Tea has more caffeine than coffee if measured by dry weight; however, when brewed, tea has less caffeine than coffee as tea is generally not prepared to release a large amount of caffeine. An 8-ounce serving of brewed coffee contains 95 to 200 mg of caffeine, while an 8-ounce serving of black tea contains 14 to 70 mg of caffeine. An 8-ounce serving of instant coffee has 27 to 173 mg of caffeine, and a specialty drink, such as a mocha or a latte, has 63 to 175 mg of caffeine.
Green tea contains more caffeine than black tea, with every 8-ounce serving containing 24 to 25 mg of caffeine, according to Mayo Clinic. The amount of caffeine can also be determined by the color of tea, as darker colored teas contain less caffeine, while yellow or pale teas like the Japanese green tea gyokuro contain significantly more caffeine. In addition, the number of times the tea is brewed can also indicate the amount of caffeine.
In a study published on Science Direct conducted by the Department of Nutrition and Food Science at Auburn University, black, green, herbal and Formosa oolong teas were each brewed three times. Formosa oolong tea revealed the least amount of caffeine after three brews at 23.8 mg/g, and green tea revealed the highest amount of caffeine at 36.6 mg/g. The amount of caffeine in black tea was slightly less than green tea at 32.8 mg/g, while herbal tea was caffeine free.