The BMI Formula

Body Mass Index (BMI) = weight (kg) ÷ height (m)²

Or in imperial: BMI = (weight in pounds ÷ height in inches²) × 703

Step-by-Step Calculation

  1. 1

    Measure your height in metres

    Convert if needed: 175cm = 1.75m. Multiply your height in metres by itself: 1.75 × 1.75 = 3.0625.

  2. 2

    Divide your weight (kg) by that number

    Example: 70kg ÷ 3.0625 = 22.9. That is your BMI.

  3. 3

    Interpret your result

    Under 18.5 = Underweight. 18.5–24.9 = Healthy weight range. 25.0–29.9 = Overweight. 30.0–34.9 = Obese Class I. 35.0–39.9 = Obese Class II. 40+ = Obese Class III (morbidly obese).

Quick BMI Examples

  • 60kg at 1.60m → 60 ÷ 2.56 = BMI 23.4 (Healthy)
  • 80kg at 1.75m → 80 ÷ 3.06 = BMI 26.1 (Overweight)
  • 90kg at 1.80m → 90 ÷ 3.24 = BMI 27.8 (Overweight)
  • 100kg at 1.70m → 100 ÷ 2.89 = BMI 34.6 (Obese)
BMI has real limitationsBMI does not distinguish between fat and muscle — a heavily muscled athlete may have a BMI in the "overweight" range while being in excellent health. It also does not account for where fat is distributed — abdominal fat carries higher health risks than fat elsewhere. BMI is a screening tool, not a diagnosis. A doctor considers BMI alongside many other factors.
Waist circumference is also importantHealth risk increases significantly with waist circumference above 94cm for men and 80cm for women (Australian and international guidelines). Waist-to-height ratio (waist circumference divided by height, both in the same unit) below 0.5 is generally considered healthy.

Frequently Asked Questions

BMI uses the same formula regardless of sex, but men and women have different body compositions — women naturally carry more body fat than men at the same BMI. Some organisations use sex-specific BMI cutoffs. The WHO and Australian guidelines use the same ranges for both sexes, noting BMI is a population-level screening tool rather than an individual diagnostic measure.
Not directly — for children and teenagers, BMI is interpreted using age- and sex-specific percentile charts rather than the adult categories. A BMI number that is "healthy" for an adult may mean different things for a 10-year-old versus a 16-year-old. Use a paediatric BMI calculator specifically designed for children.