Sudoku: play, solve and generate unique-solution grids

How the solver solves and rates a sudoku grid

Nikoli's rule: every row, every column and each of the nine 3 × 3 blocks contains each digit from 1 to 9 once. A well-posed grid has exactly one solution, and it needs at least 17 clues.

Solving

For each empty cell, the solver works out the digits still possible: those missing from its row, its column and its block at the same time, which covers 20 neighbouring cells. It fills the most constrained cell first, tries its candidates one by one and backtracks at the first contradiction. It stops as soon as it has found 2 solutions, which is enough to tell whether the grid has none, exactly one or several.

Example on the starting grid

Cell at row 8, column 3: the row holds 9, 4, 8, 1 and 2; the column 3, 6, 8 and 5; the block 9 and 4. The digits already taken are 1, 2, 3, 4, 5, 6, 8 and 9, so only 7 is left, and it is indeed the answer. This is a naked single.

Generating and rating

The generator fills a complete grid at random, then removes digits one by one in random order. A removal is kept only if the grid still has a unique solution and does not require a technique harder than the target level. The difficulty is an in-house estimate: easy if naked singles are enough, medium if hidden singles are needed (only one possible place for a digit in a row, column or block), hard if locked candidates are needed, expert when these techniques are no longer enough. The clue floors of 36, 30, 26 and 22 are settings of the tool.

Limits

• The rating ignores advanced techniques (pairs, X-wing), so two expert grids can differ a lot.
• Solving a typed grid stops after 150,000 attempts; the tool then says the calculation was interrupted.

Sources and dates: Nikoli, sudoku rules; McGuire, Tugemann and Civario, There is no 16-Clue Sudoku (arXiv, 2012, revised 2013) (checked on 29/10/2026)

Frequently Asked Questions

What is the minimum number of clues for a sudoku?

17. An exhaustive computer search (McGuire, Tugemann and Civario, 2012) found no 16-clue grid with a single solution. The grids generated here have 22 to 36 clues depending on the level.

How can I check that a sudoku has only one solution?

Type it into the Solve tab. The solver looks for up to 2 solutions: if it finds only one, the solution is unique; if it finds 2, the grid is ill-posed and the tool shows one of them.

How is the difficulty estimated?

By the hardest technique required: easy with naked singles only, medium with hidden singles, hard with locked candidates, expert beyond that. It is an in-house estimate, not an official scale.

How do I note several possible digits in a cell?

Turn on notes with the pencil button or the N key, then type digits: each one is added to or removed from the cell. When you place a digit, it is removed automatically from the notes of the 20 neighbouring cells.

Is this tool really free?

Yes, all tools on Kristools are 100% free and accessible without registration.

Is my data protected?

Yes. All processing is performed directly and securely inside your browser. No personal data is sent to our servers.