I follow some Japanese based websites, and I've read that "Japan population is going to reduce by half in 50 years". The estimates in this article are more optimistic, according to the graph it seems to estimate around 30 millions lose in population in the same period of time.
Then, I looked into this chart in Wikipedia about countries population growths. Some European countries like Spain and Portugal seems to have much worse population growth than Japan. Japan -0.12 , Spain -0.21 , Portugal -0.45 . Then I read this article, and they say Spain will lose 5.6 millions inhabitants in the following 50 years. This is proportionally a much lower figure than Japan, in spite of it is stated by that Wikipedia article Spain decreased much more than Japan in the period of 2010-2015 . I suppose many factors are taken into account for calculating population growth aside births and deaths such as immigration, emigration, etc.
I would like to know if there are mathematical equations to calculate population growth or how are they predicted?