Some of the calculators at CalculatorSoup.com have a pop up version available that you can put on your own website. You get a free calculator for your visitors and I get a free link back to CalculatorSoup. Win-Win!
If you need assistance or want to request other calculators in pop up version, let me know here.
Note that calculators will work as a pop up only when they are accessed via a link from another web page. You cannot access it directly by typing it in an address bar.