user23209: For  layman, can someone explain how you calculate the number of available positions in a chess game, and how do you get the huge number of 10^120 or 10^46?

user17100: One way to do it is think about what the smallest computer encoding of a chess position (in bits). The maximum size for a given encoding provides an upper-bound on the number of chess positions. For some really tight upper bounds check out https://codegolf.stackexchange.com/questions/19397/smallest-chess-board-compression/19446#19446. This gives an upper bound of 2^160 (10^48) positions, which is a bit high, but not that much higher than the true value.

user33776: Assume that only checkmate or stalemate ends the chess game. It implies that every legal position in the classical chess remains legal. Then, the number of legal positions in chess does not exceed 53005918951815626260641171706639462843262017900515420, please see Lemma 16 in
http://ssrn.com/abstract=3978669
The upper bound in Lemma 16 is not minimal because takes into account many positions which are impossible to achieve in a game.
For every p \in {0,...,8}, any position with p white pawns cannot contain 2+8-p white bishops on the squares of the same colour.
For every p \in {0,...,8}, any position with p black pawns cannot contain 2+8-p black bishops on the squares of the same colour.
If the white king is in check then Black cannot have a move.
If the black king is in check then White cannot have a move.

user9192: We can easily get a reasonably good upper bound on the number of positions. At any point in time, each player has 16 pieces of which the 8 pawns can perhaps be promoted to a knight / bishop / rook / queen. Consider all captured pieces to be off the board but part of the position (this would increase the number of positions but that is fine because we only want an upper bound). Thus in each position the number of combinations of pieces each player has is less than 58. Place the kings on the board first. There are less than 642 ways to do that. Line up the 30 other pieces in any order and place them one by one on the board or off the board (captured). There are at most 6330 ways to do that. This gives a total of less than (58)2×64^2×63^30 < 6×1068 positions. Double that to include whose turn it is. I had ignored castling rights; you can check that it is negligible by similar calculations.
This bound is very much smaller than what you gave in your question, even though it is an extremely loose upper bound, because you had a misconception. The number of possible positions is not at all the same as the number of possible games. For example, there are less than 642 positions with just the 2 kings, but at least 2100 possible games starting from any such position (under the 50-move rule but ignoring 3-fold repetition rule for easy estimation). Moreover, the 10120 estimate is assuming the game lasts 40 moves (2 plies) and each move has about 1000 possibilities. The real total number of possible games (including stupid ones) is obviously way more than that, since we already have 2100 possible KK games alone!

23209: <p>For  layman, can someone explain how you calculate the number of available positions in a chess game, and how do you get the huge number of 10^120 or 10^46?</p>
 17100: <p>One way to do it is think about what the smallest computer encoding of a chess position (in bits). The maximum size for a given encoding provides an upper-bound on the number of chess positions. For some really tight upper bounds check out <a href="https://codegolf.stackexchange.com/questions/19397/smallest-chess-board-compression/19446#19446">https://codegolf.stackexchange.com/questions/19397/smallest-chess-board-compression/19446#19446</a>. This gives an upper bound of 2^160 (10^48) positions, which is a bit high, but not that much higher than the true value.</p>
 33776: <p>Assume that only checkmate or stalemate ends the chess game. It implies that every legal position in the classical chess remains legal. Then, the number of legal positions in chess does not exceed 53005918951815626260641171706639462843262017900515420, please see Lemma 16 in</p>
<p><a href="http://ssrn.com/abstract=3978669" rel="nofollow noreferrer">http://ssrn.com/abstract=3978669</a></p>
<p>The upper bound in Lemma 16 is not minimal because takes into account many positions which are impossible to achieve in a game.</p>
<p>For every p \in {0,...,8}, any position with p white pawns cannot contain 2+8-p white bishops on the squares of the same colour.</p>
<p>For every p \in {0,...,8}, any position with p black pawns cannot contain 2+8-p black bishops on the squares of the same colour.</p>
<p>If the white king is in check then Black cannot have a move.</p>
<p>If the black king is in check then White cannot have a move.</p>
 9192: <p>We can easily get a reasonably good upper bound on the number of positions. At any point in time, each player has 16 pieces of which the 8 pawns can perhaps be promoted to a knight / bishop / rook / queen. Consider all captured pieces to be off the board but part of the position (this would increase the number of positions but that is fine because we only want an upper bound). Thus in each position the number of combinations of pieces each player has is less than 5<sup>8</sup>. Place the kings on the board first. There are less than 64<sup>2</sup> ways to do that. Line up the 30 other pieces in any order and place them one by one on the board or off the board (captured). There are at most 63<sup>30</sup> ways to do that. This gives a total of less than (5<sup>8</sup>)<sup>2</sup>×64^2×63^30 &lt; 6×10<sup>68</sup> positions. Double that to include whose turn it is. I had ignored castling rights; you can check that it is negligible by similar calculations.</p>
<p>This bound is very much smaller than what you gave in your question, even though it is an extremely loose upper bound, because you had a misconception. The number of possible positions is <strong>not at all</strong> the same as the number of possible games. For example, there are less than 64<sup>2</sup> positions with just the 2 kings, but at least 2<sup>100</sup> possible games starting from any such position (under the 50-move rule but ignoring 3-fold repetition rule for easy estimation). Moreover, the 10<sup>120</sup> estimate is assuming the game lasts 40 moves (2 plies) and each move has about 1000 possibilities. The real total number of possible games (including stupid ones) is obviously <strong>way</strong> more than that, since we already have 2<sup>100</sup> possible KK games alone!</p>
