How does America elect its president?

Voters look at the 2008 electoral college map. Credit: Reuters

At the centre of the US electoral system is the magic number 270.

This is the amount of electoral college votes needed to win the presidential election and claim that prized seat in the Oval Office.

But how does the electoral college system work?

A guide by ITV News presenter Mark Austin

Under the system, US voters do not elect their president and vice-president directly, but instead appoint electors who formally elect them on their behalf.

A group of 538 electors is chosen from the various states - winning 270 or more of these votes will win the election.

The number of electoral votes each state receives is equal to the number of Senators plus the number of House Representatives in the state.

The number of senators is fixed at two but the number of Representatives for each state varies depending on the state's population.

The number of representatives that a state has is determined by the population.

After the official census is completed every ten years, Congress adjusts the number of Representatives based on the new population figures.

For example California, has the largest amount of electoral votes with 55.

Alaska and Wyoming are among eight states who all have the lowest amount of electoral votes: 3.

For a full run-down of each state and its number of electoral votes click here.

U.S. presidential candidates are listed as an elections official demonstrates a touch-screen voting machine. Credit: Reuters

Should none of the candidates win 270 electoral votes, the 12th Amendment kicks in and the election is decided by the House of Representatives.

The combined representatives of each state get one vote and a simple majority of states is required to win.

This has only happened twice. Presidents Thomas Jefferson in 1801 and John Quincy Adams in 1825 were elected by the House of Representatives.

Voters cast their ballots at a Franklin County polling location on the first day of in-person absentee voting in Columbus, Ohio. Credit: Reuters

Under the current US system it is possible for a candidate to lose the popular vote and still be elected president by the Electoral College.

Four presidents have been elected in this manner: John Quincy Adams in 1824, Rutherford B. Hayes in 1876, Benjamin Harrison in 1888 and George W. Bush in 2000.