Welcome to iMore! iPad doesn't have a native calculator app so if you want to get rid of ads you'll likely need a premium calculator app from the App Store.
CALC Smart is the preferred choice for many professionals and school goers alike because of it's flexibility and continuous improvements based on end-user feedback. However, you don't have to take our word for it - there are over 7'500 reviews world-wide on the app and almost 2k reviews in the US alone... https://itunes.apple.com/us/app/id576675190?at=10l3Vy