Your BMI
BMI Chart — WHO Weight Categories
| BMI range | Category | What it means |
|---|---|---|
| Below 18.5 | Underweight | Weight may be too low for your height; consider a check-up |
| 18.5 – 24.9 | Normal weight | Healthy range for most adults — maintain it |
| 25.0 – 29.9 | Overweight | Slightly above the healthy range; small changes help |
| 30.0 and above | Obese | Higher health risk; medical guidance is recommended |
How to Calculate Your BMI
Choose metric (kilograms and centimeters) or imperial (pounds, feet and inches) — the calculator converts everything for you.
Type your measurements. The BMI value, category and healthy weight range update live as you type — no button to press.
The color gauge shows where you sit on the WHO scale, and BMI Prime tells you how close you are to the healthy limit of 25.
What Is Body Mass Index?
Body mass index (BMI) is the most widely used screening measure for body weight relative to height. It was devised in the 1830s by the Belgian statistician Adolphe Quetelet and is calculated by dividing your weight in kilograms by the square of your height in meters (kg/m²). If you use pounds and inches, the formula becomes 703 × lb / in² — exactly what this calculator applies when you switch to imperial units.
The World Health Organization groups adult BMI into four main categories: underweight (below 18.5), normal weight (18.5–24.9), overweight (25–29.9) and obese (30 and above). Large population studies link BMI values outside the normal range to higher risks of cardiovascular disease, type 2 diabetes, joint problems and some cancers — which is why BMI remains the first number doctors look at during routine check-ups.
That said, BMI is a screening tool, not a diagnosis. It cannot distinguish muscle from fat: a muscular athlete may register as "overweight" while carrying very little body fat, and an older adult can have a "normal" BMI with low muscle mass. For a fuller picture, pair BMI with waist circumference (a marker of abdominal fat), body-fat percentage, and how you actually feel and perform. Any health decision should involve a qualified professional.
Alongside your BMI, this calculator also reports two complementary metrics — BMI Prime and the Ponderal Index — explained in the sections below. Everything runs locally in your browser: your height and weight are never transmitted, stored or shared.
What Is BMI Prime?
BMI Prime is your body mass index divided by 25 — the upper limit of the healthy WHO range — expressed as a simple ratio. A BMI Prime of exactly 1.0 means you sit right on that upper limit; a value below 1.0 means you are within or under the healthy range, while a value above 1.0 tells you, as a fraction, how far over the limit you are.
Because it is a dimensionless number, BMI Prime makes progress easy to track and to compare between people: a BMI Prime of 0.92 is 8% below the healthy ceiling, whereas 1.15 is 15% above it. The formula is BMI Prime = BMI ÷ 25, so a BMI of 27.5 gives a BMI Prime of 1.10. The calculator above shows your BMI Prime to two decimals the moment you enter your height and weight.
What Is the Ponderal Index?
The Ponderal Index (also called the Corpulence Index) divides your weight in kilograms by your height in meters cubed (kg/m³), rather than squared as in BMI. For most healthy adults it lands in roughly the 11–15 kg/m³ range, though there is no single universally agreed cut-off.
Its advantage shows at the extremes of height. Because BMI scales weight against height squared, it tends to overstate body fat for very tall people and understate it for very short people; the Ponderal Index uses the cube of height, which stays more consistent across unusual heights — one reason it is often preferred for infants and very tall athletes. The formula is Ponderal Index = weight (kg) ÷ height (m)³, and the calculator above reports it in kg/m³ alongside your BMI and BMI Prime.
BMI Calculator — FAQ
How do I calculate my BMI?
BMI = weight in kilograms divided by height in meters squared (kg/m²). With imperial units: BMI = 703 × weight in pounds ÷ (height in inches)². This tool computes it instantly for both systems.
What is a healthy BMI?
For most adults a healthy BMI is 18.5–24.9 (WHO). Below 18.5 is underweight, 25–29.9 is overweight, and 30+ is classed as obese. The healthy weight range shown above is derived from these thresholds and your height.
Is BMI accurate for everyone?
No measure is perfect. BMI doesn't separate muscle from fat, so muscular people can score "overweight" while lean. It's a screening tool — combine it with waist circumference and professional advice for real conclusions.
Does this calculator work for children?
It uses the adult WHO classification (18+). Children and teens need age- and sex-specific BMI percentiles, so use a dedicated pediatric percentile calculator for ages 2–17.
Is my data uploaded anywhere?
No. All calculations run locally in your browser with JavaScript. Your height and weight never leave your device.
What is BMI Prime?
BMI Prime = your BMI ÷ 25 (the upper healthy limit). Under 1.0 means you're within or below the healthy range; above 1.0 shows how far over it you are — e.g. 1.10 means 10% above.
What is the Ponderal Index?
The Ponderal Index (Corpulence Index) = weight in kg ÷ height in metres cubed (kg/m³). Unlike BMI it cubes height instead of squaring it, so it stays more reliable for very tall or very short people. The calculator shows it automatically.