Coffee has about twice the amount of caffeine as tea. According to the
Bluebird Tea Company, a cup (12 oz) of coffee has about 210mg of caffeine, while the same amount of black tea would have about 120mg, and green tea 70mg. In the case of tea, it is also noted that the temperature and amount of time the tea was steeped in affects how much caffeine is leached into the water from the tea leaves.