Swipe to the right to access your iPhone's search.
Type your calculation out using figures and symbols on your keyboard. Add using + Subtract with - Multiply with x Divide with / and use (parenthesis) for complex calculations like (5 - 21.9) / 3.7
Tap "Search Web"
Watch Google do its magic. Voila! Calculations!
Try parenthesis for complex operations. Multiply the fun!