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.

Lucian Apostol

You can add the plugin as a widget, dragging and dropping it from the WordPress administration panel, Appearance->Widgets section, or with a short tag directly in your blog posts or pages using the [calotor] shortcode.
You can also add it directly into your template files.

Calotor Calorie Counter

Provide your visitors the ability to figure out their basal metabolic rate, based on their age, sex and weight.

Healthy BMI Calculator vs Calotor Calorie Counter