for a conversation with your doctor — not as a diagnosis.
Why use this BMI Calculator?
BMI Pulse is built to be the calmest, clearest body mass index calculator on the web — a real BMI calculator you can trust. You get instant results as you type, a visual BMI scale, your healthy weight range, BMI Prime, and plain-language context — all clinically referenced, with your numbers calculated privately in your browser. No accounts, no sign-up: just an accurate, free online BMI calculator whenever you need it.