Google of course

I haven't seen a meta study so if you have some different data I'd like to see it.
Atheist Population - Conservapedia
I'd trust Conservapedia about as much as I'd trust a wire mesh condom. If they told me that water was wet, I'd want it verified independently,
twice.
The
2012 WIN-Gallup Global Index of Religion and Atheism found that 13% of the world's population identify as atheists. The
Pew Forum on Religion & Public Life gives a slightly higher number for 'Unaffiliated' at 16% - although that's a broader measure which does include some believers in an unidentified 'higher power'. University of Cambridge, surveying the top 50 countries with a lack of belief in god/deities (accounting for just under 60% of the global population), came up with a figure of 505-750 million in 2007. Adding the rest of the world, even if the rates of the irreligious were under 5%, would add another 140-150 million.
Buddhism, a non-theistic religion, accounts for around 475-535 million practitioners - that's somewhere between 6.5% and 7.5% of the global population.
So already, you have somewhere north of 20% of the globe's population that have no belief in gods/deities.
Then you have smaller groups which identify as religious, but again have no belief in god/deities. There's Jainism, the various atheistic sects of Hinduism, Toaism, Animism, the practitioners of traditional African and Latin American shamanistic religions, and lots of different belief systems that are 'theistic' but don't involve a deity.
You can, conservatively, chuck in at least another 100 million, or about another 1.5% of the world's population.
It depends on how you slice the numbers, but if you have a broad category of 'non-believers' to include atheists, agnostics, secularists and humanists, then conservatively you can estimate the global population at about 850-900 million. If you add the non-theistic religions/subsects, then its easily above 1.5 billion.