Learn how to estimate the number of grains of sand on all of Earth’s beaches. Here's Math Dude's trick to quickly estimate tough-to-calculate numbers.
Step 2: Estimate the Sand Grains Per Volume
The first sub-problem we need to solve is to figure out how many grains of sand there are in a volume of beach. We could measure this with any unit of volume we’d like—grains per cubic mile, grains per cubic kilometer, or whatever—but those would all be difficult estimates to make since they’re all really large volumes. And since the point of breaking the original problem up into sub-pieces is to come up with easier-to-solve problems, a better plan is to start by estimating the number of grains within a small volume—such as the number of grains per cubic centimeter.
How can we do that? Well, we can just gather up a bit of sand and count the grains. But instead of actually gathering a cubic centimeter of sand and counting each individual grain (which, while accurate, would also be tedious and slow), let’s make a quick and dirty estimate by lining up a bunch of grains in a row and measuring the length of the line in centimeters. The number you get will depend upon the type of sand, where it’s from, and a bunch of stuff like that—but if you do this you should get somewhere between 15 and 25 grains per centimeter (please email me and let me know what you get!). So if we assume that an average of 20 grains will fit along each side of a 1 cubic centimeter box of sand, we find that there are about 20 x 20 x 20 = 8,000 grains per cubic centimeter. Which, after a bit of fun converting units, we find is equivalent to 8,000,000,000 grains of sand per cubic meter.
Step 3: Estimate the Volume of Sand on Earth’s Beaches
Now that we know how many grains of sand there are in a cubic meter of beach, we need to figure out how many cubic meters of beach there are in the world. To find the volume of the Earth’s beaches, we need to know the typical width and depth of a beach and the total length of Earth’s coastlines. In truth, this is a complicated problem since not all beaches are the same width or depth and not all of Earth’s coastline is covered by beaches. But the name of the game here is to come up with a ballpark estimate, so let’s see what we can figure out.
In truth, I’m really not sure what the average width of a beach is, but it seems reasonable to estimate that a typical beach is about half a football field wide—or approximately 50 meters. As for the average depth, it seems reasonable that a typical beach is something like half as deep as it is wide—since deeper than that doesn’t really seem like “beach” anymore—which is about 25 meters.
As for the length of Earth’s coastlines, I’m going to assume that the coastline of each of the classic seven continents is long enough to circle the Earth twice. Why? Because that’s how I’m deciding to make my educated guess. There are lots of other ways you could do it, and my method could certainly be half as big as it should be or maybe twice as big as it should be, but I can live with that uncertainty. The trick is to come up with some method for calculating a good educated guess for the number you’re trying to find (this is where a bit of art comes into estimation), and the particular method I’ve chosen does exactly that. Which means that, since Earth’s circumference is roughly 40,000,000 meters, there are approximately 14 x 40,000,000 = 560,000,000 meters of coastline on Earth. So the volume of Earth’s beaches is about 50 x 25 x 560,000,000 = 700,000,000,000 cubic meters!