Population simulation tool: model demographic change in the 21st century
An interactive tool that lets you test how changes in fertility rates, life expectancy, and migration rates will affect future populations.
May 18, 2026
How will populations change in the future? Demographers model their scenarios using assumptions about key demographic changes, most notably fertility rates, life expectancy, and migration rates.
In our work, we often present the projections published by the United Nations. But no one knows with certainty what the migration rate will be in decades, or how many children people will have. So it’s worth asking what the population will look like if things turn out differently from what the UN or other demographers assume.
We have built a population simulation tool that helps answer this question.
First, select your country from the dropdown in the title. Second, in the first panel, change any of the three inputs: fertility rates, life expectancy, and net migration rates.
Then, in the second and third panels, you’ll see what the output of your assumptions would mean for population numbers and age structure in the country.
A note on methodology
We’ve built this tool to be deliberately simple: small enough to run in a browser and nevertheless capable of modeling whole populations based on the key demographic inputs.
It is less detailed than the models expert demographers use, and our aim is not to replace them. We hope it’s useful for developing an understanding of the difference that changes in demographic variables can make to the size and structure of future populations.
In our technical documentation, we provide a much more detailed walkthrough of our methodology and its limitations. Here, we’ll provide a much more brief summary.
The model draws on historical population and demographic data from the United Nations (UN).
The tool uses the UN’s historical data for 2023 for these metrics, then allows the user to select values for three metrics — fertility rates, life expectancy, and net migration — at three points in time: 2030, 2050, and 2100.
The model then shows what would happen to the total population and age structure of that country over the course of this century.
Our model applies the cohort-component method. This approach is also used by the UN in its forecasts, and is commonly used by expert demographers, researchers, and statistical offices.
The approach follows groups of people — cohorts — over time, and applies the expected inputs for fertility, mortality, and migration to each cohort.
Cohorts are broken down by age and sex. The projected change for the first year is then based on four changes: deaths, aging, migration, and births. The output of this model run for one year then becomes the population cohort input for the next year, and this repeats.
Contributors & acknowledgements
By Daniel Bachler, Sophia Mersmann, and Hannah Ritchie.
With contributions from Marwa Boukarim.
Daniel Bachler worked on the initial concept and prototype of the model. Sophia Mersmann built the visualization. Hannah Ritchie worked on the model and wrote the text. Marwa Boukarim provided input on design.
Many thanks to Mallika Snyder for her valuable feedback and suggestions on this tool. And thanks to Max Roser and Edouard Mathieu for comments and editorial feedback.
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Daniel Bachler, Sophia Mersmann, and Hannah Ritchie (2026) - “Population simulation tool: model demographic change in the 21st century” Published online at OurWorldinData.org. Retrieved from: 'https://ourworldindata.org/featured-viz/population-simulation-tool' [Online Resource]BibTeX citation
@article{owid-population-simulation-tool,
author = {Daniel Bachler and Sophia Mersmann and Hannah Ritchie},
title = {Population simulation tool: model demographic change in the 21st century},
journal = {Our World in Data},
year = {2026},
note = {https://ourworldindata.org/featured-viz/population-simulation-tool}
}
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