how to add javascrpit to a webpage

I’ve been at this for about 2 hours and had no progress. I know barely anything about javascript but this seems fairly straight forward, probably a matter of putting the code in the correct place.

I want to put the calculator from this page – https://javascript.about.com/library/blfat2.htm on my page which is here – https://completemuscleandfitness.com/index.php/body-fat-calculator/

I would really appreciate it if someone could tell me where in my header.php i need to put the following – <script type="text/javascript" src="fatcalc.js"></script>

I have placed it in a number of places and still havent managed to get this calc to work.

The actual fatcalc.js is curently placed in – htdocs/wp-content/themes/theme-name/fatcalc.js

Should it be placed somewhere else?

Thanks in advance for your help….