Sudoku Game
Enter a number
Keyboard Shortcuts
Arrow keys — navigate cells
1–9 — place digit
N — toggle notes mode
H — hint
Ctrl+Z — undo
Backspace / 0 — erase
Classic 9x9 Sudoku puzzle — four difficulty levels, notes, hints, and auto-save
Sudoku is one of the world's most popular logic puzzles, enjoyed by millions of people every day across every continent and age group. The name comes from a Japanese abbreviation meaning 'the digits must remain single,' and while the puzzle reached mainstream popularity via Japan in the 1980s and 1990s, its mathematical foundations trace back to the Latin squares studied by the Swiss mathematician Leonhard Euler in the eighteenth century. Today, Sudoku appears in newspapers, dedicated apps, and now right here in your browser — completely free, with no download or account required. The rules are beautifully simple. A standard Sudoku puzzle is a 9×9 grid divided into nine 3×3 boxes. At the start of the puzzle some cells are pre-filled with digits from 1 to 9; these are called 'givens' or 'clues' and you cannot change them. Your task is to fill every empty cell so that each row, each column, and each 3×3 box contains all of the digits from 1 through 9 exactly once. Every valid Sudoku puzzle has exactly one unique solution — no guessing is ever required if you approach it systematically. This free online Sudoku game offers four difficulty levels to suit every skill level. Easy puzzles begin with 36–46 clues, leaving roughly a third of the grid blank and requiring only basic single-candidate logic to solve. Medium difficulty (28–35 clues) introduces more complex reasoning and is ideal for players comfortable with the basics who want a genuine challenge. Hard puzzles (22–27 clues) demand advanced techniques such as naked pairs, hidden pairs, and pointing pairs. Expert difficulty, with just 17–21 clues, approaches the theoretical minimum of 17 clues needed for a uniquely solvable puzzle and will test even seasoned solvers. The game includes a full set of features designed to make your solving experience smooth and enjoyable. Pencil notes (also called candidate marks) let you record which digits might belong in a cell when you are not yet certain — a technique used by every competitive solver. The undo button lets you step back through your moves without penalty. A hint button reveals the correct value for the selected cell when you are truly stuck. The elapsed timer starts automatically on your first move and records exactly how long each puzzle takes you. All of these features are available on desktop and mobile alike. Beyond the core gameplay, this implementation adds several quality-of-life features that competitive Sudoku sites charge for or hide behind accounts. Auto-save means your puzzle progress is stored in your browser's local storage — refresh the page and your game continues exactly where you left off. Same-digit highlighting lights up every other cell on the board that contains the same number as the one you have selected, helping you quickly see where a digit is already placed. Peer highlighting shows the entire row, column, and 3×3 box of the selected cell in a softer shade, making it effortless to spot which digits are already used in those regions. The number pad also shows how many placements remain for each digit, so you can see at a glance when a number has been fully placed and can grey it out. Full keyboard support makes this Sudoku game accessible to power users and those who prefer not to use a mouse. Arrow keys navigate from cell to cell, the digit keys 1–9 place numbers, Backspace or Delete erases a cell, N toggles notes mode, H triggers a hint, and Ctrl+Z undoes the last move. The keyboard shortcut panel is always visible on the controls panel for quick reference. Whether you are a first-time Sudoku player learning the ropes on Easy difficulty, a commuter polishing your Medium and Hard times, or an advanced solver chasing personal bests on Expert, this free online Sudoku game has everything you need in one clean, fast, and ad-light interface.
Understanding Sudoku
What Is Sudoku?
Sudoku is a logic-based number-placement puzzle played on a 9×9 grid. The grid is subdivided into nine 3×3 boxes. The starting puzzle contains a subset of digits (called clues or givens) already filled in. The solver must complete the grid such that every row, every column, and every 3×3 box contains each of the digits 1 through 9 exactly once. A well-formed Sudoku puzzle has one and only one valid solution. No arithmetic is involved — the digits are used purely as symbols, and you could replace them with any nine distinct characters. The challenge is purely logical: deducing where each digit must go based on the constraints imposed by the clues and the rules.
How Are Puzzles Generated?
This game generates fresh, unique Sudoku puzzles on demand using a backtracking algorithm. First, a complete valid solution grid is created by filling the 9×9 grid with digits using a recursive depth-first search combined with random shuffling to ensure variety. Second, cells are removed one at a time in a random order. After each removal, a solver checks that the remaining puzzle still has exactly one solution. If removing a cell creates ambiguity (two or more solutions), the cell is restored. This process continues until the target number of clues for the selected difficulty is reached. The result is a guaranteed-solvable puzzle with a unique solution every time you click New Game.
Why Play Sudoku?
Sudoku is widely recognized as one of the best exercises for logical thinking and concentration. Regular puzzle solving has been linked to improved working memory, stronger pattern recognition, and a sharpened ability to hold multiple constraints in mind simultaneously — skills that transfer directly to problem-solving in professional and everyday contexts. From a mental health perspective, the focused state of mind that Sudoku induces (sometimes called 'flow') is associated with reduced stress and increased feelings of accomplishment. Many players use daily Sudoku as a meditative break from screen-heavy work. At a competitive level, speed-solving Sudoku is a recognized sport with world championships, where top players complete Expert-level puzzles in under two minutes.
Tips and Limitations
Puzzle generation in the browser uses a randomized backtracking algorithm that is very fast for Easy and Medium puzzles but may take a fraction of a second longer for Expert difficulty, since removing many clues while maintaining a unique solution requires more solver iterations. The uniqueness check used here stops as soon as two solutions are found, keeping generation time minimal. One practical limitation is that the puzzles are generated fresh each time, so there is no persistent library of numbered puzzles to reference or share by ID. Progress is auto-saved per device in local storage; clearing your browser cache will erase saved progress. Finally, 'minimal' 17-clue puzzles are mathematically proven to be the hardest category, but not every 17-clue puzzle is equally difficult — some happen to yield to simple techniques while others require advanced logic.
How to Play Sudoku
Choose Your Difficulty
Click one of the four difficulty tabs at the top — Easy, Medium, Hard, or Expert — to start a new puzzle at that level. Easy puzzles provide the most clues (36–46 pre-filled cells) and are great for beginners. Expert puzzles leave just 17–21 cells filled in and require advanced logic to solve. A new puzzle is generated instantly in your browser.
Select a Cell and Enter a Digit
Click or tap any empty cell to select it. Its row, column, and 3×3 box will be highlighted so you can quickly see which digits are already used in those regions. Then click a number on the on-screen number pad (1–9) or press a digit key on your keyboard. If you are on desktop, use the arrow keys to move between cells. The cell turns red if your entry conflicts with the puzzle's solution.
Use Notes to Track Candidates
When you are unsure which digit belongs in a cell, click the Notes button (or press N) to enter pencil-mark mode. In this mode, clicking a digit places it as a small candidate note inside the cell rather than a confirmed answer. You can place multiple candidates in one cell. When you confirm a digit in that cell or an adjacent peer, the corresponding notes are automatically removed from all peer cells in the same row, column, and box.
Use Undo and Hints Freely
Made a mistake? Click Undo (or press Ctrl+Z) to step back through your moves one at a time — there is no penalty for undoing. If you are completely stuck, select a cell and click Hint (or press H) to reveal its correct value. Your time, mistake count, and hint count are all tracked and shown in the completion summary when you finish the puzzle. Click New Game at any time to start a fresh puzzle.
Frequently Asked Questions
How do I use pencil notes in Sudoku?
Pencil notes (also called candidates or pencil marks) are small digit annotations you place inside a cell to remind yourself which digits are still possible for that position. To use them, click the Notes button on the control panel — it will turn highlighted to show notes mode is active. Now clicking any digit on the number pad places that digit as a small mark inside the selected cell rather than a confirmed answer. You can add multiple candidates to one cell. When you eventually place a confirmed digit in a peer cell (same row, column, or box), this game automatically removes that digit from all peer cells' notes. Press N on the keyboard to toggle notes mode on and off quickly.
What does the cell highlighting mean?
When you click a cell, three types of highlighting appear simultaneously. First, the selected cell itself gets the strongest highlight in the primary accent color. Second, all cells in the same row, column, and 3×3 box as the selected cell are shown with a softer background — these are called 'peer cells' and they are regions that cannot contain the same digit as the selected cell. Third, if the selected cell contains a digit, every other cell on the board that already contains the same digit is highlighted in a third shade. This same-digit highlighting helps you quickly spot where a digit has already been placed across the entire board and where it still needs to go.
What happens if I make a mistake?
If you enter a digit that does not match the puzzle's unique solution, the cell text turns red to indicate an error, and the mistake counter in the top bar increments by one. Mistakes are tracked and displayed in the completion summary at the end of the game, but there is no game-over condition — you can have as many mistakes as you like and still complete the puzzle. To correct a mistake, click the cell and click the correct digit, or press Backspace/Delete to erase it. You can also use the Undo button to step back to the state before the incorrect entry was made.
Will my progress be saved if I close the browser?
Yes. This Sudoku game automatically saves your entire puzzle state to your browser's local storage every time you make a move. This includes the board, your notes, the timer, mistake count, and hint count. When you return to the page, your game resumes exactly where you left off. Note that local storage is tied to the specific browser and device you are using — your progress will not carry over to a different browser, a private/incognito window, or another device. Clearing your browser's local storage or site data will erase your saved progress.
What is the minimum number of clues needed for a valid Sudoku?
The mathematical minimum for a Sudoku puzzle that has a unique solution is 17 clues. This was proven rigorously in 2012 by Gary McGuire, Bastian Tugemann, and Gilles Civario after a comprehensive computer search. No valid Sudoku puzzle with only 16 given clues can have a unique solution. Our Expert difficulty level targets 17–21 clues, which means some Expert puzzles are right at this theoretical boundary. In practice, 17-clue puzzles are extremely difficult and even many experienced solvers need to use advanced techniques or systematic trial-and-error to crack them.
What are some basic Sudoku solving strategies?
The simplest strategy is the 'last free cell': if a row, column, or box has eight digits filled in, the ninth cell must contain the only missing digit. Next is 'single candidate': if a cell has only one digit that can possibly go there after eliminating all digits already present in its row, column, and box, place that digit. 'Hidden singles' means a digit can only go in one cell within a row, column, or box even if that cell appears to have multiple candidates. For harder puzzles, 'naked pairs' (two cells in a region that can only contain the same two digits), 'pointing pairs', and 'X-Wing' patterns become necessary. Most Easy and Medium puzzles on this site are solvable using only the first three strategies.