As the deadline looms, it's important to understand how best to fill out the survey – and why.
Because of the pandemic, we know less about the shape and size of our society than we have for decades.
The UK is facing a population crisis with birth rates at an all-time low.
There were an extra 623,000 deaths in the UK in the year to mid-2018 – an increase of 3%.
Whatever happens with Brexit, the UK population is projected to increase in size, become more ethnically diverse and shift to a structure which is older.
Understanding population density takes more than just arithmetic – that's where mapping can help reveal which countries and cities are really getting cramped.
Life expectancy has been steadily improving in the UK for 110 years. Until now.
Some scenarios put the UK population at just under 80m by 2039. Here are the facts.