The 17-year-old has signed a contract that will keep him with the Blues until the summer of 2016.
“There were rumours flying around already during the spring, but at that time there were more clubs around me,” he told iDnes.cz.
“It became more specific only after the U-17 European Championship in Liechtenstein in May. Chelsea scouts were present there.
“Then I, my dad and Olomouc were proposed a specific offer. But Mr (Frank, sporting director) Arnesen told me I had been on the list already before the Euro and they had been following me for two years.”
Kalas added, when asked who else was interested in signing him: “Bayer Leverkusen and Hoffenheim from the Bundesliga and apart from Chelsea also Arsenal.
“I have been to both German clubs and also Arsenal, but the best impression was left on me by Chelsea.
“People say that Arsenal are famous for their academy, but from what I could see during my days at Chelsea, that really impressed me.
“Even my dad thought of Chelsea as the best option. Moreover, I have been a Chelsea fan since I was a little kid.
“I am convinced I have made the right decision by joining Chelsea. After all, by this I am making my dream come true.”