4.4 million people. That's roughly the same population size as the Republic of Ireland or Croatia. Half of London. Or, put another way, four more Birminghams.
That is the increase in the UK's population over the next decade, according to the Office for National Statistics. And it throws up major questions about 'social integration' - the extent to which people in Britain interact with others from different backgrounds to their own.
Ever since Thomas Malthus published his book An Essay on the Principle of Population in 1798, there has been much public debate about population growth and what this means for life in Britain.
We have always worried about population growth but disaster hasn't struck. Malthus was concerned about what came to be called The Iron Law of Population - the idea that rising population would lead to an over-supply of working people, driving down wages and creating widespread poverty. He was wrong and right. Right that bigger populations create new challenges. But wrong as there is no Iron Law. We can handle growth, with time to prepare.
Malthus's anxieties have prevailed in another respect - that we might not rub along well together. Some have thought that there is an 'Iron Law of Diversity' - a tipping point in our ability to cope with an ever increasing variety of identities, formed around nationality, faith and ethnicity.
In extremis, these worriers have talked about 'rivers of blood'. On the contrary, the diversity evangelists preach 'Kum ba yah', insisting 'move on, no issue here'. The truth is that we will be fine, but only if we prepare. The question then is: are we preparing?
In Britain today there is a paradox at play. We are an increasingly mixed society. But we are a society that is becoming less inclined to mix with one another.
A recent survey for the Social Integration Commission found that the average Briton has fewer social interactions with people who are different to themselves, along the lines of age, ethnicity and income. It found that highly diverse areas - including London - whilst mixed, are not integrated. In short, in Britain we have achieved tolerance but not yet integration.
Why does this matter?
A lack of social integration is likely to make it harder to address the many challenges that modern Britain faces - from education, to housing to care for the elderly.
Without action to promote greater social integration, the danger grows that in the face of the many complex problems of the future, instead of asking 'how can we solve this together?' the British people will ask 'who can we blame?'
Inter Faith Week takes place across Britain next week. Communities of all faiths and none will come together to celebrate their differences. Churches, synagogues, mosques, temples and community centres will open their doors.
This is what we do in our programmes for young people as the UK's leading provider of the National Citizen Service. During the summer, we brought together 30,000 15- 17 year olds from different backgrounds to learn new skills that assist them in solving shared challenges in their communities.
Assigned to groups for the duration of a four week residential, participants are matched with other young people from their own communities but who we know to be from different income, faith and ethnic backgrounds. And this is the important thing. Because we are much less likely to mix with people who are less like we are.
Look at your own friendship group. If you are university educated, are your friends? If you are a home owner, are your friends? If you are in secure employment, are your friends? When did you last socialise with an elderly person who is not a relative? What proportion of your friendship circle belongs to a different ethnic group?
The world over, communities in which people take part in shared experiences demonstrate greater capacity to deal with adversity.
Accommodating 4.4 million people in the next decade will not be easy. We all need housing, schools, care and parks. There is no Iron Law of Population nor an Iron Law of Diversity. But unless we take preparations to make it work, our increasingly diverse neighbourhoods will become increasingly segregated communities. We must ask ourselves do we want this for our country, as we seek to take on the complex future that lies ahead?
Jon Yates is Director and Co-Founder of The Challenge - Britain's leading charity promoting social integration