Since caffeine in green tea occurs naturally, the amount depends largely on the variety of tea plant, its growing conditions and the way it is processed and brewed. The average amount of caffeine in an 8-oz (230-ml) serving of green tea is around 30 to 50 mg.
The amount of caffeine in your drink can also be influenced by the type of green tea you choose and how you prepare it. For example, bagged tea tends to be more caffeinated than loose leaf teas. This may be because the tea leaves in tea bags are crushed up, so more caffeine is extracted and infused into the drink.
Additionally, powdered green tea have a higher caffeine content than both bagged and loose green tea.
The longer you brew your tea and the hotter the water, the more caffeine will make its way into your drink.