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.

Andreas Ostheimer

Global Body Mass Index Calculator adds a clean, responsive Body Mass Index calculator to your WordPress site. Visitors can work out the BMI for an adult or a child, switch between imperial and metric units, and see a clear, plain-language result — all without leaving your page.
Drop it anywhere with the <code>[gbmicalc]</code> shortcode or the “BMI Calculator” widget/block.
Features
<ul>
<li>Separate calculators for adults and children, on two tabs.</li>
<li>Imperial (feet, inches, pounds) or metric (centimetres, kilograms) units.</li>
<li>Plain-language result with the weight-status category — and, for children, the BMI-for-age percentile.</li>
<li>Colour-customisable (card, text, tabs and button) through the built-in WordPress colour picker, with an adjustable size.</li>
<li>Responsive layout that adapts from desktop to mobile.</li>
<li>Works both as a shortcode and as a widget/block.</li>
<li>Translation-ready, with bundled translations for the 24 official languages of the European Union.</li>
<li>Accessible markup: labelled fields, results announced to screen readers, keyboard-friendly tabs.</li>
</ul>
How the categories are determined
<ul>
<li>Adults: the standard WHO BMI ranges (underweight, normal, overweight, obese).</li>
<li>Children and teens (2–20 years): the BMI is interpreted against CDC BMI-for-age percentiles.</li>
</ul>
Disclaimer
This plugin is provided for general information only and is not medical advice. BMI is a screening tool, not a diagnosis, and it does not account for muscle mass, body composition, pregnancy or other factors. Always consult a qualified healthcare professional about your health.
Links
<a href="https://www.ostheimer.at/wordpress-plugins/global-body-mass-index/" rel="nofollow ugc">Demo</a> · <a href="https://www.ostheimer.at/leistungen-preise/wordpress/wordpress-plugins/" rel="nofollow ugc">More plugins by Ostheimer</a>

Global Body Mass Index Calculator

BMI calculator for adults and children — add it with the [gbmicalc] shortcode or a widget, in a colour-customisable, fully translatable interface.

Healthy BMI Calculator vs Global Body Mass Index Calculator