An average grain of sand weighs approximately 50 micrograms, and a grain of very fine sand can weigh as little as 350 nanograms. The size of grains of sand vary because they are not uniform.
In the imperial measurement system used in the United States, a grain is a standard unit of measure that has a fixed weight, regardless of to what type of material it pertains. An imperial grain is equal to 0.0648 grams, or 64.8 milligrams. This unit of measurement was set in reference to a grain of wheat, which is much larger than an average grain of sand.