this web page runs locally on your browser. no data is transmitted to a server unless you use bookmarks, then it submits to same page (nothing active on server side) and builds a URL. it's just a simple calculator I have made provisions for making the values bookmarkable if you wish to share the information (such as with a doctor).
The Body Mass Index is used as away of measuring how much adipose tissue (body fat) there is. you can determine whether your body is in its proper range or not. This is calculated by your height and weight. a Wikipedia article shows you how to take the number you get from this calculator and the caveats of using it for certain healthy body types.
The USA government organization CDC (CDC.gov, Centers for Disease Control) has a web site which defines how to calculate this (so lots of people have calculators), and has a CDC.gov Children's & Teens BMI Calculator (which is based on mathematical curves) and has a CDC.gov Adult BMI Calculator which is more linear.
Things are different for children (please calculate BMI and then see appropriate chart.
Be sure you read the wikipedia article if you have a lot of muscles or are physically fit, because the BMI will give an improper interpretation.
INSTRUCTIONS: the units you can use and can be mixed with math. but you must use the units provided. you can even mix SI units and English units.