It is 64,000,000,000 Bytes which translates into 59.6 GB, minus ~2 GB for OS and stuff.
Edit: Just wanted to point out that it is real 64GiB not 64GB (http://en.wikipedia.org/wiki/Gibibyte)
Why do they cell it 64GB then? Why not call it 59GB? Why not put a 69GB hard drive and then properly call it 64GB?
I just purchased a 2TB western digital elements external hard drive and it has exactly 2TB (2,048GB). They stuck to their word. That's why I ask.