How many grains of sand are there in the world?

According to the University of Hawaii's answer:

There are seven quintillion five quadrillion grains of sand on all the beaches of the world. That's a 75 with 17 zeros following!

"75 with 17 zeros following" (7.5x10^18) is seven quintillion, five hundred quadrillion. But what's an extra 495 quadrillion with so many variables in the equation?

Consider more possibilities...

With oceans rising and the deserts expanding, the question of grains of sand on the beach is kind of arbitrary. I think we would do better to ponder a more global picture: an estimate of all "sand" particles on the entire earth's surface, (including that which is covered by salty, briny, or fresh seas and rivers).

Of course "sand" varies by composition, but is defined by size of particle. Tough questions then arise regarding the unexplored ocean floor, which includes a lot of silt (the same composition, but different size per particle than sand). Perhaps we could correct for that based on volume of silt, (or gravel, for that matter), per grain of sand under the sea.

With massive erosion occurring due to water runoff and flooding caused by widespread deforestation, the intractability of the problem becomes as troublesome as estimating the number of stars in the expanding universe. Isn't that the analogy that captures our imagination, anyway - stars versus grains of sand?

But another answer is:

The exact number is not known.

The exact number can't be known to mere mortals. The number of grains of sand is constantly changing as rock is worn away daily into more sand and tons of sand are melted daily to make glass and other products. Then you come to the dilemma of deciding the point at which sand ground fine enough, when do you start to call it dust.