How will populations across the world change in the 21st century? 🔧 Explore for yourself with our new interactive tool! Demographers publish projections using assumptions about key demographic changes, most notably fertility rates, life expectancy, and migration rates. But no one knows for sure how many children people will have decades from now, or how migration will shift. So it’s worth asking what the population would look like if things turn out differently from what the UN or other demographers assume. Our colleagues Daniel Bachler and Sophia Mersmann built a population simulation tool that lets you do just that — for every country in the world. Pick a country, adjust the assumptions, and see how the projections change, for both total population and age structure. For instance, what would happen if fertility rates recovered to replacement level, or migration was cut in half?