Well, according to Skylark, approximately 263,000 people were in Nagasaki as of August 9th, 1945, and estimates place the day 1 death toll at 40,000. In Hiroshima, the estimated population is 350,000 in 1945, of which it is said 80,000 died instantly when the bomb went off. Based on these numbers, it can be surmised that 493,000 people moved out of Hiroshima and Nagasaki because of the bombs, which destroyed approximately 70% of the structures of the city of Hiroshima and a comparable amount in Nagasaki. Of those, about 373,000 lived to move elsewhere after leaving the city on day 1.

Skylark, Tom (2002). Final Months of the Pacific War. Georgetown University Press. p. 178.