BMI Calculator

The free BMI calculator on CollabsTool computes your body mass index from height and weight in metric or imperial units, then places you on the WHO classification scale. Whether you are tracking fitness goals, checking a healthy weight range or screening for health risk, this BMI calculator gives an instant result with the context you need to interpret it.

The BMI formula

BMI = weight (kg) ÷ height (m)²

The imperial equivalent multiplies by 703:

BMI = (weight (lb) ÷ height (inches)²) × 703

Squaring the height is an empirical choice made by Adolphe Quetelet in the 1830s because it happened to fit the weight-to-height relationship he observed across a population. It was never derived from physiology, and it was not intended as a measure of individual health — Quetelet called it the "average man" statistic.

BMI categories — the WHO classification

This BMI calculator uses these standard WHO thresholds to classify your result and display the healthy weight range for your height.

Metric vs imperial BMI calculation

The BMI calculator supports both unit systems. In metric mode, enter kilograms and centimetres. In imperial mode, enter pounds and feet-and-inches. The result is identical regardless of input units because the 703 factor in the imperial formula is simply the unit conversion constant. Switching between metric and imperial BMI calculation is a single click.

Where BMI works well

At population scale, BMI correlates reasonably well with body fat and, more importantly, with health outcomes. It requires no equipment beyond a scale and a tape measure, it is cheap, it is comparable across studies, and it flags risk groups effectively. For epidemiology and for a quick clinical screen, the body mass index remains genuinely useful.

Where BMI fails — and what to use instead

It cannot distinguish muscle from fat. A strength athlete and a sedentary person of identical height and weight have the same BMI and completely different health profiles.

It ignores fat distribution. Visceral fat around the abdomen carries substantially more metabolic risk than subcutaneous fat elsewhere. Two people at BMI 27 can have very different risk, and waist circumference or waist-to-height ratio captures the difference while BMI cannot.

The height scaling is imperfect. Squaring rather than cubing systematically over-penalises tall people and under-penalises short ones. Some researchers advocate a 2.5 exponent for exactly this reason.

Cut-offs are population-specific. The international thresholds were largely derived from European-ancestry populations. South Asian, Chinese and Japanese populations show elevated cardiometabolic risk at lower BMI, and several national bodies use 23 as the overweight threshold.

It says nothing about fitness, diet, sleep, blood pressure, lipids or glucose. Those matter more, individually and collectively, than the index does.

Healthy weight range for your height

One of the most useful features of this BMI calculator is showing the healthy weight range. For any given height, the normal BMI band (18.5 to 24.9) corresponds to a specific weight range in kilograms or pounds. Knowing this range is more actionable than the single BMI number, because it tells you how much weight you would need to gain or lose to enter the healthy band.

A more useful framing for body mass index

Treat BMI as one weak signal among several. Pair it with waist circumference (aim for less than half your height), waist-to-height ratio, resting heart rate, blood pressure and bloodwork. If those are good, a BMI outside the "normal" band is unlikely to be telling you anything important. If they are not, BMI is not where the useful information is either.

This is general information, not medical advice. For any decision about your health, talk to a clinician who can look at the whole picture rather than one ratio.