The Rule of the Square in Chess: Can the King Catch the Passed Pawn?

A passed pawn is running for the last rank. Can the enemy king catch it? You don't have to count move by move. Just draw a square: one side runs from the pawn's square to its promotion square, and the square extends toward the enemy king. If the king, with the move, can step into the square, it catches the pawn. If it can't, the pawn promotes. When you play chess online and a pawn race comes up, this is the quickest way to judge it.

Key position

How to play it

Draw the square

The pawn on a4 is four squares from a8, so one side of the square runs from a4 to a8, and the square is the area bounded by a4, e4, e8 and a8.

White to move: the pawn goes to a5 and the square shrinks to a5, d5, d8, a8. The black king on f5 can only reach the e-file in one move. It can't get in, so it can't catch the pawn.
Black to move: the black king steps to e4, e5 or e6 and is inside the a4 square. It catches the pawn.

White moves first and pushes: the black king is always one step short, and the pawn becomes a queen.

Black moves first and steps into the square: chasing diagonally, the black king arrives on b7 just in time and takes the pawn next move. Draw.

Moving diagonally is the fastest way for a king to travel: each diagonal step gains a rank and a file at once. When you chase a pawn, head diagonally toward it.

Pawn on its starting square: it can move two

A pawn on its starting square can move two squares on its first move, so draw the square as if the pawn were already one rank further up.

The pawn is on a2, so draw the square from a3 (a3 to f8). The black king on f4 is just inside it. With Black to move, it chases the pawn down by c6, and the pawn can't escape.

Common mistakes

The rule of the square applies when it's the defender's move. The most common mistake is chasing the pawn without heading diagonally toward it, which wastes a move.

Mistake: Kf4?? The king steps down instead of toward the a-file. After a5 the black king is outside the new square and can't catch up.

Also remember: if the attacking king stands in the way of the chasing king, or the defender's own pawns block its path, you have to calculate it out instead of just looking at the square.

Practice puzzles

Black to move. The white pawn is on c5. Can the black king catch it? Which square?

Show answer

1... Kf5 Only Kf5. The square of the c5 pawn is c5, f5, f8, c8, and f5 is right on its edge. Every other move stays outside the square, and the pawn promotes.

White to move. How do you keep the black king from catching the pawn?

Show answer

1. h4 h4! The pawn jumps two squares and the square becomes d4 to h8. The black king on b4 can only reach the c-file next move, so it can't get in. After h3, the square is c3 to h8, the black king steps to c4 or c5 and catches the pawn: draw.

FAQ

What's the quickest way to draw the square?

Count the squares from the pawn to its promotion square, count the same number sideways, and the four corners give you the square. If the defending king, with the move, can step into it, it catches the pawn.

Why is a pawn on its starting square a special case?

Because the pawn can move two squares on its first move. So with the pawn on its starting rank, draw the square as if it were one rank further up, or you'll be off by one move.

Is the rule of the square always right?

With just kings and the pawn, yes. If other pieces are in the way, or the attacking king can block the chasing king's path, you need to calculate it.

Try it yourself

The rule of the square and the opposition are the two basic calculations in pawn endgames. Other endgames are in Chess endgames. To practice, go to free chess, and in your pawn endgames, draw the square first, then decide whether to push.

Practise in a game