Filling in the form, which takes about 10 minutes, can be done on paper, smartphone, tablet or computer.
For those unfamiliar with technology, assistance can be provided with filling out the form online and submitting it.
The census helps local and national government to obtain a snapshot of the entire population at one point in time.
This helps in future planning for services such as healthcare, education and transport.
Read more:
Census facts and figures
This year marks the 22nd census.
The first census was held in Great Britain in March 1801.
It recorded the combined population of England, Wales and Scotland at 10.9 million, living in 1.8 million houses.
It has been held every 10 years, except for 1941, when it did not take place because of World War II.
By the 2011 Census, the UK population – which also includes Northern Ireland – had grown to 63 million, of which 32 million were female and 31 million were male.
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