The common claim that there are ~7.5E18 (~2.5 × 2⁶⁴) grains of sand seems to originate from this back-of-the-envelope calculation:

https://web.archive.org/web/19990117001023/http://www2.hawai...

It takes only beach sand into account, and of course there's a lot of guessing involved. It could be easily off by many orders of magnitude.