"The SHA-256 algorithm is surprisingly simple, easy enough to do by hand...Doing one round of SHA-256 by hand took me 16 minutes, 45 seconds. At this rate, hashing a full Bitcoin block (128 rounds)[3] would take 1.49 days, for a hash rate of 0.67 hashes per day (although I would probably get faster with practice). In comparison, current Bitcoin mining hardware does several terahashes per second, about a quintillion times faster than my manual hashing. Needless to say, manual Bitcoin mining is not at all practical.[5]"
http://www.righto.com/2014/09/mining-bitcoin-with-pencil-and-paper.html
http://www.righto.com/2014/09/mining-bitcoin-with-pencil-and-paper.html