Up to this moment around 3,79 mln of bitcoins are lost forever and couldn't be restored. Analytical company Chainanalysis, which specializes in crypto currency have got such results.
Overall 21 mln coins exist, and all of them would be mined up to 2140 year.
Chainalysis analysts used a detailed empirical research of bitcoin blockchain. The results were obtained after the division of already issued coins into several categories. Age and activity of the transactions were key factors. The company used a statistical sample to determine the amount lost.