The Apple calculator in iOS and OS X have remained pretty much unchanged since both were released. They work fine, but the Numerical calculator for iOS (free for a limited time) takes cues from the iOS 7 design language and adds some features that are terrific, but might take some getting used to.
The app bills itself as a "Calculator without Equal' and that is literally true -- there is no equals button. You enter your numbers, like 2+2 and 4 appears.
As you work, Numerical creates a history of your calculations, and there is an undo key that lets you delete your last entries. Many actions are done using swipes, which work for undo, redo and saving an answer. The app also supports the clipboard.
Numerical for iOS
If you do something wrong like divide a number by zero, the app gives you a plain English answer as to what is wrong. That's most welcome.
The app isn't perfect -- there is no percentages button or localized decimals, but both are about to be added. It's not a specialized scientific calculator with dedicated function keys, which Apple's iOS version turns into in landscape mode. The Numerical calculator sounds can be turned off or on, and it only works in portrait orientation.
Numerical is getting good reviews from users, and it really is some fresh thinking for people who use calculators frequently. The app requires iOS 7, and it's optimized for the iPhone 5. When it's not free, it's US$2.99, so if you are interested make a quick trip to the App Store today.