The basic Calculator app that comes with iOS is good for simple calculations, and if you rotate your phone into landscape orientation, it even provides some rudimentary scientific-calculator functions. But if it’s a full-featured scientific calculator you seek—or if you’d just like a great calculator for your iPad, which inexplicably doesn’t include one—you’ll want to look elsewhere.
My personal favorite remains TLA Systems’s $10 PCalc (App Store link). It’s overflowing with features, including scientific operations, conversions, constants, and even user-defined functions. And you get it all in an interface that can look as simple or complete, and as modern or retro, as you prefer.