The rule of the square is a shortcut for pawn races: draw the square from the pawn to its promotion file, and if the lone king can step into that square it catches the pawn.
Instead of counting moves, picture a square whose side runs from the pawn to its queening square. If it’s the king’s move and the king can enter that square, it will catch the pawn; if it can’t, the pawn promotes.
The geometry works because the king moves one square in any direction, including diagonally, so it can always keep pace with a pawn once it’s inside the square. A pawn on its starting square counts from the third rank, since it may advance two.
This rule lets you judge a passed-pawn race at a glance — invaluable in time trouble and when deciding whether to push a passer or chase one. Just remember it assumes the king’s path isn’t blocked and it’s genuinely the king’s turn.
Take the file from the pawn to its promotion square as one side, then complete the square toward the king’s side of the board. If the king (on its move) can step onto any square inside it, the king catches the pawn.
Yes. A pawn that hasn’t moved can advance two squares, so you measure the square from the third rank in front of it rather than from the pawn itself, giving the pawn a head start.
BetterChess is a practice tool — we make no guarantee you'll reach 1800 or any rating. Definitions are standard chess terminology; every diagram position is checked legal with the same engine the board runs.