According to the 2011 census, the total population of the United Kingdom was around 63,182,000. It is the 22nd most populated country in the world. Its overall population density is 259 people per square kilometre (671 people per sq mi), with England having a significantly higher population density than Wales, Scotland and Northern Ireland. Almost one-third of the population lives in England's southeast, which is predominantly urban and suburban, with about 9 million in the capital city of London, the population density of which is just over 5,200 per square kilometre (13,468 per sq mi).
The population of the United Kingdom is considered an example of a population that has undergone the 'demographic transition' – that is, the transition from a (typically) pre-industrial population with high birth and mortality rates and only slow population growth, through a stage of falling mortality and faster rates of population growth, to a stage of low birth and mortality rates with, again, lower rates of population growth. This population growth through 'natural change' has been accompanied in the past two decades by growth through net international migration into the UK.
The United Kingdom's high literacy rate (99% at age 15 and above) is attributable to universal public education introduced for the primary level in 1870 (Scotland 1872, free 1890) and secondary level in 1900. Parents are obliged to have their children educated from the ages of 5 to 18 (raised from 16 in 2013), and can continue education free of charge in the form of A-Levels, vocational training or apprenticeship to age 18. The Church of England and the Church of Scotland function as the national churches in their respective countries, but all the major religions found in the world are represented in the United Kingdom.
The UK's population is predominantly White British. Being located close to continental Europe, the countries that formed the United Kingdom were subject to many invasions and migrations from the continent, especially from Scandinavia, including Roman occupation for several centuries. Historically, British people were thought to be descended mainly from the different ethnic stocks that settled there before the 11th century: pre-Celtic, Celtic, Anglo-Saxon, Viking and Norman. Although Celtic languages are spoken in Scotland, Cornwall, and Northern Ireland, the predominant language overall is English. In North and West Wales, Welsh is widely spoken as a first language, but much less so in the South East of the country, where English is the predominant language.
Roman Britain had an estimated population between 2.8 million and 3 million at the end of the second century AD. At the end of the fourth century, it had an estimated population of 3.6 million, of whom 125,000 consisted of the Roman army and their families and dependents. The urban population of Roman Britain was about 240,000 people at the end of the fourth century. Roman Britain's capital city Londinium is estimated to have had a population of about 60,000. Londinium was an ethnically diverse city, with inhabitants from across the Roman Empire, including natives of Britannia, continental Europe, the Middle East, and North Africa. There was also cultural diversity in other Roman-British towns, which were sustained by considerable migration, both within Britannia and from other Roman territories, including North Africa, Roman Syria, the Eastern Mediterranean, and continental Europe.
During the Industrial Revolution child mortality decreased dramatically. The proportion of children born in London who died before the age of five decreased from 74.5 per thousand in 1730–1749 to 31.8 per thousand in 1810–1829. According to Robert Hughes in The Fatal Shore, the population of England and Wales, which had remained steady at 6 million from 1700 to 1740, rose dramatically after 1740.