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What Is X-Sudoku? Diagonal Sudoku Rules, Explained

2026-06-22

If you've solved a few normal Sudoku and want a fresh twist, X-Sudoku (also called Diagonal Sudoku) is the easiest variant to pick up — it adds exactly one rule. You can try a board here while you read.

The extra rule

Classic Sudoku asks that every row, column, and 3×3 box contains 1–9 once. X-Sudoku adds one more constraint:

Both main diagonals (corner to corner) must also contain 1–9, with no repeats.

That's the whole difference. The "X" is the shape those two diagonals make across the grid — on ToolKoala's board they're lightly tinted so you can see them at a glance.

How it changes the solve

The diagonal rule is extra information, and good solvers learn to use it rather than fear it. The 17 cells on the diagonals get two overlapping constraints, so they often resolve earlier than in classic Sudoku — if you remember to check them. Beginners who ignore the diagonals find X-Sudoku harder; once you fold them into your scanning, it can actually be faster.

And you can't leave them out. I took a few hundred boards from ToolKoala's generator and solved them as if they were classic Sudoku: without the diagonal rule, almost every Hard and Expert board had more than one answer, and so did about nine Medium boards in ten. In those puzzles there's a square only a diagonal can decide.

A few habits help:

  • When you place a number, check its diagonal too, not just the row, column, and box.
  • The center cell sits on both diagonals — it's heavily constrained, so it's often a good place to look for a single.
  • On ToolKoala's board, selecting a square on a diagonal lights that whole diagonal, just like its row, column and box, and placing a number crosses it off the pencil marks along the diagonal too.

Is it harder than normal Sudoku?

Not because of the rule itself. A rule you can use is a clue, so with the same number of givens an X-Sudoku is, if anything, easier: more squares get pinned down. What makes it harder is that there are two more lines to scan, and that the extra rule lets a puzzle stay unique with fewer givens.

On ToolKoala it depends on the level. Easy and Medium are built so that singles alone finish them, on both pages (on the diagonal page, a single along a diagonal counts). Hard is about the same as classic Hard. Expert is where X gets tougher: it starts with 23–24 givens instead of 24–28, and about two Expert boards in three need a move beyond singles, against about two in five on classic Sudoku. If you're comfortable finding hidden singles in classic Sudoku, you're ready for X up to Hard; for Expert, read the guide's section on what to do when singles run out (pointing pairs, naked pairs).

Try it

Open Diagonal Sudoku, pick a difficulty, and keep an eye on the two tinted diagonals. Prefer pen and paper? You can print a PDF pack of diagonal puzzles too.

FAQ

What is X-Sudoku? It's Sudoku with one extra rule: each of the two main diagonals must also contain 1–9, on top of the usual rows, columns, and 3×3 boxes.

Why is it called "X"? The two corner-to-corner diagonals form an X across the grid. Those are the cells with the extra constraint.

Is X-Sudoku harder? The rule itself is extra information, so it helps you. On ToolKoala, Easy and Medium need only singles on both pages and Hard is about the same as classic; Expert X is harder, because it starts with fewer givens and about two boards in three need more than singles.

Does every X-Sudoku have one solution? Every puzzle on ToolKoala does: each is generated to have exactly one solution, diagonals included, so you never have to pick between two valid answers. That doesn't mean singles always finish it: Easy and Medium can be solved with singles alone, while Hard and Expert sometimes need pointing pairs, naked pairs or harder techniques.

— Milo 🐨