Frequently Asked Questions

Sudoku is a logic-based number placement puzzle played on a grid. The most common version uses a 9×9 grid divided into nine 3×3 boxes. The goal: fill every row, every column, and every box with the digits 1–9 so each digit appears exactly once in each unit. No mathematics involved — the numbers are just symbols. Every well-formed Sudoku puzzle has exactly one valid solution.

To play on SudokuMaster.org: click an empty cell, then click a number from the numpad or press a key. The puzzle is solved when all cells are correctly filled. Start with Easy to get familiar, then progress through Medium, Hard, and Expert.

The three rules are simple: (1) Every row must contain the numbers 1–9 exactly once. (2) Every column must contain the numbers 1–9 exactly once. (3) Every 3×3 box must contain the numbers 1–9 exactly once. These three constraints together guarantee a unique solution. If two solutions are possible, the puzzle is incorrectly constructed — all SudokuMaster puzzles are algorithmically verified.

Not at all. The digits 1–9 are just symbols — you could replace them with letters or shapes and the puzzle would work identically. Sudoku is entirely about logical deduction and pattern recognition. You never add, subtract, or multiply anything. Many children learn Sudoku before learning multiplication, and it has been shown to improve focus and working memory regardless of mathematical ability.

Mathematically, a valid 9×9 Sudoku with a unique solution requires at least 17 given clues. This was proven by exhaustive computer search in 2012. Our Extreme difficulty uses approximately 10–12 clues (12% of 81 cells), which is close to this theoretical minimum and makes it extraordinarily challenging.

No. SudokuMaster.org requires zero registration. No email, no password, no social login. Visit the site and play immediately. Everything — your streak, best times, settings, current puzzle, and theme preference — is stored automatically in your browser's local storage on your device. Nothing is sent to a server.

Six sizes: 4×4 (16 cells, numbers 1–4 — ideal for children and complete beginners), 6×6 (36 cells, numbers 1–6 — natural step up), 9×9 (81 cells, numbers 1–9 — the classic Sudoku format), 12×12 (144 cells, numbers 1–12 — much harder than 9×9), 16×16 (256 cells, numbers 1–16 — expert level), and 25×25 (625 cells, numbers 1–25 — an extreme challenge for the most dedicated players). All sizes use the same three rules.

Six difficulty levels for the 9×9 grid: Easy (55% filled — most cells already given), Medium (40% — requires basic scanning), Hard (30% — needs pencil marks and naked pairs), Expert (22% — requires X-Wing and pointing pairs), Master (17% — deep technique needed throughout), Extreme (12% — only about 10 clues, may require hours even for experienced players).

Click the Notes button below the board or press N on your keyboard to enter Notes mode. In Notes mode, clicking a number adds a small candidate digit to the cell rather than filling it definitively. This is called "pencil marking" and is essential for Medium difficulty and above — it lets you track multiple possibilities simultaneously. Notes are automatically removed when the matching correct number is placed in the same row, column, or box. Click Notes or press N again to exit Notes mode and resume filling cells normally.

Arrow keys — move between cells. Tab / Shift+Tab — jump to next/previous empty cell. 1–9 — fill selected cell. Backspace or Delete — erase cell. N — toggle Notes mode. H — use a hint. P — pause/unpause timer. Ctrl+Z (Cmd+Z on Mac) — undo last move. Esc — close any open modal. The game is fully playable without a mouse or trackpad.

Yes, with no limit. Click Undo or press Ctrl+Z to undo your last move. Undo restores the previous cell value including any pencil marks that were in the cell before you overwrote them. You can undo all the way back to the start of the puzzle if needed.

Yes. Your current puzzle is auto-saved every 15 seconds. Return within 24 hours on the same device and browser and your puzzle, timer, pencil marks, and undo history will be fully restored. After 24 hours the saved game expires to keep storage clean. Your best times, streak, and statistics are saved permanently until you clear browser data.

Seven colour themes: Purple (default), Ocean (blue), Forest (green), Rose (pink/red), Amber (warm orange), Dark (dark mode), and Midnight (green-on-black). Click the 🎨 button in the top navigation to cycle through themes. Themes apply to every page — including the daily puzzle, leaderboard, and learn pages. Your choice is saved automatically. On first visit, your OS dark/light mode preference is detected and the appropriate theme applied.

Every day at midnight UTC, a new 9×9 Medium difficulty puzzle is generated using that day's date as a seed. This means the exact same puzzle is shown to every player worldwide on the same day — making it a shared daily challenge. The daily puzzle is found at /daily/. It resets every 24 hours automatically.

Your streak counts how many consecutive days you have completed the daily puzzle. Complete today's puzzle → streak increases by 1. Miss a day → streak resets to zero. Your current streak and all-time best streak are displayed on the daily puzzle page and in the sidebar on the main game. Playing regular puzzles does not affect the streak — only the daily puzzle counts.

No — only today's puzzle is available. Each puzzle is generated deterministically from its date, so it changes automatically at midnight UTC each day. This keeps the daily challenge fair and consistent for all players. If you missed yesterday's puzzle, start building your streak from today.

Your score for each puzzle type is your fastest solving time. The fewer mistakes you make and the faster you finish, the better your personal record. There is no points system — pure elapsed time is the measure. Best times are tracked separately for each combination of grid size and difficulty (e.g., 9×9 Hard has its own record, separate from 9×9 Easy).

Your personal best times are displayed on the Leaderboard page. It shows your fastest solve for every grid size and difficulty combination you've attempted. The leaderboard is personal — your records are stored locally in your browser and are private to your device. No global ranking is available (no account = no server-side storage).

Go to the Printable Sudoku page. Choose your grid size (4×4, 6×6, or 9×9), difficulty, and number of puzzles per page (2 or 4). Click Print Puzzles → in the print dialog, set the destination to "Save as PDF" to download a PDF, or select your printer to print directly. No download, no login, no software needed — works in all modern browsers.

Yes — printable puzzles are freshly generated each time you load the print page. They are not the same as the online puzzles you play interactively. Every puzzle is algorithmically verified to have exactly one unique solution. Easy printable puzzles use the same 55% fill ratio as Easy online puzzles.

Yes. The site is fully responsive with three distinct layouts: mobile (optimised for portrait phones), tablet, and desktop. On phones you see a large board, full-screen numpad, and bottom navigation bar. All game features — Notes mode, Undo, Hints, Undo, themes — are fully accessible on touch devices. Tested on iPhone, Android, and iPads of all sizes.

On Android (Chrome): tap the browser menu (⋮) → "Add to Home Screen" → Install. On iPhone (Safari): tap the Share button (□↑) → "Add to Home Screen" → Add. The app opens full-screen with no browser bar, loads instantly from cache, and works offline. It appears on your home screen like a native app.

Yes, after your first visit. The site uses a service worker that caches all game files, pages, and assets on first load. Once cached, you can play completely offline — all 37 pages, all grid sizes, and all difficulty levels. The daily puzzle uses today's date to generate its puzzle (no server needed) but won't update until you're back online.

Yes. Full keyboard navigation (no mouse needed). ARIA roles and labels on every interactive element. Skip-to-main-content link. Screen reader support for the game grid with cell-level labels (e.g. "Row 3 Col 7 — empty"). prefers-reduced-motion CSS — all animations suppressed for sensitive users. High-contrast Dark and Midnight themes. If you encounter any accessibility barrier, please email sudokumaster.org@gmail.com.