The best number I could find is that over 500 hours of video are uploaded every minute. That’s 720,000 hours of video uploaded per day or 263 million hours per year.
At a modest estimate of around 1.5GB/hour for 1080p video (I think this is a very low estimate, since YouTube has much higher quality available now, and also stores different versions of each video), this works out to 376PB per year!
Other factors to consider:
* Transcoding. YouTube servers are constantly transcoding video into many different formats, given a single input file.
* Serving. YouTube isn’t served from one central data centre. They operate a huge CDN in order to serve users at the edge of the network. This means some percentage of that 376PB figure must be duplicated many times across many different data centres.