Explanation
Adult body mass index
BMI is a weight-to-height screening measure for adults aged twenty and older. It is not a direct body-fat measurement or an individual diagnosis. Next to the number, this calculator names the WHO adult category it falls in: underweight below 18.5, healthy weight from 18.5 to under 25, overweight from 25 to under 30, and obesity from 30, split into class I (30 to under 35), class II (35 to under 40) and class III (40 or more). The category is compared on the exact BMI, before rounding for display.
Choose the question and enter values
Measure body weight in kilograms and height in centimetres. Enter adult age to confirm that the adult formula is the intended context. Height is squared, so measurement errors matter.
BMI = 10000 × w ÷ h², with w in kilograms and h in centimetres.
| w | Body weight |
|---|---|
| h | Height |
| A | Age |
Example: 70 kg at 2.00 m
| Body weight | 70 |
|---|---|
| Height | 200 |
| Age | 30 |
| Result | 17.5 kg/m² |
70 kg × 10000 ÷ (200 cm × 200 cm) = 17.5 kg/m². Age 30 confirms the adult context.
17.5 kg/m² falls in the WHO adult range below 18.5: Underweight.
Example: 64 kg at 1.60 m
| Body weight | 64 |
|---|---|
| Height | 160 |
| Age | 40 |
| Result | 25 kg/m² |
64 kg × 10000 ÷ (160 cm × 160 cm) = 25 kg/m². Age 40 confirms the adult context.
25 kg/m² falls in the WHO adult range 25 to under 30: Overweight.
Example: 81 kg at 1.80 m
| Body weight | 81 |
|---|---|
| Height | 180 |
| Age | 50 |
| Result | 25 kg/m² |
81 kg × 10000 ÷ (180 cm × 180 cm) = 25 kg/m². Age 50 confirms the adult context.
25 kg/m² falls in the WHO adult range 25 to under 30: Overweight.
Diagram of this result
Ticks and alternating shades mark the WHO adult cut-offs at 18.5, 25, 30, 35 and 40 kg/m²; the needle sits in the band named above. The dial ends at 15 and 40 for scale.
The exact numbers and their units are stated in the visible text. Predictions and scenario inputs are not medical measurements or prescriptions.
Common mistakes
Do not enter metres in the centimetre field. BMI does not distinguish muscle from fat and should not replace a broader assessment.
Assumptions and limits
Adult ages 20–120, weight 20–350kg and height 100–250cm. These bounds are input controls, not validation that the model is appropriate. Children, pregnancy and individual clinical assessments require other methods. Educational information only, not medical advice, diagnosis or an individualized nutrition plan.
Does a BMI number diagnose health?
No. The WHO category is a population screening band, not a diagnosis. Medical interpretation also needs the individual’s history, body composition and other observations.
Method and references
The metric BMI formula and adult context are documented by CDC: https://www.cdc.gov/growth-chart-training/hcp/using-bmi/calculating-bmi.html . Screening limitations: https://www.cdc.gov/bmi/about/index.html . WHO adult BMI categories: https://www.who.int/data/gho/data/themes/topics/topic-details/GHO/body-mass-index .