I had the misfortune of being in a McDonalds one day during the peak lunchtime rush when the power went out all of a sudden. That killed all of the registers. The manager handed out pocket calculators so the people working the registers could at least determine the correct amount of money to charge, and the correct amount of change to give back in an attempt to keep things rolling for a bit. Surprise, surprise, surprise. Not one person could even use a calculator! The girl working at my line tried 4 times to give me the correct change. I could have easily walked out with more than I went in with, but instead told her what the correct amount of change was. By then the lines were so backed up that they closed the store until the power came back on several hours later. All of these handy on-line converters and apps are great. I use the conversion program called Convert by Josh Madison at work all the time. But you should still be able to go about things the old school way just to keep your witts about you. That's why I like to do the math out when trying to explain things.