Why does my phone shut off at 15%

[Kaylito]

Ok, so, I've had my HTC one m8 for almost a year now. Recently, my phone has been shutting off out of nowhere whenever it reaches below 15%. In the past, when I didn't have a charger around me I could still use my phone perfectly until it reached 1%. I've tried resetting my phone. Using a different launcher. I don't understand the issue. Do I need a whole new battery? How can I fix this somebody help please thanks