I was trying to use an online sha-256 calculator to verify blocks, but I have had no luck. I was just stringing together the "header" and applying the sha-256 algorithm twice. Then I noticed that I get different results to "hello" than is displayed on the wiki. It shows two rounds of the hash algorithm. I get a match for the first round, but not on the second. Any idea what is going on here? Thanks

https://en.bitcoin.it/wiki/Protocol_specification

Any chance you could show me how you did those hashes? If it were an endian problem, I'd expect to get the wrong hash for "hello" on both cases, not just one.

For the block you posted, I didnt get the right hash in either case. Am I to remove the the " previous block" text and just leave the values when I do the computation?

Keep in mind that the most significant bits are on the right.  The first hash has a leading zero, which is actually a trailing zero.  It is important to the second round.