I was also confused about the missing calculator in iOS 8. I recently (5-2015) bought an iPad and read the user manual. In the document titled "iPad User Guide for iOS 8.3" from Apple, on page 47 about the Control Center, it does mention a Calculator accesable from the Control Center. I can only assume that Apple (at one time) intended to include a calculator, but forgot to update the user manual for the iPad (someone said it's on the iPhone). I found a free one called Calculator X that also does conversions. It has some ads, but they are at the top and bottom and not too intrusive.
So stop looking for the missing calc, and find a better one for free. I still am looking for one that "sits" in the Control Panel...
Or is the Control Panel off limits to developers?