BMI Calculator
Calculate body mass index from height and weight. Your work stays on this device whenever possible.
How to use BMI Calculator
- Choose Metric (kg, cm) or US / Imperial (lbs, ft, in) in the Unit system selector; the fields underneath swap to match.
- On the metric side, enter Weight (kg) and Height (cm); on the imperial side, use Weight (lbs), Height (feet) and Height (inches) separately.
- Press "Calculate BMI" to see BMI in kg/m², the BMI Prime figure and a named category.
- Read the healthy weight range for your height and the line telling you how many kilograms and pounds separate you from it.
- Change only the weight and recalculate to see how each kilogram shifts the index and category.
Worked example
Select Metric, enter Weight 90 kg and Height 180 cm. Height in metres is 1.80, squared 3.24, so BMI = 90 ÷ 3.24 = 27.8 kg/m², in the Overweight (Pre-obesity) band, with BMI Prime 1.11. The healthy range is 59.9 to 80.7 kg, and the tool shows -9.3 kg (-20.6 lbs) to reach 24.9.
When to use this tool
- Checking a starting index before setting a gradual weight target with a clinician.
- Converting a height in feet and inches and a weight in pounds without converting by hand first.
- Finding what weight range corresponds to the normal band for your height.
Important limitations
BMI is a statistical screening ratio (weight divided by height squared), not a direct diagnostic measure of body composition, bone density, or metabolic health. Consult a healthcare provider for medical evaluations.
Frequently asked questions
What is BMI Prime?
It is your BMI divided by 25, the upper limit of the normal band. A value below 1.0 is under the overweight threshold; above 1.0 means the BMI is over 25. It offers a quick ratio against that cut-off.
Which categories does the tool report?
It splits thinness into severe, moderate and mild, then normal, overweight, and three obesity classes at 30, 35 and 40. The cut-offs follow World Health Organization adult ranges (who.int/data/gho/data/themes/topics/topic-details/GHO/body-mass-index) and CDC guidelines. An optional toggle switches to the WHO Asian-population cut-offs. Results are a screening estimate, not medical advice.
Is BMI reliable for athletes or older adults?
Not always. BMI uses only height and weight, so it cannot tell muscle from fat or account for bone density, and the same cut-offs apply regardless of age. Treat it as a screening estimate, not medical advice or a diagnosis.
Can I use it for children?
No. The tool applies adult cut-offs and does not ask for age or sex. Paediatric BMI is judged against growth percentiles for age, which need a different chart and a health professional's input.
How is the healthy weight range calculated?
It multiplies your squared height in metres by 18.5 for the lower limit and 24.9 for the upper, then converts to pounds. If you fall outside, the difference to the nearest limit is shown.