Numbers of Ultra-Js

Answers to Quora Questions by Yair Davidiy (1 January 2017, 14 Tevet, 5778)

How many non-ultra-Orthodox Jews become ultra-Orthodox? Have most ultra-Orthodox adults been raised in ultra-Orthodox households?
 https://www.quora.com/How-many-non-ultra-Orthodox-Jews-become-ultra-Orthodox-Have-most-ultra-Orthodox-adults-been-raised-in-ultra-Orthodox-households/answer/Yair-Davidiy

Surveys have been made in the past several times over. Some-one who is prepared to spend time surfing the net can probably find reasonably reliable figures. From what I remember about 20% of Ultra-Orthodox adults in Israel came from outside. In most cases you would not notice this since the price of becoming Ultra-Orthodox is to belong as completely as possible. The newcomers include they who were once National Religious (i.e. Modern Orthodox) and they who were not Jewish at all. Their children however are less successful in the religious sense than the oldtimers. Between a fifth to a third of them drop out. Their dropout rate is about twice as high as that of the others: A good portion of they who leave Ultra-Orthodoxy belong to families who were not in it for that long. The numbers of Ultra-Orthodox are on the increase due to large families and new blood coming in.

As we said some leave but the net increase is still significant. On the other hand the National Religious lose a lot, maybe from 40% to 50%, or maybe less but still high. This loss is partly made up by secular Jews becoming NRs and by the larger families of they who remain within the fold. A related phenomenon is for the more learned among the NRs to draw closer to the Ultra-Orthodox. 
The two main centers of Jewish Orthodoxy today are in Israel and the USA. In America the situation is different but here too the Ultra-Orthodox are growing.