McDonald's, for instance, had 9.1 mg per fluid ounce, while Starbucks packed more than double that at a full 20.6 milligrams. For more on those findings, click here. The average American consumes about 200 mg of caffeine a day. According to the FDA, 80 percent of U.S. adults consume caffeine each day, with an individual intake of 200 mg. To put that in real world terms, the average caffeine-consuming American drinks two five-ounce cups of coffee or about four sodas.