You'll find all sorts of information about varying caffeine levels in different types of tea, and supposed measurements of how much caffeine there is per cup – in reality this kind of information is inaccurate. Without a laboratory to hand, there are far too many variables to be able to say with conviction how much caffeine is in a cup of tea, or which teas have more caffeine than others. However there are some general, interesting facts regarding Tea and Caffeine.
Tea vs. Coffee
Though it is true that dry tea contains more caffeine than dry coffee, the opposite is true when they are brewed. When comparing brewed coffee and tea the levels of caffeine in coffee are far higher, simply because the amount of coffee required for a cup is far more than the amount of tea.
Tea contains L-theanine, an amino acid which works with the caffeine and is said to have a calming effect without reducing caffeine alertness – this can explain why you don't "crash" when drinking tea.
Factors
The amount of caffeine in any tea depends on a number of factors, including the method and length of brewing or steeping. With tea, studies also show that leaf location on the tea plant, and growing processes could also affect caffeine content.
You might have read, or heard, that the degree of oxidation increases the amount of caffeine in tea – this claim has no scientific basis and is false.
Infusion Time
Most likely, the greatest impact on caffeine content is the water temperature and length of infusion. Black, Oolong, Green and White tea leaves themselves have similar caffeine content, but a tea steeped for five minutes in boiling water is going to transfer more caffeine to the cup than a tea steeped for less time at a lower temperature.
Decaf Tea + Caffeine-free Infusions
Decaffeinated tea is not completely caffeine free as the process used to 'wash' the caffeine off is not 100% effective.
If you are looking to eliminate caffeine completely, you need to make the switch to herbal infusions and tisanes. All real tea comes from the same plant, Camellia sinensis, which always contains caffeine. Infusions like Chamomile, Rooibos and Peppermint, are made from botanicals that are naturally caffeine free.
