Sand comes in many different sizes. Very fine sand is defined as containing particles that are no smaller than one twentieth of a millimeter and no larger than one tenth of a millimeter, while coarse sand is defined as containing particles that are from one half a millimeter to one millimeter in size. For purposes of our calculation let us suppose that all the particles of sand on the beach we are considering are one quarter of a millimeter in diameter. This is the boundary between what is called fine sand and medium sand. Let us also suppose that the grains of sand are tiny cubes, not spheres. In one millimeter, we can line up neatly in a row four particles of this kind of sand, and over one hundred in one inch. Sixteen such grains of sand can be placed in one square millimeter, more than 10,300 into one square inch, and well over one million fit into one cubic inch. There are 1,728 cubic inches in one cubic foot. So each cubic foot of sand would contain somewhat more than 1.8 billion grains of sand of this size. With this in mind. we can roughly estimate the number of grains of sand on an imaginary beach.

Let's assume that our beach is one mile long and hundred feet wide. This makes the area of our beach 528 thousand square feet. Assuming a layer of sand to the depth of only one foot, for a total of 528 thousand cubic feet in the entire beach, the number of grains of sand on this beach would be over 950 trillion, or close to one quadrillion.

This may sound like a lot of grains of sand. However, as far as soil goes, sand particles are fairly large. Most soils are made up of particles that are much smaller. More than 60% of all soils have particles smaller than one twentieth of a millimeter. And clay, which is the type of soil found on about 16% of the land, is composed of particles that are smaller than one five hundredth of a millimeter. One cubic foot of clay, even of the type with the largest particles, contains far more than 3 quadrillion particles of soil, more than three times the number of grains of sand on a beach one mile long and one hundred feet wide.