A population measure, shown with the caveat it needs. It is a good description of a thousand people and a poor description of one.
Your numbers
1
What you weigh now. First thing in the morning is the steadiest reading.
2
Without shoes. Centimetres, or feet and inches if that is what you know.
2 still to fill in
Your BMI
Fill in the boxes and press Calculate. Nothing is worked out until you ask for it.
Bodyweight
Height
Nothing is sent anywhere. The sum happens in this tab.
Two people, one number. BMI knows your height and your weight and nothing else.
A worked example
Somebody weighing 75 kg at 178 cm has a BMI of 23.7, which is the middle of the healthy band. The sum is 1.78 x 1.78 = 3.1684, then 75 / 3.1684 = 23.7.