BMI Calculator

Calculate body mass index and see where it falls on the WHO classification.

Please note: For general information only. This is not medical advice — speak to a doctor or registered dietitian before changing your diet, training or medication.

What the BMI Calculator does

Body mass index divides weight by height squared, giving a single number that scales body mass to frame size. It is a screening tool for populations, not a diagnosis for individuals — useful as a starting point, poor as a final verdict.

Formula

  • BMI = weight (kg) ÷ height (m)²
  • Imperial: BMI = 703 × weight (lb) ÷ height (in)²

Inputs explained

InputUnitRequiredNotes
Unitsone of 2 optionsYes—
Weightkg or lbYesAccepts 1 or more.
Heightcm or ft + inYesAccepts 1 or more.

How to use it

  1. Choose Units.
  2. Enter Weight and Height.
  3. Select Calculate.

Worked example

Someone weighing 75 kg at 175 cm tall.

Weight
75 kg
Height
175 cm

75 ÷ 1.75² = 24.5 — within the healthy range.

Reading the result

  • BMI is a screening ratio, not a body-composition measurement. It places you in a population band; it does not measure fat.
  • The standard adult bands are under 18.5 underweight, 18.5-24.9 healthy, 25-29.9 overweight, and 30 or above obese. They apply to adults of both sexes aged 20 and over.
  • Health risk rises continuously, not at the band edges. A BMI of 24.9 and one of 25.1 describe near-identical bodies.

Assumptions and limitations

  • The formula assumes weight scales with the square of height, which fits average adult builds but overstates BMI for tall people and understates it for short people.
  • It cannot distinguish muscle from fat, so athletes and heavily muscled people often read as overweight while carrying little fat.
  • The cut-offs were derived largely from white European populations. Several health bodies use a lower overweight threshold, around 23, for people of South Asian, Chinese and other Asian descent.
  • It is not valid for children, teenagers, pregnant people, or anyone with significant fluid retention.

Common mistakes

  • Mixing unit systems — entering height in centimetres while the calculator expects metres shifts the result by orders of magnitude.
  • Treating a single reading as a diagnosis. BMI is one input among blood pressure, waist measurement, blood markers and activity level.
  • Using adult bands for a child. Children are assessed on age-and-sex percentile charts instead.

Frequently asked questions

Is BMI accurate for athletes?

Often not. Muscle is denser than fat, so a muscular athlete can register as overweight or obese while carrying very little body fat. Use body fat percentage instead.

What about children?

Children need age- and sex-specific BMI percentile charts, not the adult thresholds shown here.

Is waist measurement better?

For cardiovascular risk, waist-to-height ratio is generally a stronger predictor because it reflects where fat is stored.

Method and sources

Method. Quetelet index — weight in kilograms divided by height in metres squared — classified against the WHO adult cut-off points.

Assumptions

  • The reader is a non-pregnant adult aged 20 or over; the adult cut-offs do not apply to children, who are assessed on age-and-sex percentiles.
  • Weight scales with the square of height, which is an approximation that drifts at the extremes of stature.

Limitations

  • BMI measures neither body fat nor its distribution, so it misclassifies muscular builds as overweight and can miss excess fat in people of normal weight.
  • The 18.5 / 25 / 30 boundaries were derived largely from European-ancestry populations. WHO notes that health risk rises at lower BMI in many Asian populations, and several countries use lower national cut-offs.

Sources

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