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.

Adnan Shahid

FitCalc is a simple BMI calculator supporting Metric (cm, kg) and Imperial (ft/in, lbs) units. Accessible, responsive, translation ready.

FitCalc – BMI Calculator

Lightweight BMI calculator with Metric and Imperial units. Use the shortcode [fitcalc_bmi].

Healthy BMI Calculator vs FitCalc – BMI Calculator