How is BMI calculated?
Body mass index (BMI) is a single number that relates your weight to your height. In metric units the formula is weight in kilograms divided by height in meters squared. In imperial units it is 703 times weight in pounds, divided by height in inches squared.
This calculator then places the result into one of four bands: below 18.5 is labeled Underweight, 18.5 up to (but not including) 25 is Healthy weight, 25 up to 30 is Overweight, and 30 or above is Obesity. The result is shown rounded to one decimal place.
BMI is a screening number, not a diagnosis. It does not measure body fat, muscle, bone density, age or where weight is carried on the body, so it can misjudge athletic people, older adults and people with different body builds.
When is a BMI check useful?
BMI is mostly useful as a quick, consistent reference point. People use it to see roughly where they sit before a conversation with a doctor, to track a general trend over months, or to fill in a form that asks for it.
It works best for comparing the same person over time, using the same scale and the same height. It is much less reliable for judging an individual in isolation, especially someone with a lot of muscle or someone who has lost muscle mass.
If a result surprises you, treat it as a prompt to ask a healthcare provider, not as a conclusion about your health.
How to calculate your BMI with this tool
- Choose Metric or Imperial with the buttons at the top.
- In metric mode, enter your height in centimeters and weight in kilograms. In imperial mode, enter height as feet and inches in two boxes, plus weight in pounds.
- The result updates as you type. The large number is your BMI and the colored label underneath is the band it falls in.
Worked example (metric): 70 kg and 175 cm. Height is 1.75 m, and 1.75 x 1.75 = 3.0625. Then 70 / 3.0625 = 22.86, which displays as 22.9 and falls in Healthy weight.
Worked example (imperial): 5 ft 7 in is 67 inches, and weight is 143 lb. 703 x 143 = 100,529, and 67 x 67 = 4,489. Dividing gives about 22.4, also in Healthy weight.
Common mistakes and limits
The most frequent error is mixing units, such as typing pounds while the Metric tab is selected. Check the labels on the boxes before reading the result. In imperial mode, enter feet and inches separately, so 5 ft 7 in is 5 and 7, not 57.
The calculator uses one set of adult thresholds for every input. It does not adjust for age, sex or ethnicity, and it is not designed for children, whose BMI is normally interpreted against growth charts.
If you also want to convert your measurements first, the weight converter and length converter can turn stones, ounces or feet into the units used here.