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.

HTS Labs

HTS Labs Pregnancy Calculator adds a responsive calculator that estimates:
<ul>
<li>estimated due date</li>
<li>current gestational age</li>
<li>estimated conception date</li>
<li>estimated fertility window</li>
</ul>
The free edition focuses on the first day of the last menstrual period (LMP) method only.
<h4>Features</h4>
<ul>
<li>LMP-based pregnancy due date calculation</li>
<li>custom in-plugin calendar date picker</li>
<li>shortcode support: <code>[pddc_due_date]</code></li>
<li>tabbed admin settings UI</li>
<li>layout and styling controls for the main calculator/result cards</li>
<li>adjustable colors and disclaimer text</li>
<li>responsive frontend layout</li>
</ul>

HTS Labs Pregnancy Calculator

A professional LMP-based pregnancy due date calculator for WordPress with a polished frontend and streamlined admin settings.

Healthy BMI Calculator vs HTS Labs Pregnancy Calculator