I've been wondering this myself.
Election - Voting Eligible Population (VEP) - Turnout - % Turnout of VEP
1932 - 75,768,000 - 39,817,000 - 52.6%
1936 - 80,174,000 - 45,647,000 - 56.9%
1940 - 84,728,000 - 49,815,000 - 58.8%
1944 - 85,654,000 - 48,026,000 - 56.1%
1948 - 95,573,000 - 48,834,000 - 51.1%
1952 - 99,929,000 - 61,552,000 - 61.6%
1956 - 104,515,000 - 62,027,000 - 59.3%
1960 - 109,672,000 - 68,836,000 - 62.8% (Highest turnout for Kennedy / Nixon)
1964 - 114,090,000 - 70,098,000 - 61.4%
1968 - 120,285,000 - 73,027,000 - 60.7%
1972 - 140,777,000 - 77,625,000 - 55.1%
1976 - 152,308,000 - 81,603,000 - 53.6%
1980 - 163,945,000 - 86,497,000 - 52.8%
1984 - 173,995,000 - 92,655,000 - 53.3%
1988 - 181,956,000 - 91,587,000 - 50.3%
1992 - 189,493,000 - 104,600,000 - 55.2%
1996 - 196,789,000 - 96,390,000 - 49.0% (Lowest Turnout for Clinton / Dole / Perot)
2000 - 209,787,000 - 105,594,000 - 50.3%
2004 - 219,553,000 - 122,349,000 - 55.7%
2008 - 229,945,000 - 131,407,000 - 57.1%
2012 - 235,248,000 - 129,235,000 - 54.9%
2016 - 230,931,921 - 136,669,276 - 59.2%
2020 - 239,247,182 - ??? - ???