US election: Who's winning so far - and when will we know for sure?

Donald Trump is projected to have won the first two of the seven swing states to be declared - narrowing Kamala Harris's routes to the White House.

Trump and Harris both need to hit the magic 270 Electoral College votes to become the next US president.

Trump wins are projected in they key battlegrounds of North Carolina and Georgia.

Losing there could indicate that any route for Kamala Harris has to go through the "blue wall" states - Pennsylvania, Michigan and Wisconsin - which are all tight battlegrounds.

Votes are still being counted, and counting is generally more complicated in the US than in the UK, largely due to the country's sheer size; the varying time differences alone make for a longer process.

Polls began closing at 11pm UK time and the last of them closed at 4am UK time.

In many past elections, it's at around this time that a winner has been declared.

Election latest: Live updates as results come in

But US election results can take days - or even weeks.

Even with Donald Trump ahead in early battleground voting, factors like high turnout and voting by post can mean leads later change hands in crucial states.

Before we get into those factors in detail, you need to know how a winner is declared.

How calling the election works

When a winner is declared, whether it's hours or days from now, it will initially be a "projected" winner.

It's projected because the official results are typically only confirmed by state officials after 7-30 days, depending on the state.

Sky News has access to the most comprehensive exit poll and vote-counting results from every state, county and demographic across America through its US-partner network NBC.

Big voter turnout

There are some 240 million people eligible to vote in the US.

And if voter turnout is particularly high, it means vote counting will take longer than usual.

This was the case in 2020, when a record 158.5 million people voted - the highest turnout since 1900.

It isn't yet known how many voters have visited the polls on election day, but more than 77 million Americans had already cast their votes by Monday, according to the University of Florida's Election Lab - 42,195,018 returned in person and 35,173,674 by mail.

In the 2020 presidential election, it took four days for Mr Biden to be called the winner, but that wasn't just put down to the turnout.

Many also said it was because more people - around 100 million - voted by post than usual due to the COVID-19 pandemic, and those votes took longer to count.

Election officials have been optimistic that the 2024 vote count will be smoother without the many challenges the pandemic posed to officials in 2020, according to NBC News.

Tight race will lead to a longer wait

For months, polls have suggested this will be an incredibly tight election - particularly in a few battleground states: Arizona, Georgia, Michigan, Nevada, North Carolina, Pennsylvania and Wisconsin.

It's been so tight that most experts have refused to predict the outcome - and exit polls have done little to separate the candidates.

But you'll have seen overnight that Ms Harris and Mr Trump were projected winners in certain states early on after polls closed.

These are states where votes are so clearly leaning in one direction as they get counted that there is too much ground for the trailing candidate to make up.

In those swing states, however, voting is going to be far closer, and a winner will not be projected until the NBC News decision desk is certain of an outcome.

That could mean waiting until practically all the votes in some of the states are in to project a winner - which could take days.

Key terms to listen out for as the count progresses

There are a number of key stages between now and reaching a projected winner, NBC News says. Listen out for these calls:

• Too early to call: This can mean two things: there might be a significant margin for one of the candidates, not enough to meet NBC's statistical standards to project the race; or there is not enough data to determine the margin with certainty.
• Too close to call: This means the final margin between the candidates will be less than five percentage points. NBC News' decision desk will not use this characterisation until it has statistical confidence that the race will be this close.
• Leaning: This status is introduced when the decision desk is confident that the candidate who is ahead is going to win, but the statistical threshold for calling the race has not yet been met.
• Projected winner: NBC News has made a projection that a candidate will win.

How long has it taken in the past?

It's common in the US for it to take a couple of days to know who the next president will be.

The 2020 election's four day wait was an anomaly, but it paled in comparison to the 2000 election, when it took weeks.

Florida and its 25 Electoral College votes (it now has 30) were set to decide the contest between Democrat Al Gore and Republican George W Bush, whose brother was Florida governor.

On election night on 7 November, TV networks called the state for Gore before polls had closed everywhere in the state. Later that evening, they reversed their stance and said it was too close to call, then called it for Bush and then returned to "too close to call".

A preliminary vote tally the day after the election had Bush ahead by around 1,700 votes in Florida - so close that state laws triggered an automatic machine recount. The first recount winnowed Bush's lead down to just 317 votes.

The issue was around Florida's punch-hole ballots and hanging chads - punched holes that might still have a corner intact - and how these were counted.

A legal battle ensued that went right up to the US Supreme Court, which in a 5-4 decision along ideological lines ruled that any solution to the recount issue could not be put in place by the deadline, thus handing the state to Bush.

What happens after a winner is declared?

Whether a winner is declared on the night or in the following days, they aren't officially elected until the electoral votes are formally cast and counted.

Electors cast them on 17 December, they are counted and certified by Congress on 6 January and then the new president will take office after being inaugurated on 20 January.

Unlike the general election in the UK, there is a transition period between the election result and the new president taking over.

During the gap, Mr Biden will continue as president, with the election winner known as the president-elect until the transfer of power in January.

(c) Sky News 2024: US election: Who's winning so far - and when will we know for sure?