## How Many People Have Been Alive Since 1970?

Today my friend Steve Spalding asked me a surprisingly deep question: How many people have been 21 years of age or older since 1970? He posited that if I couldn’t come up with a good way to estimate that, another option would be, how many people have been alive since 1970?

This turns out to be a really easy question to answer if you have the right data set, and nearly impossible otherwise. I initially started by looking up the population in 1975 and the population today, shooting for the ‘alive at any time’ number. This didn’t work out as well as I had hoped - the number I initially came up with, which was right off, was about 9.2 billion.

If, however, you go to the actual UN Data set that most of the links you find are based on ^{1}, you can download a file called “WPP2012_POP_F07_1_POPULATION_BY_AGE_BOTH_SEXES.XLS” ^{2}.
This file has estimates of the world population every year since 1950, every five years, by five year age groups.

So, we can answer the question of how many people have been aged 20 or older since 1970- start with the population aged 20 or older in 1970 (1,935,700,000) Then add the “20-24” population for every five year estimate since. Each person can be in the “20-24” group only once, so we aren’t double counting anyone. Finally, add the “Medium Fertility” estimate for the 20-24 age group for 2015, as the data was published in 2012.

The total? About 6,432,521,000 have been aged 20 or older in the world since 1970.