Does Sudoku Go Diagonal? The Rule, Explained

No. In classic sudoku the diagonals do not count. The only rule is that each row, each column and each of the nine 3×3 boxes contains the digits 1 to 9 exactly once. A digit can appear twice (or more) on a diagonal and the grid is still correct. The rule that the two long diagonals must also hold 1 to 9 belongs to a separate variant called diagonal sudoku, or Sudoku X.

Nikoli, the Japanese publisher that named the puzzle, states the whole rule set in two lines on its official sudoku rules page: place a number from 1 to 9 in each empty cell, and make sure every row, column and bold 3×3 block contains all the numbers from 1 to 9. Diagonals are not mentioned.

Why people think the diagonals count

The confusion usually comes from two places. First, a finished grid looks so regular that it feels like every straight line should work. Second, diagonal sudoku is common in puzzle books and apps, often without a clear label, so someone who learned on that variant carries the extra rule into classic puzzles.

The grid above is the solution to the example puzzle on Wikipedia's sudoku article. Check any row, column or box and you will find 1 to 9 once each. Check the diagonal from top left to bottom right and you will find repeats. Most published classic grids look like this, because nothing in the rules pushes the diagonals toward 1 to 9.

Diagonal sudoku (Sudoku X): when the diagonals do count

In diagonal sudoku, the two main diagonals are treated as two extra units. Each of them must contain 1 to 9 exactly once, on top of the normal row, column and box rules. The World Puzzle Federation uses this variant in its competitions and lists it in the rules booklets for the World Sudoku Championship. A diagonal puzzle is usually marked with shaded diagonals or an X in the title. If you see neither, assume the diagonals are free.

The extra constraint makes a puzzle easier to finish from the same number of clues, because each diagonal cell sits in four units instead of three. Setters compensate by giving fewer clues. Our guide to sudoku variants covers Sudoku X next to killer, jigsaw and hyper sudoku.

Other rule questions people ask

Can a number repeat in sudoku?

Only across different units. A digit appears exactly nine times in a finished grid, once in every row, once in every column and once in every box. It cannot repeat within any row, column or box. It can repeat along a diagonal, as shown above, and it obviously repeats across the grid as a whole.

Do you have to use every number from 1 to 9?

Yes. Every row, column and box has nine cells and must hold nine different digits, so each one uses all of 1 to 9. The digits are just symbols. You could replace them with letters or colours and the puzzle would work the same way, which is why sudoku needs no arithmetic.

How many boxes are in a sudoku?

A standard grid has 81 cells, arranged as 9 rows, 9 columns and nine 3×3 boxes. That makes 27 units in total. Each cell belongs to exactly three of them: its row, its column and its box. In diagonal sudoku there are 29 units.

What is the minimum number of clues?

Seventeen. A 2012 exhaustive computer search by Gary McGuire, Bastian Tugemann and Gilles Civario showed that no 16-clue puzzle has a unique solution (arXiv:1201.0749). Newspaper and app puzzles usually give between 22 and 35 clues. We cover the search and the 6.67 sextillion possible grids in how many sudoku puzzles are there.

Must a sudoku have only one solution?

By convention, yes. Nikoli's two-line rules do not say it, but every reputable publisher only prints puzzles with exactly one solution, and solvers rely on that. A grid with two valid answers forces a guess at some point. Every puzzle in Sudoku Royale is checked for a unique solution before it is served.

Is guessing allowed?

Nothing forbids it, but a well-made puzzle never requires it. Each step can be found by logic, from hidden singles up to patterns like the X-Wing. The full sudoku rules guide walks through a first puzzle step by step.