My theory is that Paul Morphy's rating in todays standards, if he happened to come from the grave and was playing as well as he was in his prime from the 1800s his fide rating would be about 2638; while not world champion level, he could still be considered a strong grandmaster.  I came to this conclusion by looking at Morphy's match/tournament scores of his games that were not played at odds or other handicaps.  Also I only mentioned opponents that Morphy played a substantial amount of games against (at least 12) in order to increase the accuracy of my conclusion (this also factors in the idea that chess players often have 'bad days', and with all the traveling players did they often were sick for the first few games in a match). 

His record against Eugene Rousseau in match play was 45/50; which would make his rating at least 358 points higher than Rousseau.

Against Louis Paulsen in tournament and match play his record was 9.5/12; making his rating at least 226 points higher than Paulsen.

Against George Hammond in a match Mophy scored 15/16; making his rating at least 412 points higher than Hammond.

Against John William Schulten in a match Morphy scored 23/24; making his rating at least 457 points higher than Schulten

Against Henry Edward Bird in a match Morphy scored 10.5/12; making his rating at least 320 points higher than Bird.

and finally Morphy scored 14/17 in matches against Adolf Anderson; making Morphy at least 257 points higher than Anderson.

This astounding record is 117/131; 89%.

The rating points are based on his win percentages against those mentioned players, also I only choosed players  that Morphy played a substantial amount of games against in order to get accurate results, and all of the players I listed were pretty good.

I estimated that this lot of masters today would have an average fide rating of todays standards of at the 'very least' 2300, so 2300+338=2638 (338 is average amount of points Morphy is higher than his "rivals.") Also when i say 'very least' it should be noted Adolf Anderson in particular was 'estimated' to be over 2600 by Aprad Elo (the inventor of the rating system in the first place).  One should also note that Aprad Elo himself believed Morphy was 2690.   

On top of this evidence it is said that Morphy often played 8 blindfold games at a time while still managing to win the great majority of the games.  I asked one of my freinds who is a Grandmaster (2496 fide atm) how many blindfold games he could play while still keeping most of his strength, and he replied that he was sure he could play 3 at the same time and maybe a 4th without over exerting himself.  Morphy being able to easily play twice as many blindfold games further proves that he was not just any 'weak grandmaster, strong international master level', and that he could in fact be a strong 2600 rated Grandmaster.

I came to the idea of how much rating points Morphy was stronger than an opponent by plugging in his win percentage into this chart that corresponds with elo ratings http://www.chessville.com/Reference_Center/WinPercentageExpectancies.htm.

1. ^ Edo Historical Chess Ratings – Morphy, Paul
2. ^ Jeremy Spinrad, Collected results 1836–1863
3. ^ C. Sericano, I grandi matches 1850–1864

which are summarized in http://en.wikipedia.org/wiki/Paul_Morphy

I think this is a solid guess.  I forgot what rating chessmetrics (similar to what you just now did but ridiculously in depth) put Morphy at but I think it was around that.

Adolf Anderson, Louis Paulson, & Henry Bird all would have been rated higher thatn 2300, enough so - IMHO - that your average of 2300 is waaaay too low.  I think you need more empirical evidence than just your supposition.

The Chessmetrics site estimates (scientifically) that Morphy's peak rating would have been 2743.  Other peak ratings from your list above:

Anderssen = 2744
Bird = 2635
Hammond = not given
Paulson = 2710
Schutten = not given

Chessmetrics uses a method to compensate for rating inflation, but it is seriously flawed.  Neither Anderssen, Bird, nor Paulson played at any such level.  Play over their games - read the notes which have been tested over more than a century and see the sort of mistakes they made.  Compare to players of that rating today - it doesn't work.  Today's 2744 players don't make nearly the dumb mistakes Anderssen did.  Maybe he was 2544 at his peak, which was probably London 1851.

Seriously, look at the Chessmetrics ratings for the lesser master through the late 19th and early 20th centuries, you will find them all laughably overrated.

Morphy's strength cannot be approximated with any scientific accuracy because he was too much stronger than the players he faced.  Remember, his chess career was a way for him to kill time - having passed the bar exam at 18, he had to be 21 to practice in Louisiana, so he played chess.  He never accepted money for playing, except when Anderssen insisted on the traditional stakes and Morphy quietly returned the purse to Anderssen's wife. 

I think the chessmetric ratings are just used to determine the dominance of a player in his time, and that we can compare this dominance from era to era?

It's not strength per se that is being measured.

We'll also find that the ratings are not equivalent to FIDE ratings, if we just look at the numbers for modern players whose ratings we are well aware of.

For the record chessmetrics ratings are only meant to compare players of the same time period and thus are highly inaccurate for comparing players of different periods.

Chessmetrics doesn't take into account the different standard practices of the 19th century,  or, more precisely, it treats 19th century player the same as modern players, which tends to ignore many of the things that not only made the 19th century more charming but actually defined many players' relative skill - the very measuring tool, I would imagine of retro-rating. While I'm by no means a statician, I am firmly in EDO chess'  retro-rating camp particularly when it comes to pre-20th century players (EDO only rates players up to the year 1900). see: http://www.edochess.ca/Edo.explanation.html to help understand the differences in systems.

Appropos to this discussion, you might read down on the link provided above to a section called "Results (updated 2010)" in which Morphy is talked about in some detail. Rod Edwards explains why Morphy is assigned a peak rating of 2821. 

I think I remember seeing an estimate of 2680-90 somewhere, but that seems too low.  I'd hazard a guess around 2730s today accounting for inflation.  If he got up to date with theory he'd be even higher.

Morphy may have played at 2600 strength in the openings that were popular in his day, the King's Gambit, Giuoco Piano, Evans Gambit etc. But if he played against modern GMs he would likely never even get any of these openings on the board. It's a bit like saying that Newton was on the level of a modern physicist based on his accounts of Newtonian physics while today's field is much broader.

 As stated in the original post he in fact did score about 88/100.  It should be noted many of Morphy's 'blunders' were made on purpose in attempts on his part to finish the game faster, as stronger players often do against much weaker opposition. 

I agree with NM tonydal. You can't possibly compare players of different times. You can only compare them relative to their contemporaries. And it gets paricularly difficult in comparing 19th and 20th or 21st century players. 19th century players often rated themselves through piece odds (there was no real chess clock until the latter part of the century, so even time odds wasn't possible) and these games were perfectly valid though, in truth, they required a different skill set, casual games were usually treated as seriously as match games  (there were few tournaments until the last quarter of the century).  Players were generally amateurs who devoted relatively little time to the game (compared to professionals of today). There was no real chess instruction, few books (and books were expensive) or public libraries, no databases other than what was in the books few people had or in magazine.

It was indeed a very different world back then.

John William Schutten

Just for the record: John William Schulten

It's like saying Napoleon sucks as a general because he didn't know how to deploy nuclear weapons.  If you took a modern grandmaster and magically erased from his knowldege,  all the advances in chess since 1850, and made him play against Morphy in a match without seconds; I think most mordern grandmasters would be in deep trouble.

For some reason the OP left out a match that ought to have been included: Morphy-Harrwitz 5.5-2.5.

exactly, to analyse Morphy's games in light of modern theory is completely ridiculous. Ratings should only reflect how they fared against their contemporaries, not their play in terms of modern theory. And in that regard, only Capablanca, Fischer and Kasparov come close to Morphy, the only players with an aura of invincibility about them.

If Morphy came back from his grave playing like he did in the 1800s his rating would be 2100 at the most