Current capacity (lots of math)
1 Inches (in) = 25400000 Nanometers (nm)
so... 1 square inch is 645160000000000 square nm
so now divide that by capacity they're shooting for (1TB - which is 1 followed by a bunch of zeros as far as the hard drive companies are concerned)
so... 645160000000000 / 1000000000000 = 645.16 square nm
now take the square root to find the length that the 13nm pits take up.
this is 25.4nm (which is probably correct since the 13nm pits need some area around that isn't a pit to support the structure and provide that magnetic shielding)
now... do some algebra... 25.4 / 13 = N / 100
13 is the target, 100 is the current working prototype.
N is 195.384615... lots more decimal places... anyhoo...
we get this...
38175.147928994... again... lots more decimal places... now divide the nm area by this and we have the capacity in bytes.
645160000000000 / 38175.147928994... = 16900000000
16.9 GB. Far cry from 1 TB, but it's getting there.
and hopefully I didn't miss copying a zero somewhere.