BMI Calculator
Calculate body mass index (BMI)
BMI Calculator tool
BMI Calculator
BMI Calculator: key facts
- What it does
- Calculate body mass index (BMI)
- Category
- Health & Fitness Calculators
- Cost
- Free, with no account, sign-up, or install.
- Your data
- Runs entirely in your browser — the files and text you enter are never uploaded to a server.
- Last reviewed
- . Report an incorrect result.
About the BMI Calculator
Body Mass Index is the quickest standardized way to see where your weight sits relative to your height. The BMI Calculator takes those two numbers and returns your BMI along with the category it falls into — underweight, normal, overweight, or obese — color-coded so the result is readable at a glance.
It is widely used as a starting point: by people tracking a fitness goal, by clinicians for a fast screen, and by anyone curious how their numbers compare to standard ranges. You can enter measurements in metric (kilograms and centimeters) or imperial (pounds and inches) with a single toggle, so there is no manual unit conversion.
BMI is a screening figure, not a diagnosis — it does not distinguish muscle from fat — but it remains a useful, universally understood reference point.
How to use it
- Choose Metric or Imperial units with the toggle.
- Enter your weight (kilograms for metric, pounds for imperial).
- Enter your height (centimeters for metric, inches for imperial).
- Read your BMI value, shown to one decimal place.
- Note the color-coded category beneath it, and copy the result if you want to keep a record.
The formula, and the two unit paths
BMI is weight divided by height squared, and the only complication is units. In metric mode your height in centimetres is converted to metres and the division is direct. In imperial mode pounds are divided by inches squared and the result multiplied by 703, which is the conversion factor that reconciles the two systems.
The index was devised in the nineteenth century by Adolphe Quetelet as a way of describing populations, not individuals, and it was never intended as a measure of an individual's health. It survives because it needs only two measurements that anyone can take accurately, which makes it useful for screening large groups cheaply.
- The default entry — 70 kg at 175 cm — gives a BMI of 22.86, inside the range conventionally described as healthy.
- The same person in imperial units, roughly 154 lb at 69 in, gives 22.74; the small difference is rounding in the conversion, not a different calculation.
- Adding 15 kg at the same height moves the figure to 27.76, crossing into the overweight band.
The adult bands, and what sits behind them
The four bands this calculator colour-codes against are the adult thresholds published by the National Heart, Lung, and Blood Institute at the US National Institutes of Health. They are fixed numbers rather than percentiles, which is what makes them quick to apply and also what limits them: one boundary is asked to describe every adult body.
The NHLBI is explicit that the index is a screening tool and "does not necessarily diagnose body fatness", and points to waist circumference as a companion measurement rather than treating BMI as sufficient on its own. That is the spirit to read the colour in: it tells you which conversation to have, not what is true about your body.
| Category | BMI range | What the number does and does not tell you |
|---|---|---|
| Underweight | Below 18.5 | Flags possible undernutrition; says nothing about cause |
| Healthy weight | 18.5 – 24.9 | The reference band; a trained athlete can sit above it |
| Overweight | 25.0 – 29.9 | Screening signal only; fat distribution is invisible here |
| Obesity | 30.0 and above | A reason for clinical follow-up, not a diagnosis in itself |
Why use this version
- One toggle switches the whole form between metric and imperial, applying the correct formula for each — no converting your height to meters first.
- The result is classified into the standard adult categories published by the NHLBI with color coding (under 18.5 underweight, 18.5–24.9 healthy weight, 25–29.9 overweight, 30 or above obesity) so the number has immediate context.
- It calculates the moment you have valid weight and height, with no submit step.
- All math is done locally in the browser, so your health figures are never transmitted or stored.
Where BMI misleads
Because the formula sees only mass, it cannot distinguish muscle from fat. A heavily trained athlete routinely registers as overweight or obese on BMI alone while carrying very little fat, and someone sedentary with low muscle mass can sit inside the healthy band while carrying a great deal. Fat distribution is invisible to it too, though abdominal fat carries more metabolic risk than the same weight on hips and thighs.
The standard cut-offs were derived largely from European-descended populations, and health bodies in several countries apply lower thresholds for people of South and East Asian descent, where cardiometabolic risk appears at lower BMI values. The index is also unsuitable for children and adolescents, who need age- and sex-specific percentile charts, and it is not meaningful during pregnancy.
Read it as one crude screening number among several. A waist measurement, blood pressure, and a blood panel tell a doctor considerably more, and a BMI outside the usual range is a reason for a conversation rather than a conclusion in itself.
Frequently Asked Questions
What is a healthy BMI range?
A BMI between 18.5 and 24.9 is classified as normal weight. Below 18.5 is underweight, 25 to 29.9 is overweight, and 30 or above is obese. These are general screening ranges for adults and do not account for individual factors like muscle mass.
How is BMI calculated?
In metric units, BMI is your weight in kilograms divided by your height in meters squared. In imperial units, it is your weight in pounds divided by your height in inches squared, multiplied by 703. This calculator applies the right formula automatically based on the unit toggle.
Is BMI accurate for athletes or very muscular people?
Not always. BMI does not distinguish between muscle and fat, so muscular individuals can register as overweight despite low body fat. For a fuller picture, pair BMI with a body fat percentage estimate.
Does this work for children?
The categories used here are for adults. BMI for children and teens is interpreted against age- and sex-specific percentiles rather than fixed ranges, so consult a pediatric chart or clinician for under-18s.
Sources
- NHLBI, National Institutes of Health — Overweight and Obesity: Symptoms and Diagnosis — the adult BMI bands used here, and the statement that BMI screens rather than diagnoses
