How US Election day 2016 will unfold: The key states and times you need to know

After a long and fiercely fought campaign, US election day on Tuesday will see millions of Americans head to the polls to vote for their next commander-in-chief.

We should know in the early hours of Wednesday UK time whether Donald Trump or Hillary Clinton has won the race for the White House.

Here's our guide at how election day - and night - will unfold, with the key times to know if you're planning to stay up through the night.

Watch the result unfold live on ITV and itv.com/news

  • How does the voting system work?

In a nutshell, rather than voting directly for a presidential candidate, Americans vote for electoral college members - or electors - who have pledged allegiance to their preferred candidate.

There are 538 electors and each US state has a different number depending on the size of its population. For example, California has a population of 38.8 million people and gets 55 electoral college votes.

The candidate with the most votes in each state is the one it will support for president overall and award all of its electoral votes.

  • The swing states to watch out for

Both Trump and Clinton can pretty much bank on certain states voting for them. For example, California will more than likely vote Democrat while traditionally Republican Texas looks set to go for Mr Trump.

So the election will be won and lost in the so-called swing states - the states where the vote could go either way.

The key swing states to watch - including their respective electoral college votes - are:

  • Florida - 29

  • North Carolina - 15

  • Virginia - 13

  • Colorado - 9

  • Nevada - 6

  • Ohio - 18

The magic number is 270 electoral college votes - the first out of Clinton or Trump to reach that number will automatically become president.

  • What time do polls open?

Voting in all 50 US states will cross six time zones with the majority of polling stations opening between 6am and 7am local time and closing between 7pm and 8pm.

Voting booths can be found set up in the strangest places. Credit: Reuters

  • What time will the states declare?

2300 UK time

The first polling stations close after 7pm local time (2300 UK time), starting with Indiana and Kentucky.

0000 UK time

All eyes in both Hillary Clinton and Donald Trump's camps will be on Florida and Virginia when projected results from those states - based on exit polls - will be made. These swing states could offer an early indication of whether Clinton or Trump will become president.

0030 UK time

Projections from two more swing states - Ohio and North Carolina - are expected.

0100 - 0200 UK time

A raft of projections will come in from traditionally Democrat or Republican leaning states such as New Jersey, Illinois and Mississippi and Texas.

A projection is also expected during this time from swing state Colorado, which has got nine college votes.

Donald Trump will be clinging to the hope that traditional Republican states will stick with him. Credit: Reuters

0300 - 0500 UK time

By this time projections for the majority of states will be made - including another swing state, Nevada - and Washington DC.

Hillary Clinton will be looking for voters to thwart Donald Trump's White House ambitions. Credit: Reuters

  • When will we know who has won?

US television networks will 'call' whether Hillary Clinton or Donald Trump has won the presidency, based on the projections they have made earlier in the night.

Once the election has been called, the losing candidate will call the other to concede.

Then both Mrs Clinton and Mr Trump will give either a concession or victory speech.

Who will be heading to the White House? Credit: Reuters

  • What if there is no clear on the winner at the end of election night?

There is always the possibility of no clear winner if neither Mrs Clinton or Mr Trump reaches the magic number of 270 electoral college votes.

If this happens the House of Representatives would elect the new President from among the top three candidates under the Twelfth Amendment system.

The only time this has happened was during the 1824 election when John Quincy Adams became President this way.

The House of Representatives could decide who becomes President if no candidate get 270 electoral votes. Credit: Reuters

  • Can the candidates challenge the election result?

Yes, but only if there are sufficient grounds. In 2000, Florida's electoral college 29 votes famously decided the election in favour of George W Bush over Al Gore.

It followed a protracted and bitter legal battle after Bush lost the national popular vote which ended after a Supreme Court ruling.

A swing state famously decided the 2000 election in favour of George Bush. Credit: Reuters