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Kent's population is projected to increase by 125,800 by 2026, according to the latest figures.
The last recorded statistics in 2016 put the county's population at 1,540,400 - excluding Medway.
But it is expected to reach 1,666,200 within eight years, a rise of 8.2%.
The figures come from the Office for National Statistics, which biennially estimates how the population of England will change over the next 25 years.
Statisticians study birth and death rates, and look at how the county's population is ageing.
However, the ONS figures do not include Medway, which had a population of 277,000 in 2016.
In Kent, the percentage of the population made up by pensioners is also expected to rise - from 19.9% in 2016 to 22.2% 10 years later.
And by 2036 the ONS thinks over 65s will make up more than a quarter of the area's residents.
People are living longer due to improvements in healthcare and technology, however, this puts greater pressure on the NHS and social care.
Caroline Abrahams, Age UK's charity director, said: "The fact there are growing numbers of older people is a cause for real celebration, but these figures underline, once again, the need to ensure that our health and social care system is fit for the future.
"These figures underline, once again, the need to ensure that our health and social care system is fit for the future" - Caroline Abrahams
"The Government's decision to produce a social care green paper was welcome but realistically it will be years before its proposals impact in local areas, so action is needed now to shore up what remains of our care system."
When calculating the population estimates for Kent the ONS expects the birth rate to outstrip the death rate.
Figures also show the male population is expected to grow slightly faster than the female population.
The statisticians then factor in people moving around England and immigrating from across the world, which is called internal and international migration.
The ONS estimates there will be an overall net increase of 105,200 migrants by 2026.
The most significant increase was in migrants from other parts of England, with a net rise of 77,500 estimated.
Andrew Nash, from the ONS, commented: "While the overall populations of all regions in England are projected to increase over the next decade, reasons for these increases vary greatly depending on where you live.
"For instance, projected population change in London is mainly caused by natural change - the difference between the number of births and deaths - and not migration.
"This is because London's net inflow of international migrants is offset by a similar number of people moving to other parts of the UK.
"That contrasts with the North East, where growth is mostly down to migration.
"What's also clear is that the population is ageing in all regions, with the number of people aged 65 and over growing considerably faster than younger age groups."
The average growth rate across England was 5.9%.
And the ONS predicts that by the year 2041 the population of Kent will have grown to 1,816,900.