It is very difficult to put an exact figure on Green Tea caffeine. Like other drinks from cultivated plants (tea, coffee), there are numerous variables that can alter caffeine content.

Length of brewing time also affects caffeine content - the longer the brew - the higher the caffeine.

One study compared the content of dry matter from tea leaves and found Green Tea had 10-20mg per 1 gram dried matter. Black Tea had 22-28mg per 1 gram dried matter.

So as a general rule, Green Tea has less caffeine than Black Tea.

Another study (here) showed that Green Tea (in Japan) has 30mg caffeine per 200ml cup.

The figure we have included in our caffeine database is an average of these two.

Tea makers Lipton list a figure of 45mg caffeine for their 100% Green Tea drink. They claim this is from 1 to 2.5 minutes of brewing (see source below)

Green tea is also high in the amino acid L-theanine. This has a calming effect and aides with concentration. There's a myth that green tea has more caffeine than coffee, but this isn't true. L-theanine is most likely responsible for this. People will have a different level of alertness when drinking green tea which people mistake for the caffeine's effect when it is really the effect of L-theanine.

Matcha green tea can have more caffeine because of the unique way it is made. You can find the caffeine content of Matcha here.

Caffeine in Other Green Tea Based Products