What size is the average human memory, in terms of computer storage sizes?

How many megabytes, gigabytes or terabytes does the average human memory contain?
Update: I know that the human mind does not work like a computer. Maybe you could answer this by saying, for example, if the memory could store 10,000,000,000 words, you could put that as if there was 10,000,000,000 words in a word document. Just some advice.
Update 2: I realise that this is a hard question to answer, so don't worry if you can't answer. I thought it up in school today and couldn't wait to find out! Thanks to everyone who's answered already! You're all really quick. =)
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