BMI Calculator

How BMI is calculated

Body Mass Index is weight in kilograms divided by height in metres squared. Enter your figures in metric or imperial and you get the value plus the World Health Organization category it falls into. The imperial formula multiplies pounds divided by inches squared by 703, which is simply the unit conversion folded into the arithmetic.

The categories are: below 18.5 underweight, 18.5 to 24.9 normal, 25.0 to 29.9 overweight, and 30.0 and above obese. Those boundaries are the WHO thresholds used in most of the world.

What BMI was designed for — and what it was not

BMI was devised in the 1830s by Adolphe Quetelet, a mathematician studying the statistical characteristics of populations. He was explicit that it described groups, not individuals. It became a clinical shorthand much later, largely because it is cheap: it needs two measurements and no equipment beyond scales and a tape measure.

That origin explains its central limitation. BMI takes no account of what your weight is made of. Muscle is considerably denser than fat, so a muscular athlete can land in the overweight or obese range while carrying very little body fat. The reverse also occurs — someone within the normal range can carry a high proportion of fat and little muscle, a pattern sometimes called normal-weight obesity, which carries real metabolic risk that BMI misses entirely.

Where the number is least reliable

BMI is at its weakest for exactly the people most likely to look it up:

• Athletes and anyone with substantial muscle mass, who are routinely misclassified as overweight.
• Older adults, who lose muscle with age, so a normal BMI can conceal low muscle and higher fat.
• Children and teenagers, for whom adult categories do not apply at all — paediatric assessment uses age- and sex-specific percentile charts.
• Pregnant people, where the standard interpretation is not meaningful.
• People of different ethnic backgrounds. Health risk rises at lower BMI values for people of South Asian, Chinese and some other Asian descent, so several health bodies apply a lower overweight threshold, often around 23, for those populations.
• Anyone very tall or very short, since the height-squared term does not scale perfectly across the range.

Better signals to look at alongside it

Waist circumference is the most useful single addition, because it captures visceral fat around the organs — the fat most strongly associated with cardiovascular and metabolic risk. As a rough guide, risk rises above about 94 cm for men and 80 cm for women, with lower thresholds for some Asian populations.

Waist-to-height ratio is another good measure and pleasingly simple: keeping your waist under half your height works across a wide range of body sizes without needing separate thresholds by sex.

Beyond measurements, the things that actually predict health outcomes are blood pressure, blood lipids, blood glucose, cardiorespiratory fitness, sleep and diet quality. Someone with a slightly high BMI who is active and has good markers is generally in better health than someone with a normal BMI and poor ones.

How to use your result

Treat it as one rough data point, useful mainly for spotting a trend over time rather than judging a single day. It cannot diagnose anything, and a number slightly outside the normal band is not in itself a cause for alarm.

This is a general-information calculator, not medical advice. If your result concerns you, or if you are planning a significant change to diet or exercise, speak to a doctor or a registered dietitian — they can interpret the number in the context of your actual health history, which no calculator can do.