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.

Waterloo Plugins

Add a body mass index calculator to your blog by using the <code>[bmi]</code> shortcode. See the screenshots for previews.
There are 3 built-in styles: material design, Bootstrap, and default (your theme’s styles).
Options:
<ul>
<li>theme – <code>material</code>, <code>clean</code>, <code>default</code>, or <code>none</code></li>
<li>metric – Label text for metric section</li>
<li>imperial – Label text for imperial section</li>
<li>height – Label text for height input</li>
<li>weight – Label text for weight input</li>
<li>heightPlaceholder – Placeholder text for height input</li>
<li>weightPlaceholder – Placeholder text for weight input</li>
<li>submit – Text for submit button</li>
<li>result – Text for result, use <code>%bmi%</code> to represent the calculated BMI</li>
</ul>
Examples: 
 [bmi theme=”clean”] 
 [bmi theme=”material” heightPlaceholder=”Enter your height” submit=”Calculate your BMI!” result=”Your BMI is %bmi%”]

BMI Calculator Plugin

Great!

Add a simple, stylable BMI calculator to your WordPress blog. Comes with several stylish themes.

Healthy BMI Calculator vs BMI Calculator Plugin