Sandy McFadden

This is a very simple plugin which requires almost no configuration.
All you need to do is go to the Appearence then Widgets area in the Admin Dashboard. 
Once you are there drag the BMI Calculator widget onto the widget area you would like it added to.
The only two settings are the title and the color. The color is the hex color you would like the results background to be.
A lot of the code was adapted from 
<a href="http://codereview.stackexchange.com/questions/21581/calculating-body-mass-index-bmi" title="Inspiration Code" rel="nofollow ugc">Stack Exchange Thread</a>

Healthy BMI Calculator

Adds a shortcode and widget that will display a simple BMI Calculator on your site. Works with both metric and US style measurements.

guelben

A BMI (Body Mass Index) calculator free and easy to use. It is responsive and adjusts to any size of screen. The user receives his BMI and medical advices according to the World Health Organization. You can insert it using a widget or a shortcode. You can customize its appearence at the settings page.

Responsive BMI Calculator

A BMI (Body Mass Index) calculator, free, responsive and easy to use. The user receives his BMI and medical advices according to the World Health Orga …

Healthy BMI Calculator vs Responsive BMI Calculator