The Shannon number, 10¹²⁰, is an estimation of the game-tree complexity of chess. It was first calculated by Claude Shannon (father of information theory).

The game-tree complexity of chess is now evaluated at approximately 10¹²³ (the number of legal positions in the game of chess is estimated to be between 10⁴³ and 10⁵⁰). As a comparison, the number of atoms in the Universe, to which it is often compared, is estimated to be between 4×10⁷⁸ and 6×10⁷⁹.