You needn’t dig around for the iPhone’s Calculator app to perform basic arithmetic because you can run numbers in Spotlight Search, just as in OS X. Type a calculation into the search bar, using * for multiply and / for divide.
Just type a mathematical expression into Spotlight, and it will return the answer. It’s best for doing quick arithmetic, using the +, -, *, and / symbols. And for slightly more complex equations, you can use parentheses to set the order of operations. Other functions I found: type “sqrt(256)” to find the square root of a number. Or type “pi” and Spotlight returns the number pi to 10 decimal places.