The number of atheists in the world is not known within any narrow margin, and the reasons are worth setting out, because confident figures circulate freely in both directions.
Most of the large numbers quoted are not atheist numbers. Pew Research Center put the religiously unaffiliated at roughly 1.9 billion in 2020, about a quarter of humanity and the third largest category after Christians and Muslims. But unaffiliated is an answer about membership, not about belief. Pew's own later work on the same population found that large shares of people who report no religion nevertheless report belief in God or a higher power, in spirits, in fate or in an afterlife. Treating that figure as a count of atheists overstates it by a wide multiple.
The United States shows the gap plainly. About 28 per cent of American adults told Pew in 2023 that they had no religion, but only around 17 per cent of that group described themselves as atheists, which puts atheists near 5 per cent of all adults.
The larger difficulty is China, which supplies the majority of the world's unaffiliated in every dataset and therefore drives the global total. Chinese respondents commonly report no religious affiliation while making offerings to ancestors, visiting temples and holding beliefs a Western questionnaire would score as religious, because the Chinese term rendered as religion carries a sense of formal institutional membership that the practices do not involve. Pew publishes a separate methodological report on the problem. Where and how the surveys are conducted adds further uncertainty.
Then there is stigma, which cuts both ways. In states where unbelief is criminal or socially ruinous it is not declared to strangers with clipboards. In others the word atheist carries a combativeness that people who simply do not believe prefer to avoid, and they answer agnostic or nothing in particular instead. Any global figure should be read as a floor.