Bitcoin production follows a geometric progression:


Every four years, half of the remaining bitcoins are mined. The number of bitcoins in existence gets ever closer to, but never quite reaches, 21 million. If bitcoins were infinitely divisible, the number of bitcoins in circulation would keep increasing forever, but still never reach 21 million. But since bitcoins are not infinitely divisible, creation of new bitcoins will cease when the block subsidy drops from 0.00000001 to 0.00000000, at which point the number of bitcoins will be fixed at exactly 20999999.97690000. This will happen sometime around the year 2140.