How many people have ever lived on Earth?

How to get close to this number

Integrate population over time. World population was tiny for most of human history and has only been in the billions for a couple of centuries, so the cumulative total is dominated by a very long stretch of small numbers plus a very short stretch of large ones. That integration gives a total in the low hundreds of billions.

The share alive now is the framing that makes it useful: around seven percent of everyone ever born is alive today. That is far higher than most people guess, and it follows directly from the population explosion of the last two centuries — a species that grew eightfold in two hundred years concentrates a large share of its total history in the present.

The uncertainty is very large and honestly flagged. The estimate depends on assumptions about prehistoric birth rates, infant mortality and where to start counting, none of which can be observed. Changing the assumed starting date or birth rate moves the answer by tens of billions, which is why the figure is quoted with wide error bars and a named methodology.