Story highlights 21 U.S. presidents have won re-election to a second term.

One U.S. presidents served two full terms in office non-consecutively

Since 1900, six Democratic presidents have won a second term in office

Nine presidents changed vice presidents between their first and second terms.

On January 20, 2013, President Barack Obama will be sworn in for his second term in office. (Because that is a Sunday, the public ceremony will be January 21.) Less than half of U.S. presidents who have served have been re-elected to a second term in office.

Here's a look back, by the numbers, at who and when:

21 -- Number of U.S. presidents who Number of U.S. presidents who won election to a second term. This includes four men who assumed the office after the death of the sitting president, and later won election in their own right.

13 -- Number of U.S. presidents who Number of U.S. presidents who served two full terms in office. This includes Franklin D. Roosevelt, who was elected four times but died before finishing his fourth term. Making up the distinguished group are: George Washington, Thomas Jefferson, James Madison, James Monroe, Andrew Jackson, Ulysses S. Grant, Grover Cleveland, Woodrow Wilson, Franklin Roosevelt, Dwight Eisenhower, Ronald Reagan, Bill Clinton and George W. Bush.

3 -- Number of U.S. presidents who died in office Number of U.S. presidents who died in office during their second (or later) terms . They are Abraham Lincoln (assassinated), William McKinley (assassinated), and Franklin Roosevelt (died of illness in his fourth term).

1 -- Number of U.S. presidents who served two full terms in office Number of U.S. presidents who served two full terms in office non-consecutively . Grover Cleveland was president from 1885 to 1889 and again from 1893 to 1897.

1 -- Number of U.S. presidents who resigned Number of U.S. presidents who resigned during their second terms . Richard Nixon resigned in 1974, two years after winning re-election.

2 -- Number of U.S. presidents who Number of U.S. presidents who took the country to war during their second or subsequent terms. Woodrow Wilson and Franklin Roosevelt both campaigned on the platform of keeping America out of wars in Europe but were forced to go back on that promise.

6 -- Number of Number of Republican presidents elected to a second term in office since 1900. William Taft, Herbert Hoover, Gerald Ford and George H.W. Bush did not win re-election and Warren Harding died in office.

6 -- Number of Number of Democratic presidents elected to a second term in office since 1900. Jimmy Carter did not win re-election and John F. Kennedy died in office.

46 -- The age of Theodore Roosevelt in 1904, the The age of Theodore Roosevelt in 1904, the youngest man elected to a second term as president

73 -- The age of Ronald Reagan in 1984, the The age of Ronald Reagan in 1984, the oldest man elected to a second term as president

9 -- The number of presidents who The number of presidents who changed vice presidents between their first and second terms. This was very common in the 1800s. Before 1916, George Washington and James Monroe were the only two-term presidents who did not change their vice presidents.

3 -- Since 1900, the number of Since 1900, the number of Democratic second-term presidents whose party also controlled Congress. They are Franklin D. Roosevelt, Harry Truman and Lyndon B. Johnson.

3 -- Since 1900, the number of Republican second-term presidents whose Since 1900, the number of Republican second-term presidents whose party also controlled Congress . They are Theodore Roosevelt, Calvin Coolidge and George W. Bush.

100 -- of the Electoral College vote won by George Washington 1792, to win a second term. Percentage of the Electoral College vote won by George Washington 1792, to win a second term.

98.49 -- of the Electoral College vote won by Franklin D. Roosevelt in 1936, securing him a second term. He won 523 votes out of a possible 531. Except for George Washington and James Monroe, who ran almost unopposed to win second terms, it is the largest margin of victory. Percentage of the Electoral College vote won by Franklin D. Roosevelt in 1936, securing him a second term. He won 523 votes out of a possible 531. Except for George Washington and James Monroe, who ran almost unopposed to win second terms, it is the largest margin of victory.