How to use it
- Choose metric (cm, kg) or imperial (feet/inches, lb). Switching converts the values you already typed.
- Enter your height and weight. The BMI, its WHO category and the colour bar update as you type.
- Read the healthy weight range for your height and how many kilograms or pounds you are above or below it.
- Optionally add your age: under 20 the tool warns that adult categories do not apply; at 65+ it adds a note on the evidence for older adults.
- Copy the summary or the link – the link restores your inputs so you can compare later.
Example
Height 180 cm, weight 80 kg: 1.80² = 3.24, and 80 ÷ 3.24 = 24.7, which is normal weight. The healthy range for 180 cm is 18.5 × 3.24 = 59.9 kg to 24.9 × 3.24 = 80.7 kg, so this person is 0.7 kg below the top of the band. BMI prime is 24.7 ÷ 25 = 0.99. In imperial units the same person is 5 ft 10.9 in and 176.4 lb; the calculator converts to metric internally, so the result is identical.
Formula, categories and limits
BMI = weight (kg) ÷ height (m)²
Imperial shortcut: BMI ≈ 703 × weight (lb) ÷ height (in)² (703 is 0.45359237 ÷ 0.0254², rounded)
BMI prime = BMI ÷ 25
Healthy weight range = 18.5 × height² … 24.9 × height²| BMI (kg/m²) | WHO category |
|---|---|
| below 18.5 | Underweight (WHO subdivides: severe below 16, moderate 16–16.9, mild 17–18.4) |
| 18.5–24.9 | Normal weight |
| 25–29.9 | Overweight (pre-obesity) |
| 30–34.9 | Obesity class I |
| 35–39.9 | Obesity class II |
| 40 and above | Obesity class III |
BMI is a screening measure for populations, not a diagnosis. It cannot separate fat from muscle or say where fat is stored; waist circumference and waist-to-hip ratio add that information. The WHO expert consultation of 2004 noted that many Asian populations carry higher cardiometabolic risk at a given BMI and suggested public health action points at 23 and 27.5 kg/m². For adults aged 65 and over, a 2014 meta-analysis (Winter et al.) found the lowest all-cause mortality between about 23 and 30. For ages 2–19, BMI must be compared with growth charts (CDC or WHO BMI-for-age percentiles), which this tool does not implement. Pregnancy, very high muscle mass and loss of height with age all distort the number.
Sources: WHO – A healthy lifestyle: BMI classification, CDC – Adult BMI, WHO Expert Consultation 2004, The Lancet, Winter et al. 2014, Am J Clin Nutr.
Results are estimates for healthy adults and are not medical advice. Talk to a doctor or dietitian before changing your diet or activity, especially if you are pregnant, under 20, over 65 or have a medical condition.
What the number can and cannot tell you
BMI tracks total body mass relative to height, and across large groups it correlates with body fat and with the risk of diabetes, heart disease and some cancers. For an individual it is only a starting point: two people with the same BMI of 27 can be a strength athlete with 12% body fat and an inactive office worker with 30%. If your BMI is near a boundary or you train a lot, combine it with a body fat estimate and your waist measurement.
Questions
- Is BMI calculated differently for men and women?
- No. The formula and the WHO adult cut-offs are identical for both sexes. Women naturally carry a higher body fat share at the same BMI, which is one reason BMI is a rough screen rather than a measure of fat.
- What BMI should I aim for?
- For most adults a BMI between 18.5 and 24.9 is associated with the lowest health risk. The calculator converts this band into a weight range for your height, for example 59.9–80.7 kg at 180 cm.
- How accurate is the imperial 703 factor?
- The exact conversion factor is 703.07. This calculator converts feet, inches and pounds to metres and kilograms first, so there is no rounding error from the shortcut.
- Can I use this for my child?
- Only the BMI value itself. For ages 2–19 the result must be compared with age- and sex-specific percentile charts, which this tool does not include; a paediatrician or the CDC child BMI calculator does that.