From reading the wiki (https://en.bitcoin.it/wiki/Block_hashing_algorithm), a block hash is 256 bits.

Out of curiosity, I copied a block hash (block 125551, just a random pick) and turned it into binary, see:
$ echo 00000000000008a3a41b85b8b29ad444def299fee21793cd8b9e567eab02cd81 | xxd -b -c 4
0000000: 00110000 00110000 00110000 00110000  0000
0000004: 00110000 00110000 00110000 00110000  0000
0000008: 00110000 00110000 00110000 00110000  0000
000000c: 00110000 00111000 01100001 00110011  08a3
0000010: 01100001 00110100 00110001 01100010  a41b
0000014: 00111000 00110101 01100010 00111000  85b8
0000018: 01100010 00110010 00111001 01100001  b29a
000001c: 01100100 00110100 00110100 00110100  d444
0000020: 01100100 01100101 01100110 00110010  def2
0000024: 00111001 00111001 01100110 01100101  99fe
0000028: 01100101 00110010 00110001 00110111  e217
000002c: 00111001 00110011 01100011 01100100  93cd
0000030: 00111000 01100010 00111001 01100101  8b9e
0000034: 00110101 00110110 00110111 01100101  567e
0000038: 01100001 01100010 00110000 00110010  ab02
000003c: 01100011 01100100 00111000 00110001  cd81

Now I'm counting 4 columns (2nd untill the 5th), 16 lines, and 8 characters per set. 4*16*8=512 bits.

I must be missing something obvious, but haven't found it, yet. Pointing out the obvious is appreciated. Thanks.