"On June 18, 1945, Admiral Leahy pointed out that, if the "Olympic" invasion force took casualties at the same rate as Okinawa, that could mean 268,000 casualties (about 50,000 dead) on Kyushu. It nonetheless appears likely that post-war estimates of a half million American deaths were too high, but many tens of thousands of dead were a real possibility.
Judgement at the Smithsonian, page 49.
Now for the three different battle plans the U.S. might have used:
"*The most likely, an autumn 1945 attack on southern Kyushu, followed by the Tokyo plain (in early 1946)- about 40,000 American dead, 150,000 wounded, and 2,500 missing.
*the least likely, an autumn 1945 attack on southern Kyushu, followed by northwestern Kyushu- 25,000 Americans dead, 105,000 wounded, and 2,500 missing;
*an autumn 1945 attack on southern Kyushu, followed by northwestern Kyushu and then (in early 1946) the Tokyo plain- 46,000 American dead, 170,000 wounded, and 4,000 missing in action"
Judgement at the Smithsonian, page 180