In a study done by the Joint Chiefs of Staff in April, the figures of 7.45 casualties/1000 man-days and 1.78 fatalities/1000 man-days were developed. This implied that a 90-day Olympic campaign would cost 456,000 casualties, including 109,000 dead or missing. If Coronet took another 90 days, the combined cost would be 1,200,000 casualties, with 267,000 fatalities.
A study done by Adm. Nimitz's staff in May estimated 49,000 casualties in the first 30 days, including 5,000 at sea. A study done by Gen. MacArthur's staff in June estimated 23,000 in the first 30 days and 125,000 after 120 days. When these figures were questioned by Gen. Marshall, MacArthur submitted a revised estimate of 105,000, in part by deducting wounded men able to return to duty.
In a conference with President Truman on 18 June, Marshall, taking Luzon as the best model for Olympic, thought the Americans would suffer 31,000 casualties in the first 30 days (and ultimately 20% of Japanese casualties, which implied a total of 70,000 casualties). Adm. Leahy, more impressed by Okinawa, thought the American forces would suffer a 35% casualty rate (implying an ultimate toll of 268,000). Adm. King thought that casualties in the first 30 days would fall between Luzon and Okinawa, i.e., between 31,000 and 41,000.