What Is The Longest Possible Game Of Chess?

What Is The Longest Possible Game Of Chess?

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The longest chess game ever played in an official tournament as of writing this post was Nikolić vs Arsović played in Belgrade in the year 1989, This game lasted 269 moves and took 20 hours to complete! The game ended up being a draw:

When I first heard this, two things came into my mind. First, how did this game end up being more than five times longer than the average game? Second, How long is the longest possible chess game and how would we figure this out? To discover this, we need to take a look at a few of the rules that will play a big factor in finding the answer.

Rule #1: The 50 move rule

Preventing the game from playing indefinitely, in this rule the capture of a piece or a pawn move must be made within 50 moves of the previous pawn move or capture otherwise either player can claim a draw and in this case if a draw can be claimed it will be.

Rule #2: 3 Move Repetition

Simply put, if a position is repeated on the board 3 times a draw can be claimed so we will also be avoiding this carefully.

Rule #3: Insufficient Material

If there isn't sufficient material on the board to checkmate either of the kings, the game will immediately be declared a draw, in this case we want our final piece to be a rook or a queen so we can add another 50 moves to our total move count.

Rule #4: Checkmating and the capture of the kings

The capture of a king is NOT a possible move, however if a king is in check (currently under attack by an enemy piece) and cannot escape then the game will be ended, our dream position is having every single piece off of the board except for the kings.

Next we must also take into account the fact that one move is equivalent to two tempi, tempi meaning an individual piece moving from white or black. when both white and black use their tempi moves, the move counter will go up by one. This is very important because later we will need to look closely at how many tempi will need to be lost in order to find out how long the longest possible chess game can be.
When we take into account the fact that we can make 50 moves before needing a pawn move or capture, which moving forward we will now call a "reset" or "refresh" of this number as it counts up to 50. But how many times can we reset this rule before we are no longer able to? Well we need to look at the value of the pieces and how many refreshes each one offers!
It is important to remember that the king cannot be captured!
With all of the information we have looked at we can deduce an equation to see how long the longest chess game could hypothetically be:
To clarify, 50 represents how many moves we can make before we have to use a reset by capturing a piece or moving a pawn. The square bracket represents how many available refreshes there are, 30 being the amount of pieces able to be captured, 16 being the amount of pawns on the board and 6 being the amount of moves they can make before promoting and no longer being able to refresh the 50 move rule other than being captured as they are already accounted for in the number 30.
But is 6300, the number we get when we use this equation the actual answer? No! The problem is that the pawns aren't able to make it to the other side of the board without capturing a piece or a pawn! This means our number is too high, but by how much?
There is actually a solution to this! The most optimal way to continue past this point is waste 8 refreshes, how is this done? Well we know that pawns are the most valuable pieces so it is vital that we promote as many as possible and in this case we can promote every single one by giving up refreshes! We do this by sacrificing pieces in order to turn the pawn structure into the following:
Unfortunately there is no way around this and since a pawn move and a capture both counts as a reset and both are done at the same time, we now conclude that 8 refreshes must be lost. When we do this we can now update the previous equation:


Here the 50 also represents the maximum moves available before a reset and 8 represents the amount of resets we have lost to achieve the desired structure. Using this equation we now are at the number 5900 and we are almost at the final answer, but what are we missing? Well if we recall back to the idea of tempi and how each move requires two tempi, we also know that the "50 move rule" then has 100 tempi, and if we were to capture a piece or move a pawn one tempi too early we lose half a move, well in order to play out the entire game there will be several points in which we lose a tempi, lets look at them!

Tempi Loss #1

We lose our first tempi at move 150 when white has no more pieces to sacrifice as if white moves a pawn to sacrifice more pieces we lose a tempi, instead we now have black sacrifice four of their pieces since they are now able to move 4 pieces out of their pawn chain to give white the desired structure:

Tempi Loss #2

The second tempi loss will occur on move 349 when white has achieved the desired structure and now must return the favour and fix all of blacks pawn structure.

Once this happens it is now turn for black to also capture all of whites pieces (except for the pawns and king of course!) and finally promote all of their pawns.

Tempi Loss #3

The stage at which this tempi is lost may be different for others depending on whether or not black decided to capture whites remaining rook, however I did decide to keep the piece on the board. Black has now promoted all of their pawns, I decided to make every single pawn a knight as any other piece makes it more difficult to prevent a potential checkmate.

It is now whites turn to capture all of blacks pieces and to start promoting all of their pawns

Tempi Loss #4

The fourth and final tempi will be lost on move 5498 as now it is blacks turn to capture all of whites pieces

Now black captures all of whites remaining pieces and the game concludes on the move 5898.

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