A population projection applies assumptions about future fertility, mortality, and migration to a base population, then rolls the numbers forward year by year. The UN World Population Prospects is the most widely cited set of projections.

The UN publishes several scenarios. The "medium variant" assumes fertility converges toward replacement; it is the headline number most people quote. The "high" and "low" variants bracket the plausible range. For 2100, the range spans from about 9.0 billion (low) to 12.5 billion (high) — a difference larger than the entire population of Africa today.

Most of the uncertainty comes from Africa. If African fertility falls as fast as it did in Asia, the population peaks lower and sooner. If it falls more slowly, the peak is higher and later. Education, especially for girls, is the single strongest predictor of how fast fertility falls.

Projections are not predictions. They describe what would happen if assumptions hold. In practice, fertility has often fallen faster than projected (as in Bangladesh and Iran) and sometimes slower (as in parts of West Africa).

For our 20-year projection tool, we apply each country's current growth rate forward. This is a simple linear extrapolation, not a full cohort-component model. It is useful for building intuition but should not be treated as an official forecast.