My iPhone 6 has been dropping from 15% to 1-2%. If you live near an Apple Store they can do a test on your battery to verify if it is bad. They do it via a diagnostic app they use. Just setup an appt first via the Apple Store app or search the web for Apple Genius Bar appt. I agree with the other comment that the battery may need to be swapped. Apple can also do that in-store now.
I've had my iPhone 5 for almost 4 years and had to get my battery replaced in December 2014 because my battery life was horrible and would shutoff anywhere from 10-40% and other times would jump to 1% and stay there for hours. I was lucky enough, if that's what you want to call it, that Apple was replacing batteries for iPhone 5's that were sold from September 2012 to something like January or February 2013. If you think you got your phone somewhere around that time, go get it checked. If not, it may be time for a new phone since it's pretty expensive to get a new battery.