The rule of the square is a visual shortcut that tells you at a glance whether a king can catch an opposing passed pawn, without calculating a single move. You mentally draw a square whose one side runs from the pawn to its promotion square: if the defending king can step into that square on his move, he catches the pawn; otherwise, the pawn strolls home to promotion.

How to draw the square

Take the pawn’s square and its promotion square: those are two opposite corners of the square. The other two corners follow from completing a perfect square. For example, a white pawn on a5 promotes on a8: the square has corners a5, a8, d8, and d5 (four files wide, since there are three ranks left to travel plus the starting rank). If the black king is already inside that square, or can step into it in one move, he catches the pawn. Otherwise, it’s lost for him.

The a5 pawn promotes on a8: the square covers a5-a8-d8-d5. The black king on a1 is far outside it, and he will never catch the pawn, even moving first.

Using the rule in a game

This rule saves you from calculating a pawn-versus-king race move by move, which can be slow and error-prone in a tense middlegame. It has two limits worth knowing: it assumes nothing else is happening on the board (no piece blocking the pawn or helping the king), and whoever is to move counts double: a king just outside the square can sometimes still step in if it happens to be his move. Always check who is to move before drawing a conclusion. This rule pairs well with lessons learned back in the openings about controlling the center, since a solid center is often what produces the passed pawns that decide the endgame.

Frequently asked questions

Does the rule of the square work for any pawn?

Yes, including an edge pawn (a-file or h-file), as long as no other piece interferes on the board. The square is drawn the same way regardless of which file the pawn is on.

What happens if the king enters the square just in time?

If he steps into the square exactly on his move, he’s guaranteed to catch the pawn, even if it still takes him several more moves to actually reach it. Once inside the square, the king can always keep pace with the pawn.

Does the rule also apply to a king escorting the pawn?

No, it only covers the race between the pawn and the opposing king trying to stop it. If the king escorting the pawn also has to worry about a nearby enemy king, you need to calculate that position case by case.