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.

solokco

<h4>ENGLISH</h4>
This is a basic calculator that displays the users BMI (Body Mass Index) depending on their Height and Weight
You can select to display the input options as 
* Metric (Centimeters and Kilograms) 
*Imperial (Inches and Pounds)
It’s easy to configure different options from the wordpress panel
<h4>SPANISH</h4>
Esta es una calculadora sencilla que muestra el IMC (Indice de Masa Corporal) de los usuarios dependiendo de su Peso y Altura
Puedes seleccionar mostrar el formulario en 
*Sistema métrico (Centímetros y Kilogramos) 
*Sistema imperial (Pulgadas y Libras)
Fácil de configurar desde el panel de wordpress

BMI / IMC Calculator

A simple calculator to show your users BMI (Body Mass Index)

Healthy BMI Calculator vs BMI / IMC Calculator