Hi there, I found a bunch of JavaScript calculator implementations on codepen:
https://codepen.io/tag/calculator
Each implementation has HTML, CSS, and JavaScript that you can put in place in Qualtrics. I liked this one (credit vaishak101) and have adapted it into a Qualtrics survey. If you import the attached QSF file into a blank survey project, you should be all set.
SurveyCalculator.qsf

Also you might check out this thread to have the calculation fill in the Text Entry Field:
https://community.qualtrics.com/XMcommunity/discussion/2421/add-a-calculator-on-the-page-so-that-subjects-can-use-it-to-do-some-calculation-manually

Thank you so much!

Hi, I have the same query but haven’t been able to access the attached file that you reference (the .qsf seems to just feed me back to this thread). Apologies if I’m missing something but could you please reupload this if you still have access? Thanks!

@Raf here you go. I added the keith-wood calculator from the other thread in there too.

Thank you very much for the attachment and the swift response! This is exactly what I needed

If you need a specialty calculator or converter (think converting KG to LBS for example) I have a video that shows how to do that in Qualtrics with JavaScript.  For a regular calculator like you might find on your iPhone though, the one provided is perfect.  If you want a pop-up link though for people to calculate specific values (as in you want to provide the formula for them), you may find this helpful.  Code provided in the link as well. 

Video: