Democratic challenger Joe Biden has won the US presidential election.

The BBC projects that he has reached 273 electoral college votes, meaning that he’ll become the president in January, pending the outcome of any legal challenges.

Pennsylvania has just been projected as a win for Biden.

And that's it, after a long wait, he is now over the threshold of the 270 electoral college votes needed.

Under the US system, voters in each state pick electors, who then gather a few weeks after the election to decide the winner.

Each elector equals one electoral vote, and the number of electoral votes per state is roughly in line with its population - so bigger states like Texas (38) and California (55) have more, while smaller states like Delaware (3) and Idaho (4) have fewer. In total there are 538 electoral votes, and a candidate needs 270 to win.

Pennsylvania was enough to get Biden over that halfway line. With leads in Georgia, Arizona and Nevada, he may well end up winning far more than needed - taking those three states would give him a total of 306.