score:20
Wikipedia's page on conscription actually gives a pretty good breakdown.
In WWII, males between 18 and 64 were required to register, but the selections were limited by executive order on December 5, 1942 from ages 21β45 to 18β38. Assuming the citation (George Q. Flynn, The Draft, 1940β1973) is correct, 50 million men from 18 to 45 were registered. The Selective Service statistics page gives the total inductions during WWII as 10,110,104, which would indicate that the percentage would be somewhere around 20% were drafted based on registration.
By the Vietnam war, the draft age had changed to 18-26 by the Selective Service Act. The draft pool was around 27 million (apparently lifted verbatim from a paragraph from this paper), and per the Selective Service page linked above, there were 1,857,304 inductions during Vietnam. Note that Morris gives the number as 2,215,000 - this may reflect a difference in start and end dates used but they are fairly close. This would indicate that roughly 7-9% of the draft pool was conscripted.
Upvote:2
Page 40 of the 1940 US Census report shows there were only 32m males aged between 15 and 44 (including 4.4m in the 40-44 cohort, and 27.5m under 40).
The WW2 Museum states that 17.8m US personnel served in the war, of which 6.3m were volunteers, and 11.5m were draftees. 98% of those who served were male. Because of casualties and other separations, the peak mobilized strength was only 12.2m (1945).
So if you were a man aged 15-44 in America in 1940, you had a 55% chance of serving, and a 36% chance of being drafted. If you exclude men over 40 (in 1940) from this estimate, the odds change to a 63.5% chance of serving and a 42% chance of being drafted.