Butland, 20, will be sad to leave St Andrew's after coming through the ranks with the Blues.
"It was a very, very important decision to make about when and where to move," he said. "It took a lot of thinking and unfortunately Birmingham were in the position they were.
"I have been there for a long time, learned a lot there. I've had my spells in League Two and my taste of the Championship, but I feel that if I really want to push Joe Hart and the senior keepers for the senior spot then I need to be playing Premier League football.
"I feel capable of doing it and feel capable of doing it with Stoke. It is a very good set-up there.
"The goalie coach is very good and I am very pleased I've made that move and at this time. I am really at ease with the decision and really looking forward to it."