Sudoku

Sudoku is the world-famous logic-based number-placement puzzle. Fill a 9×9 grid so each row, column, and 3×3 box contains the digits 1 through 9 exactly once.

About Sudoku

Sudoku is the most popular logic puzzle of the modern era, with hundreds of millions of players worldwide. The rules are deceptively simple: fill a 9×9 grid with digits 1-9 such that every row, every column, and every 3×3 box contains each digit exactly once. Some cells start filled (the clues); the rest are blank. The puzzle has exactly one solution and can always be deduced through pure logic — never guessing. Our version offers Easy, Medium, and Hard difficulties, with input via keyboard or the on-screen number pad. The selected cell is highlighted along with its row, column, and box, helping you spot conflicts.

How to Play Sudoku

1. Click an empty cell to select it. The cell is highlighted along with its row, column, and 3×3 box.
2. Type a digit (1-9) on your keyboard, or click the number pad below the grid.
3. A digit appears in blue if it is your input. Pre-filled clues appear in black.
4. If your digit conflicts with the unique solution, it shows in red — that means it is wrong somewhere.
5. Press Backspace or Delete (or the eraser key) to clear an incorrect entry.
6. Fill every cell correctly to win. Pick a new puzzle anytime from the difficulty menu.

Tips and Strategy

• Always start by scanning rows and boxes for cells that have only one possible digit. These are called “naked singles”.
• Look for digits that can only fit in one cell within a row, column, or box — even if other digits could also fit there. These are “hidden singles”.
• Use pencil marks (mental notes) for cells with 2-3 possibilities. This is essential for medium and hard puzzles.
• Watch for “naked pairs”: two cells in a row, column, or box that contain the same two candidates. Those candidates can be eliminated from other cells in that unit.
• Solve in waves. Find all easy cells first, then come back for the harder cells once the grid is more constrained.
• If you get stuck, look at digits with the fewest occurrences in the grid. They often have the most constraints.
• Never guess in classical Sudoku. If you cannot find a logical move, you are missing a technique. Step away and look again.

History and Origin

Sudoku’s ancestor is the 18th-century Latin Square puzzle by Swiss mathematician Leonhard Euler. The modern Sudoku format was created by retired American architect Howard Garns in 1979 for Dell Magazines under the name “Number Place”. Japanese publisher Nikoli refined it in 1984 and renamed it Sudoku (“single number”). It exploded into mainstream global popularity around 2004-2005 when newspapers began publishing daily puzzles. There are 6,670,903,752,021,072,936,960 possible Sudoku grids and roughly 5.5 billion essentially distinct puzzles.

Variations and Game Modes

Variants include 4×4 mini-Sudoku for kids, 16×16 Sudoku with hex digits, Killer Sudoku (cages with sums), Jigsaw Sudoku (irregular regions instead of 3×3 boxes), Samurai Sudoku (5 overlapping grids), and X-Sudoku (additional diagonal constraints).

Frequently Asked Questions

How do you play Sudoku?

Fill a 9×9 grid with digits 1-9 so that every row, every column, and every 3×3 box contains each digit exactly once. Some cells start filled; deduce the rest using logic.

Is Sudoku always solvable without guessing?

Yes, in classical Sudoku. Every published puzzle has exactly one solution that can be reached through pure logic. If you find yourself guessing, you are missing a technique.

How many digits should be filled in a Sudoku puzzle to start?

A valid Sudoku puzzle must have at least 17 starting clues to have a unique solution. Easy puzzles typically have 35-45 clues, medium 28-34, and hard 22-27.

What is the difference between Easy, Medium, and Hard Sudoku?

Easy puzzles have more starting clues and require only basic techniques (naked/hidden singles). Medium adds pairs and pointing techniques. Hard requires advanced strategies like X-Wing, Swordfish, and chains.

How long should a Sudoku take to solve?

Easy: 5-10 minutes. Medium: 10-25 minutes. Hard: 30+ minutes. Speed varies hugely with experience — competitive players solve hard puzzles in under 5 minutes.

Why is my Sudoku entry showing in red?

A red digit means it does not match the unique solution. Check the row, column, and 3×3 box of that cell — somewhere there is a conflict, possibly with a digit you entered earlier.

Can the same digit appear twice in Sudoku?

Yes — across the whole grid, each digit appears exactly 9 times. But within a single row, column, or 3×3 box, each digit appears exactly once.

Does Sudoku improve cognitive function?

Studies suggest regular Sudoku and similar puzzles can help maintain cognitive flexibility and concentration in older adults. The mental benefits are modest but real, especially when puzzles are challenging.