The moment the Archbishop of Canterbury placed St Edward's Crown on the King
The ceremony, which saw the King and Queen Camilla crowned, was broadcast simultaneously across a range of channels between 11:00 and 13:00 BST.
Viewing peaked at 20.4 million as the King was crowned just after midday, audience measurement group Barb said.
The figures are smaller than when an average of 26.5 million viewers tuned in for Queen Elizabeth II's funeral.
An average of 18.8 million people tuned into watch the Coronation across 11 channels and services, including BBC One, Two, ITV and Sky News.
On BBC One, a peak of 13.4 million viewers tuned in to the broadcast led by Huw Edwards and the channel had an average audience of 11.9 million, the BBC said.
ITV said an average of 3.3 million viewers watched the ceremony on ITV1 between 10:45 and 13:00.
Sky News had an average of 568,000 viewers during its broadcast of the Coronation service, while GB News had an average of 176,000 and Talk TV had an average of 14,000 viewers.
Channel 4 opted to show film Johnny English Strikes Again instead of the Coronation with an average of 138,000 people preferring to the watch Rowan Atkinson film. Meanwhile, on Channel 5 children were entertained with The Adventures of Paddington Bear and SpongeBob SquarePants.
Millions of people are thought to have watched the Queen's coronation on television in 1953 - but there are no reliable figures, making it difficult to measure against this year's ceremony. Based on surveys carried out by the BBC at the time, it is estimated that more than 20 million adults in the UK watched it.
The funeral service for the King's ex-wife Diana, Princess of Wales in 1997 was seen by 31 million on BBC and ITV - making it the highest TV audience on record.
-- Courtesy of BBC News