Free Sudoku Solver & Sudoku Game Online

A Short History of Sudoku

Number-placement puzzles with Sudoku-like Latin square constraints appeared in French newspapers in the late 19th century, though without the modern box constraint. The puzzle in its current 9×9-with-3×3-boxes form is generally credited to Howard Garns, an American puzzle constructor, whose design was first published in a US puzzle magazine in 1979 under the name "Number Place." The puzzle was popularized in Japan during the 1980s by the publisher Nikoli, who gave it the name Sudoku, and it became a global phenomenon in the mid-2000s after syndication in newspapers across the UK, US and beyond.

What made Sudoku spread so quickly is the same thing that makes it a good fit for a browser page: the rules take under a minute to learn, a single puzzle is self-contained, and the difficulty can be tuned smoothly from a two-minute warm-up to an hour-long, technique-heavy challenge — which is exactly the range this page's 1,000 campaign levels aim to cover.

Sudoku Rules Explained

A standard Sudoku grid is 9 rows by 9 columns, subdivided into nine non-overlapping 3×3 boxes. Some cells start filled in with "given" digits; the rest are empty. To solve the puzzle you fill every empty cell so that:

• Each row contains the digits 1–9 exactly once.
• Each column contains the digits 1–9 exactly once.
• Each 3×3 box contains the digits 1–9 exactly once.

A properly constructed puzzle has exactly one valid solution reachable through logical deduction — no guessing required, though guessing (with backtracking if it fails) will also eventually find the answer, which is exactly how this page's solver falls back when logic alone stalls.

Naked & Hidden Singles

A naked single is a cell with only one remaining candidate digit — every other digit is already used somewhere in its row, column or box. A hidden single is a digit that can only go in one cell within a row, column or box, even if that cell still shows other candidates. These two techniques alone solve the majority of Easy and Medium puzzles.

Naked & Hidden Pairs and Triples

When two cells in the same row, column or box share exactly the same two candidates, no other cell in that group can hold either digit — those candidates can be safely eliminated elsewhere. The same logic extends to triples with three cells and three shared candidates.

Pointing Pairs & Box-Line Reduction

If a candidate digit within a box only appears in cells that share a single row or column, that digit can be eliminated from the rest of that row or column outside the box — and the reverse also holds, eliminating candidates from a box based on a row or column pattern.

X-Wing & Beyond

Harder puzzles need pattern-based eliminations like X-Wing, where a candidate forms a rectangle across exactly two rows and two columns, letting you eliminate that candidate from the rest of those columns. The Hint engine on this page recognizes these patterns where possible, and falls back to a guided guess-and-check search for the toughest Grandmaster puzzles.