Sudoku X, also called diagonal sudoku, adds one rule to the usual three: the two long diagonals, from corner to corner, must each contain the digits 1 to 9 exactly once. This Sudoku X solver enforces all five rules, proves that your puzzle has one solution, and explains how to reach it step by step.
How to use it
- Type the clues into the grid (the diagonals are shaded) or paste 81 characters with dots for blanks.
- Press Solve for the whole answer with every step listed, or Next hint for one move at a time.
- Switch to Solve it yourself to enter your own digits; Check my progress marks any that are wrong.
What changes with the diagonals
The diagonals act as two extra units. A cell on a diagonal has more neighbors, so singles appear sooner, and the center cell sits on both diagonals, its row, its column and the middle box. The hints treat a diagonal like a row: a hidden single can be the only place for a digit on the main diagonal, and an intersection removal can work between a diagonal and a box (if a digit's only places on a diagonal fall inside one box, it is removed from the rest of that box).
The other techniques carry over: pairs, triples, X-wings, XY-wings and simple coloring all use the diagonal as one more set of cells that cannot repeat a digit. Uniqueness techniques such as the unique rectangle are switched off on this page, because their logic assumes only rows, columns and boxes.
A worked example
The example has only 19 clues, because the diagonals do some of the work. Press Next hint a few times: early steps are hidden singles in boxes, then box/line reductions; later ones include an XY-wing and simple coloring, which gives it an Expert rating.
Tips
- Start with the center cell and the four corner boxes, where the diagonals cross the most units.
- If the solver reports two solutions, check that your source really is a diagonal sudoku; a classic puzzle entered here may become unsolvable, and the reverse can leave more than one answer.
To play diagonal grids, open sudoku X online, with three levels and a puzzle of the day.
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Questions
What is Sudoku X?
A sudoku where both main diagonals must also contain 1 to 9 once, in addition to every row, column and box.
Is Sudoku X harder than normal sudoku?
Not necessarily. The extra rule gives you more information, so puzzles often have fewer clues; difficulty depends on the techniques needed, which the solver rates for you.
Can I add windoku boxes or even/odd cells?
Those rules are available on the sudoku solver page under the rule chips, together with the diagonal rule.
Why does the hint mention the anti-diagonal?
The anti-diagonal runs from the top-right corner to the bottom-left. Like the main diagonal, it must hold each digit once.