In the United States, the average shot glass contains 44 milliliters of fluid. The U.S. government has not set a standard for the size of a single shot.
Shot glasses are small, thick-walled glasses used to serve hard liquors, such as whiskey. These glasses are used all around the world, and vary a bit in size between 20 to 60 milliliters in volume. However, the standard single shot is about 44 milliliters in the U.S. A double shot is about 89 milliliters in volume.
While the U.S. government has not set a standard size for shots, the state of Utah did set the standard size at 44 milliliters or 1.5 ounces.