Sudoku

Solve unique 9×9 Sudoku grids in Easy, Medium, Hard, or Expert difficulty. Use pencil notes, hints, auto-check, undo and redo, pause the timer, play a UTC-based daily challenge, and keep local win, time, hint, and streak statistics.

How to Use the Sudoku

Complete the 9×9 grid so every row, column, and 3×3 box uses digits 1 through 9 exactly once. Each generated puzzle has a unique solution.

  1. Choose Easy, Medium, Hard, or Expert. Select New Puzzle for a random grid or Daily Challenge for the UTC day's grid at that difficulty.
  2. Select an empty cell, then tap a number or press 1–9. Use arrow keys to move around the grid; 0, Backspace, or Delete erases an editable entry.
  3. Toggle Notes or press N to enter pencil candidates instead of final digits. Press N again to return to normal entry.
  4. Keep Auto Check enabled to highlight entries that do not match the solution, or turn it off to solve without immediate error marking.
  5. Use H or the Hint button to fill one cell. Use Ctrl+Z for undo, Ctrl+Shift+Z or Ctrl+Y for redo, and Pause or Space while paused to resume.
  6. Fill all 81 cells correctly to stop the timer and record the win and best time for that difficulty.

Solving strategies

  • Scan each row, column, and box for a digit that has only one possible cell.
  • Use notes for candidates, then remove candidates when a digit is fixed in the same row, column, or box.
  • Compare intersecting rows and columns across neighboring boxes before guessing; the game does not require trial-and-error.

Common mistakes

  • Do not repeat a digit in a row, column, or 3×3 box; Auto Check also marks a non-conflicting digit if it is not the solution value.
  • Changing difficulty starts a new puzzle in the current random or daily mode and replaces the saved current grid.
  • Restart clears entries, notes, and elapsed time for the same puzzle but does not remove accumulated statistics.

Daily mode, persistence, and privacy

Daily grids change at 00:00 UTC and are deterministic by date and difficulty. Progress and statistics use localStorage, so streaks and best times are device- and browser-specific. There is no account sync, online leaderboard, puzzle import, or server backup.

FAQ

What are the rules of Sudoku?

Fill every empty cell with 1–9 so each row, column, and 3×3 box contains every digit once. Starting digits cannot be changed, and the puzzle is won only when the grid matches its unique solution.

What changes between Easy, Medium, Hard, and Expert?

All modes use a 9×9 grid with a unique solution. The generator targets about 36–40 clues for Easy, 30–34 for Medium, 26–29 for Hard, and 22–25 for Expert.

What is the Daily Challenge?

It is generated from the current UTC date and selected difficulty. Players worldwide receive the same grid for that difficulty on that UTC day, and a first daily win updates the local streak.

How do notes (pencil mode) work?

Toggle Notes or press N, select an editable cell, then enter candidate digits. Placing a final digit clears that candidate from notes in the same row, column, and box.

What do Auto Check and Hint do?

Auto Check highlights entries that differ from the stored solution. Hint fills one empty cell, preferring a naked single when available, and increases the all-time hints-used statistic.

Is my Sudoku progress private and saved?

The current puzzle, entries, notes, timer, options, best times, wins, hints, and daily streak stay in this browser's local storage. They are not uploaded or synced and are lost if site data is cleared.