Sudoku X Solver and Diagonal Sudoku

The Sudoku X solver on this page takes a diagonal sudoku, also called Sudoku X or X-sudoku, and fills in the missing digits. You type or paste the clues, press Solve, and the grid is completed under the normal sudoku rules plus the rule for both long diagonals.

Everything runs in your web browser. The diagonal sudoku solver is free, needs no sign-up and nothing to install, and nothing is uploaded: puzzles are made on your device.

The page also has a Play tab for diagonal sudoku puzzles at easy, medium and hard levels, and a Print tab for a diagonal sudoku printable set with answers. Use the solver when you are stuck, or to check a puzzle you have made yourself.

How to use the Diagonal Sudoku Solver

1. Open the Solve tab. The page opens on it by default.
2. Type the clues. Click a cell and type its digit. Press Backspace to remove a digit, and use the arrow keys to move between cells.
3. Or paste the puzzle as text. Use one character per cell, with a dot or 0 for an empty cell. Spaces and line breaks are ignored, so you can paste the rows as they are.
4. Press Solve to fill the whole grid. Digits the solver adds are shown in colour, so you can see which ones it filled in.
5. To learn the method, press Next step instead. Each press makes one logical step and shows the technique’s name with a sentence explaining why.
6. Press Check uniqueness (useful for a puzzle copied by hand) to find out whether the puzzle has no solution, exactly one, or more than one.

The solver honours the diagonal rule for you, so you do not need to enter it. Use the ‘Show candidates (possible digits)’ button to see which digits are still possible in each empty cell. Press Clear to empty the grid, or Example to load a sample puzzle and try the solver on it first.

Sudoku diagonal rule

A diagonal sudoku is a normal sudoku with one extra rule: both main diagonals must also contain the digits 1 to 9 once each. The two long diagonals run from corner to corner, and on the grid they are drawn as faint lines, so you can see which cells they cover.

The normal rules still apply. Every row, every column and every 3x3 box holds 1 to 9 once each. The diagonals add two more groups of nine cells, which makes the puzzle more tightly constrained than ordinary sudoku.

How to solve sudoku X

Solve sudoku X with the same logic as ordinary sudoku, and treat the two diagonals as two more groups to check. Start by listing the possible digits for each empty cell, then fill in any cell that has only one.

A cell on a diagonal can often be ruled out sooner, because its digit must also be unused on that diagonal. The centre cell is a good place to look: it lies on both diagonals as well as its row, column and box, so it is often the most constrained cell on the grid.

Rules of sudoku diagonal

Each digit from 1 to 9 must appear exactly once in every row, every column, every 3x3 box and both main diagonals. No digit may repeat within any of these groups, and every group is complete when the puzzle is solved.

On a 6x6 diagonal sudoku the same idea applies, with digits 1 to 6 in each row, column, box and diagonal. A well-formed puzzle has exactly one solution. The Check uniqueness button tests this, which is useful for a puzzle you have made by hand.

Why use a Sudoku X solver instead of a normal one?

A normal sudoku solver ignores the diagonals. On a diagonal puzzle it may return a grid in which a diagonal repeats a digit, or report several solutions for a puzzle that is unique once the diagonal rule counts. Diagonal puzzles are often published with fewer clues for exactly that reason: the two extra groups do part of the work. The solver on this page treats both diagonals as groups in every step, in Next step explanations as well as in the full solve.

Frequently asked questions

Can I print diagonal sudoku?

Yes. Open the Print tab and choose 1, 2, 4 or 6 puzzles per page, with answers included. A preview of page 1 and its answer page appears first. Press ‘Print / Save as PDF’ to open the browser print dialog, and choose ‘Save as PDF’ as the destination to get a file.

Can I print the same diagonal sudoku puzzles again later?

Yes. Each printed puzzle has a label such as ‘Sudoku — Medium — #3’, and the page footer shows the set’s seed. Enter that seed in the Seed field under ‘Repeat or share this set’, using the same settings, and the same puzzles will be generated.

Does the diagonal sudoku solver handle 6x6 grids?

Yes. The solver works on both 9x9 and 6x6 diagonal sudoku. Choose the size that matches your puzzle before you enter the clues.

What happens if the solver takes too long?

A grid with very few clues can make the search take too long. The solver then stops and says it gave up, rather than guessing an answer. Check the clues you entered for typing mistakes, then try again.

Where can I practise diagonal sudoku?

Use the Play tab, which generates diagonal puzzles at easy, medium and hard levels. Hints and pencil marks are available while you play.

Related tools

• Sudoku solver: use it for standard sudoku puzzles without the diagonal rule.
• Hyper sudoku: use it for a variant with extra shaded regions, also known as Windoku.
• Jigsaw sudoku: use it for puzzles where the boxes are irregular shapes instead of squares.
• Killer sudoku: use it for puzzles that add cage sums to the normal rules.
• Sudoku generator: use it to make new standard sudoku puzzles.
• Printable sudoku: use it to print standard sudoku puzzles for pen and paper.
• How to play sudoku: use it if you are new to sudoku and want the basic rules and methods.