To begin to understand this we have to first look back to the founding fathers of the United States and we also need to look at the meaning of the word dollar.
The word “Dollar” actually means a weight measure of silver, 371.25 grains, to be exact.
The American silver dollars are actually heavier, since other metals were added for durability. But the 371.25 grains of silver that went into the coin, as a standard, was the dollar value.
The American silver dollar matched in weight an unbroken chain of accepted monetary units that reached back through the Spanish Milled Dollar, the Dutch Daller, back to the German Thaler , the Roman Silver Denarius.
(The German Thaler came from a silver mine that sold its product in coins of an exact weight)
The Coinage Act of 1792 defined that the United States dollar was to exactly match in weight the silver dollars in use around the world, and then defined the gold dollar to be that amount of gold which would equal the value of "silver" in a silver dollar. The gold dollar had 24.75 grains or 1/15 the weight of the silver in a silver dollar.
Calculating real value:
If you are lucky enough today to have a Roman Silver Denarius even though the Roman Empire is long gone, the money that Rome issued still has worth because the coins themselves had inherent worth. (Besides their collector value today)
Long after the collapse of the Roman Empire, their silver coins were still being used as money, because the silver in the coin itself did not depend on the issuing government for its worth. That’s right, the government that issued the coins was no longer in existence, however the money still had value, that value being 371.25 grains of silver.