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How to Play Sudoku

  1. Click a blank cell to select it, then press a digit (1–9) to place your answer.
  2. Every row, column and 3×3 box must contain each digit exactly once.
  3. Pencil mode lets you jot candidate numbers in small text without committing.
  4. Stuck? Use Hint to reveal one correct cell. Use Erase to clear a wrong entry.
  5. Check highlights any conflicting cells in red.

FAQ

No. There is no time limit — solve at your own pace.
Yes — each generated puzzle has exactly one valid solution, verified with a human-solver algorithm.
Pencil mode lets you write small candidate digits in a cell without committing to an answer. It is useful for tracking possibilities.
There is no limit on hints — use as many as you need. Each hint reveals one correctly placed digit.
Five levels: Easy (basic scanning), Medium (hidden singles), Advanced (naked & hidden pairs), Hard (X-Wing), and Expert (Swordfish). All puzzles have exactly one unique solution.

Sudoku: History, Strategy & Facts

The Origins of Sudoku

Despite its Japanese name, Sudoku was invented in 1979 by Howard Garns, a retired American architect, who called it 'Number Place'. It was published in Dell Pencil Puzzles and Word Games magazine. The puzzle reached Japan in 1984 when Nikoli, a Tokyo-based publisher, introduced it under the name 'Suji wa dokushin ni kagiru' — meaning 'the digits must remain single' — later shortened to Sudoku.

The Global Puzzle Explosion

Sudoku remained a niche Japanese pastime until 2004 when Wayne Gould, a retired Hong Kong judge, created a computer program to generate puzzles quickly. He convinced The Times of London to run a daily Sudoku, and within months the puzzle spread to newspapers worldwide. By 2005, Sudoku had become the most popular logic puzzle on the planet, spawning books, TV shows, and the first World Sudoku Championship in 2006.

The Mathematics Behind 9×9

A standard 9×9 Sudoku grid has approximately 6.67 sextillion possible completed grids. The minimum number of given clues required to guarantee a unique solution is 17 — proven by exhaustive computer search in 2012. No valid puzzle with only 16 clues has ever been found. On our Expert difficulty, puzzles start with as few as 17 givens, which represents the absolute mathematical boundary.

Solving Techniques: From Basics to Expert

Beginners solve Sudoku by scanning rows, columns, and boxes for missing digits. Intermediate players learn hidden singles and naked pairs — techniques that eliminate candidates by logical deduction. Advanced solvers use patterns like X-Wing, Swordfish, and XY-Wing, which track candidate chains across multiple rows and columns. The most challenging puzzles require a combination of these methods working together.

Pencil Marks: The Secret Weapon

Experienced players write small candidate digits in empty cells — known as pencil marks or notations. This technique transforms Sudoku from a memory challenge into a visual logic exercise. By maintaining accurate candidate lists, you can spot patterns that would otherwise be invisible. Our built-in Pencil mode makes this technique effortless, displaying up to nine tiny candidates in each cell.

Interesting Facts About Sudoku

The World Sudoku Championship has been held annually since 2006, attracting competitors from over 30 countries. The fastest recorded solve time for a competition-standard puzzle is under two minutes. Sudoku is used in cognitive research as a tool for studying logical reasoning and working memory. Unlike crosswords, Sudoku requires no language knowledge at all — the digits could be replaced with any nine distinct symbols and the puzzle would work identically.

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