William weighed 7 pounds, 1.5 ounces, when he was born June 21, 1982, as the first son of Prince Charles and Diana, princess of Wales.
Charles, prince of Wales, weighed 7 pounds, 6 ounces, when he was born Nov. 14, 1948.
The weight of the royal baby's grandmother, Queen Elizabeth II, was not announced when she was born by Cesarean section in her maternal grandparents' London home April 21, 1926, British newspaper the Daily Mail reported.
In developed countries, the average weight of a baby born around its due date is about 7.5 pounds, but Kate, the duchess of Cambridge, was about a week overdue.
In the first week, a baby's body weight usually slips 3 percent to 7 percent before increasing again.
The royal baby, whose name has not been announced, is third in line to the throne after Charles and William. He is the first prince of Cambridge born in 194 years.
The last prince of Cambridge was George, born March 26, 1819, to Prince Adolphus, the 1st duke of Cambridge -- who was also the great-great-grandfather of current Queen Elizabeth -- and Princess Augusta of Hesse-Kassel, the duchess of Cambridge, a United Press International review indicated.
Kate delivered her son at 4:24 p.m. London time after at least 10 1/2 hours of labor that "progressed as normal" in the exclusive Lindo wing at St. Mary's Hospital in London's Paddington district, said Kensington Palace, the official London residence of the duke and duchess of Cambridge and Prince Harry of Wales.
"We could not be happier," William said in a statement.
William was born in the same hospital 31 years ago.
Charles issued a statement saying: "Both my wife and I are overjoyed at the arrival of my first grandchild. It is an incredibly special moment for William and Catherine and we are so thrilled for them on the birth of their baby boy.
"Grandparenthood is a unique moment in anyone's life, as countless kind people have told me in recent months, so I am enormously proud and happy to be a grandfather for the first time and we are eagerly looking forward to seeing the baby in the near future," the statement said.
The newest royal will be styled HRH Prince (name) of Cambridge after Elizabeth issued letters patent under the Great Seal of the Realm, also known as the Great Seal of the United Kingdom, when Kate was three months' pregnant.
The decree said, "All the children of the eldest son of the prince of Wales should have and enjoy the style, title and attribute of royal highness with the titular dignity of prince or princess prefixed to their Christian names or with such other titles of honor."
As for when the name will be announced, the palace said it would happen "in due course."
The BBC said it took Charles and Diana a week to name William.