George Washington: 2865 days
John Adams: 1460 days [average: 2162.5 days]
Thomas Jefferson: 2922 days [avg: 2415.66667]
James Madison: 2922 [2542.25]
James Monroe: 2922 [2618.2]
John Quincy Adams: 1461 [2425.33333]
Andrew Jackson: 2922 [2496.28571]
Martin Van Buren: 1461 [2366.875]
William Henry Harrison: 31 [2107.33333]
John Tyler: 1430 [2039.6]
James K. Polk: 1461 
Zachary Taylor: 492 [1862.41667]
Millard Fillmore: 969 [1793.69231]
Franklin Pierce: 1461 [1769.92857]
James Buchanan: 1461 [1749.33333]
Abraham Lincoln: 1503 [1733.9375]
Andrew Johnson: 1419 [1715.41177]
Ulysses S. Grant: 2922 [1782.44444]
Rutherford B. Hayes: 1461 [1765.52632]
James A. Garfield: 199 [1737.2]
Chester Alan Arthur: 1262 [1714.57143]
Grover Cleveland: 1461 [1703.04546]
Benjamin Harrison: 1461 [1692.52174]
Grover Cleveland: 1461 [1682.875]
William McKinley: 1654 [1681.72]
Theodore Roosevelt: 2728 [1721.96154]
William Howard Taft: 1461 [1712.29630]
Woodrow Wilson: 2922 [1755.5]
Warren G. Harding: 881 [1725.34483]
Calvin Coolidge: 2041 [1735.86667]
Herbert Hoover: 1461 
Franklin D. Roosevelt: 4422 [1811.21875]
Harry S. Truman: 2840 [1842.39394]
Dwight D. Eisenhower: 2922 [1874.14706]
John F. Kennedy: 1036 [1850.2]
Lyndon B. Johnson: 1886 [1851.19444]
Richard Nixon: 2027 [1855.94595]
Gerald Ford: 895 [1830.65790]
Jimmy Carter: 1461 [1821.17949]
Ronald Reagan: 2922 [1848.7]
George H.W. Bush: 1461 [1839.24390]
Bill Clinton: 2922 [1865.02381]
George W. Bush: 2922 [1889.60465]
Barack Obama: 2922 [1913.06818]
Donald Trump: 1461 [1903.02222]
Joe Biden: pending
More than 2 terms: 1/46
2 full terms: 12/46
Between 1 and 2 terms: 7/46
1 full term: 15/46
less than 1 term: 10/46
Washington served 2 full terms, but he was sworn in as president two months after he was supposed to start, so his 2865 is less than the full 2922. His immediate successor John Adams served one full term, but because the year 1800 was not a leap year, his 1460 is one short of the regular 1461. William McKinley would have served 2921 days for the same reason had he not been assassinated. 1800 and 1900 were not leap years, but 2000 was, so George W. Bush served a full 2922. Grover Cleveland served two terms, but they are counted separately as single terms here because they were non-consecutive; he served once as the 22nd president and once as the 24th.
As of 2021, the average length of term for the president of the United States is 1903.022 days, or 5.21019 years (5 years, 77 days).
If we graph out term lengths for the first 45 presidents, we can see that, on average, they're lasting longer than before:
Line of best fit: 10.047036x + 1649.718182 (x = president number)
If we instead look at average term length over time, we'll see that while term length has trended down overall since Washington (the blue line), it has actually been trending up since reaching a local minimum at McKinley:
Line of best fit: -13.131288x + 2230.558258
If we look only at the presidents since Teddy Roosevelt, we get this:
Line of best fit: 9.677111x + 1475.141058
We can use this to try and extrapolate for Joe Biden's presidency: if we looked only at the term lengths of his predecessors, we could predict he would serve for 10.047036x46 + 1649.718182 = 2111.88184 days, or 5.78202 years (5 years, 286 days), which is considerably higher than the average of 5.21019.
Looking at how it has changed over time, we would expect the average term length after 46 presidents to be -13.131288x46 + 2230.558258 = 1626.51901 days, which is too far below the actual value to be useful. The actual average was 1903.022222 after 45 presidents, so the only way it could drop to 1626.51901 after 46 would be for Biden to serve for -10816.12554 days. Negative 11 thousand days! Negative 30 years! We'll assume that's highly unlikely, so let's instead look at the figures since Roosevelt. 9.677111x46 + 1475.141058 = 1920.288164 days. For the average to come up from 1903.022222 to 1920.288164 means Biden would be expected to serve 2697.255544 days, a much more reasonable 7.38468 years (7 years, 141 days).
In the first half the presidency, presidents died in office like clockwork. It was so common that people started believing the office was cursed; William Henry Harrison was a general known for slaughtering natives, especially at the Battle of Tippecanoe in 1811, and he became the first president to die in office in 1841. From then on, every single president elected in a year divisible by 20 also died in office; 1840 Harrison, 1860 Lincoln, 1880 Garfield, 1900 McKinley, 1920 Harding, 1940 Roosevelt, 1960 Kennedy. It was known as the Curse of Tippecanoe, or Tecumseh's revenge, and when Ronald Reagan survived his assassination attempt in 1981 people believed he had "broken" the curse. For the record, I don't believe there ever was a real curse, it was just a coincidence that failed to account for Zachary Taylor who was elected in 1848. The only significance the "curse" had was that it meant few presidents survived their full terms, meaning the vice president would assume office and serve out the remainder.
In 1974, Richard Nixon became the first president to leave office early through resignation rather than death, cutting his second term short, leaving Gerald Ford to fill out the last 2 years and some change. Since then, every single president has survived their full terms, and most of them have successfully been re-elected (save for Carter, Bush Sr. and Trump). A president is more likely to be re-elected now than at any other point in American history; the first half saw tons of single-term and partial-term presidents, bringing the average way down before FDR made it spike with 4 terms (of which he served 3 and a month before dying), followed by relatively stable lifespans ever since.
With the introduction of the 22nd Amendment during Truman's presidency, it will be near impossible for any future presidents to serve more than 2 full terms, 8 years, 2922 days. It's still theoretically possible, just extremely unlikely; the amendment says you cannot be elected president more than twice, and if you serve more than two years of a term to which someone else was elected, then you can't be elected more than once. This means that if the president died or resigned 2 years and a day into their term, their VP could serve out the remainder and still be illegible to run for two full terms of their own, for a total of 3652 days (almost 10 years exactly), so the graph will likely asymptote somewhere between 4 and 8 years (closer to 8, as re-election is much more common).
That said, who knows? Maybe we're overdue for an irregular transition and five of the next ten presidents might not make it through their full terms. That would actually be normal for America; the last 50 years of stability have been the outlier. Regularity is irregular here.
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