What are the rules of Sudoku?

Fill a 9×9 grid so that every row, every column, and every 3×3 box contains the digits 1 through 9 exactly once. Some digits are given as clues. Use logic to deduce the remaining cells.

How many difficulty levels are available?

Six levels: Easy (38–45 clues), Medium (30–37 clues), Hard (27–29 clues), Expert (24–26 clues), Master (22–23 clues), and Extreme (17–21 clues). Higher levels require more advanced solving techniques.

Can Sudoku be solved without guessing?

Easy through Expert levels can always be solved with logic alone. Master and Extreme levels may occasionally require advanced techniques like X-Wing, Swordfish, or bifurcation, though our generator aims for logically solvable puzzles at all levels.

What are pencil notes in Sudoku?

Pencil notes (or candidates) are small numbers you write in a cell to track which values are still possible. Toggle Notes mode and tap digits to add or remove pencil marks. They help you apply elimination techniques.

What is the history of Sudoku?

The modern Sudoku was created by American architect Howard Garns in 1979 as 'Number Place'. It was popularised in Japan by Nikoli in 1986 under the name 'Sudoku' (meaning 'single number'). It became a worldwide phenomenon in 2004-2005 after being featured in The Times of London.

Play Sudoku Online

The classic 9×9 number puzzle with 6 difficulty levels from Easy to Extreme. Notes mode, undo & error checking — all free.

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Select a cell and enter a number

What Is Sudoku?

Sudoku is the world’s most popular number puzzle. The goal is simple: fill a 9×9 grid so that every row, every column, and every 3×3 box contains the digits 1 through 9 exactly once. Some cells are pre-filled as clues — you deduce the rest through logic.

Despite using numbers, Sudoku is not a maths puzzle. You could replace the digits with letters, colours, or symbols and the logic would be identical. It’s pure pattern recognition and elimination.

History of Sudoku

The modern Sudoku was created by American architect Howard Garns in 1979 and published by Dell Magazines as “Number Place”. In 1986, Japanese puzzle company Nikoli introduced it to Japan under the name Sūdoku (meaning “single number”) and added two key innovations: symmetrical clue placement and a maximum of 30 given digits.

Sudoku became a worldwide phenomenon in 2004–2005 when retired Hong Kong judge Wayne Gould developed a computer program to generate puzzles and convinced The Times of London to publish them daily. Within months, virtually every newspaper in the world carried Sudoku puzzles.

Today, the World Sudoku Championship (held annually since 2006) attracts competitors from over 30 countries, and billions of Sudoku puzzles are solved every year in print, apps, and online.

How to Play

Choose a difficulty level. Easy has more clues (38–45 given digits); Extreme has very few (17–21).
Tap a cell to select it. Then tap a number on the pad to fill it in.
Use Notes mode (pencil icon) to write small candidate numbers in cells when you’re not yet sure of the answer.
Check your work with the Check button — errors are highlighted in red. The puzzle also auto-detects when you’ve solved it correctly.
Undo any move, or Erase the selected cell.

Difficulty Levels Explained

Easy (38–45 clues): Solvable with naked singles alone. Great for beginners learning the rules.
Medium (30–37 clues): Requires hidden singles — scanning rows, columns and boxes for the only place a digit can go.
Hard (27–29 clues): Needs naked and hidden pairs/triples plus box-line reduction.
Expert (24–26 clues): Advanced techniques like X-Wing, pointing pairs, and more complex elimination chains.
Master (22–23 clues): Requires Swordfish, XY-Wing, and multi-step chained deductions.
Extreme (17–21 clues): The hardest puzzles. 17 clues is the proven minimum for a unique solution. May require trial-and-error or very advanced techniques.

Strategy Tips

1. Scanning

Look at each 3×3 box and ask: “Where can this digit go?” If only one cell in a box can hold a particular number, that’s it. This is called a hidden single and is the most fundamental technique.

2. Naked Singles

If a cell has only one candidate remaining after eliminating values present in its row, column, and box, then that’s the answer. Use pencil notes to track candidates.

3. Pointing Pairs

If a candidate in a box only appears in one row (or column), you can eliminate that candidate from the rest of that row (or column) outside the box.

4. Naked Pairs & Triples

If two cells in a row/column/box share exactly the same two candidates, those two values can be eliminated from all other cells in that unit. The same logic extends to triples and quads.

5. X-Wing

If a candidate appears in exactly two cells in each of two different rows, and those cells share the same two columns, the candidate can be eliminated from those columns in all other rows. This powerful technique is essential for Expert and above.

Sudoku vs Other Puzzles

vs Kakuro: Kakuro uses sums as clues and involves arithmetic; Sudoku uses only placement logic with no maths.
vs Futoshiki: Futoshiki adds inequality constraints between adjacent cells. Sudoku uses only the row/column/box uniqueness rule.
vs Nonograms: Both are grid puzzles solved by elimination, but Nonograms reveal a picture while Sudoku reveals a number pattern.

Frequently Asked Questions

Fill the 9×9 grid so every row, column, and 3×3 box contains digits 1–9 exactly once. Some digits are given; deduce the rest by logic.

Six: Easy, Medium, Hard, Expert, Master, and Extreme. They range from 45 clues down to as few as 17.

Easy through Expert can always be solved with logic. Master and Extreme puzzles may require advanced techniques or bifurcation (educated trial-and-error).

Small candidate numbers you write in a cell to track which values are still possible. Toggle Notes mode, then tap digits to add/remove candidates.

17 clues. This was proven in 2012 by Gary McGuire and colleagues at University College Dublin. No valid Sudoku with 16 or fewer clues exists.

More Puzzle & Strategy Games

If you enjoy Sudoku, try these other free games on our site:

Kakuro

Cross-sum puzzle — fill digits so segments add up to the given clues.

Futoshiki

Latin square puzzle with inequality constraints between neighbouring cells.