Using HTML, CSS and JavaScript. It looks shit and for some reason, I can't get it to center vertically, even though I copied that piece of CSS from a website telling me this should work with flexbox. But hey, it converts kilometers into miles alright.
Have a look at it at: https://odinjorna.github.io/km-calc/
Using document.getElementById("") in javascript to get input from the HTML. Good to know for future projects.