I haven't been able to find great statistics, but you can look at state populations to get an idea. California, as an example, went from ~5 million to ~30 million in a few decades post WWII.
California is a pretty substantial outlier there, not exactly representative of the country as a whole, and even then doesn’t actually need a super high inter-state immigration rate to get those numbers. It’s worth noting that the entire country grew 2X larger during those 5 decades. So we’re looking at a 3X increase compared to the average. Which is a lot! But even ignoring the effect of people moving there in the early decades themselves having children that speed up the process…to get a 3X growth over 50 years requires an annual immigration rate of 2.2%. Which would be in 1980, .29% of the US’s population moving to California.
(And as a side note, the above numbers are ignoring immigration. In 1980 California was 15% foreign born people, about 2.5X times the national average. And people who immigrated in 1940s-1960s would themselves have children that lowered the needed inter-state moves even more)
Point is, 5 decades is a long time, only takes a small percent a year to compound. California, by far the most popular state to move to in the country historically, still doesn’t need that high a percent change to get from 5 to 30 million.
Sure, but the point of the rest of those words was to show that to get a 20 million surplus over 5 decades requires, at absolute maximum, .3% of the US to move to California per year in that time frame. (Or more accurately .3% more of the population moving to than moving away from California)
