How would you begin to work out how many grains of sand are there on earth?

I’ve seen a description of calculating the number of grains of sand on beaches.
(Does not include rivers, streams, or ocean beds beyond the visible coast.) They did studies of types of sand, coarseness and size, and how many grains were in average volumes of those types of sands. E.g. fine grain sand has many more grains in a quart, and there is less space between grains.
Then it was a matter of cataloging all the known beaches with the length and average width and depth. So you have the volume at each beach and with the type of sand can determine how many grains there.
Add ‘em all up.

Unless you meant actual gains of sand which might be an erosion calculation or some analysis of the cultural advances of the world’s sands, and I have no idea how to do that.

Unless you really had to know an exact number you could take samples of a specific size and estimate from there based on the acreage of each sandy area.

This is a perfect example of a Fermi Question or Fermi Problem. You work with assumptions and estimates. Sometimes your numbers are too high. Sometimes they are too low. If there is no bias when all the factors are used or multiplied together the result ends up being reasonably close. Physicists and engineers use this method in certain cases. They are fun.
I’d start with my assumptions.
For example:
There are X km of coastline in the world.
the average depth of sand is 3 meters deep
the average Sand bar or beach width is 100 meter.
the average grain of sand is an irregular shape with a mean diameter of 0.5 mm and estimated volume of sphere (4/3)* Pi * (0.25 mm) ^3 4/3* Pi* 0.00025m^3 ( 6.65×10^-11 grains per meter x packing density
Packing density of the irregular shape is 0.5 (50% free space)
Therefore the number of grains of sand in the world is (putting everything in meters): 1000 * X * 3 * 100 / ( volume of single grain of sand x .5)

It depends on how accurate an answer you need. If it must be exact – fuggedaboutit, but for an estimate, measure one cubic foot of sand, divide by the size of one grain, then multiply by the amount of land covered with sand. Don’t forget the deserts as well as the beaches, and include the bottom of all the oceans.

@YARNLADY Actually, once you clear the continental shelf, much of the “ocean floor is rocky“http://en.wikipedia.org/wiki/Seabed#Seabed_features, and not covered by sediment other than that left by ocean life and volcanic ash from undersea volcanoes and hydro-thermal vents.

@ETpro Oh, I did not know that.