BMI is weight divided by height squared, a formula devised in the 1830s to describe populations rather than people. The BMI Calculator computes it in metric or imperial, places you on the standard 4-band scale, and is upfront about the fact that a single number cannot tell you whether an individual body is healthy.
A coloured gauge runs beneath the figure, from 15 at the left to 40 at the right, with a marker at your position. That span is deliberate: it covers the range the categories actually distinguish, so a small change in weight produces a visible movement rather than a marker frozen in the middle of an unbounded scale.
Each band carries a sentence of context rather than a verdict, and a permanent note at the foot restates the limitation. Nothing you type is transmitted, which matters more here than on most pages, since weight and height are health data.
How the Math Differs Between Units
- Metric is direct: kilograms divided by metres squared. Enter 70 kg and 175 cm and the height is converted to 1.75 m first, giving
22.9. - Imperial needs a conversion constant. Pounds divided by inches squared, multiplied by 703, is what makes the two scales agree. 154 lbs at 5 feet 9 gives
22.7, the same band. - Feet and inches are summed before squaring, so 5 feet 9 becomes 69 inches. Leaving Inches blank is treated as 0 rather than as an error.
- The result is shown to 1 decimal place, which is as much precision as the input justifies. A kilogram either way moves the figure by about 0.3.
- Zero and negative values are rejected outright and no result appears. An earlier version let a negative weight through and confidently labelled the result Underweight.
What Each of the 4 Bands Means
- Underweight is below 18.5, Normal is 18.5 to 24.9, Overweight is 25 to 29.9, and Obese is 30 and above.
- These cutoffs come from studies of European-descended populations. The World Health Organization notes that health risk rises at a lower BMI in many Asian populations, with 23 sometimes used in place of 25.
- The gauge clamps at both ends, so any BMI under 15 or over 40 pins the marker rather than running off the bar. The number itself is never clamped.
- The scale is not age-adjusted or sex-adjusted, and it is not valid for children at all, where growth-chart percentiles are used instead.
Not a Substitute for a Medical Assessment
- BMI cannot distinguish muscle from fat. A trained athlete regularly lands in the Overweight band on body composition that is measurably healthy.
- It says nothing about where fat is carried, and abdominal fat is the distribution most strongly linked to metabolic risk. A waist measurement adds information BMI cannot.
- Because height is squared, tall people are pushed upward and short people downward relative to their actual body composition. That is a known artefact of the formula, not of your body.
- It is not valid during pregnancy, nor for anyone with significant fluid retention or reduced muscle mass.
- The tool states the boundary on screen:
BMI is a screening tool, not a medical diagnosis. Consult a healthcare professional for guidance.