Online Sudoku Game

An empty Sudoku cell belongs to a row, a column and a 3 × 3 box. A digit that fits the row can still be ruled out by the other two groups. In a completed classic puzzle, all nine rows, all nine columns and all nine boxes contain the digits 1 to 9 exactly once. The numbers are symbols to arrange, not quantities to add, so solving depends on elimination rather than arithmetic.

The starting digits are givens, or clues, and must stay unchanged. Other cells sharing a cell’s row, column or box are its peers. An empty cell’s candidates are the digits still possible after checking those peers. Pencil marks record these possibilities while you work elsewhere, but a pencil mark is not a settled answer.

• When only one digit remains possible in a cell, that digit is forced, provided the surrounding entries are correct.
• When a missing digit has only one possible position in a row, column or box, it belongs in that position even if the cell appears to have other candidates.

A mistake may not create an immediate repetition. Choosing one of two candidates without a deduction can leave the current row looking valid while making a later cell impossible to fill. Keeping uncertain digits in pencil marks avoids treating a possibility as established fact.

The help provided by starting clues depends on their positions and relationships. More blanks mean more digits left to place, not a measured level of solving technique. A puzzle with a unique solution has only one valid completed grid, but that promise does not identify the next move or prove a tentative entry correct.

How to Use This Tool:

Choose how many starting clues and reveal hints you want before beginning. There is no timer or lives limit, so a repeated digit can be corrected without ending the puzzle.

1. Choose Clue density and Reveal hints. The defaults are Gentle and Up to 3; No reveals disables cell reveals. Enter an optional Seed of up to 80 characters, or leave it blank for a new seed, then select Start puzzle. The seed is available afterward in Run details.
2. Select an editable cell and enter a digit from 1 to 9 using the keyboard or digit pad. Bold original clues cannot be changed. With a board cell focused, arrow keys move between cells without wrapping, while Home and End move to the first and last cells in that row.
3. Switch Notes on to toggle pencil marks in an empty cell, then switch it off before placing an answer. To correct a filled cell, replace its digit with Notes off or use Erase. The keyboard shortcuts are N for notes and Delete, Backspace or 0 for erasing. Erase a filled cell before trying to add notes to it.
4. Use Reveal cell when you want the selected cell’s solution digit. A successful reveal consumes one hint and fixes the underlined digit for the rest of that attempt. It supplies an answer, not a logical explanation.
   An entry that already matches the solution is not revealed again and does not consume a hint. Choose an empty cell or another entry when prompted.
5. Choose Pause or press P to hide the grid while keeping the current entries, notes and hint allowance. Switching tabs or moving to another window also pauses play; use Resume to continue. Restart puzzle instead begins the same puzzle and setup again, clearing your work and resetting the hint allowance.
6. Complete the grid to reach Sudoku solved, or choose End puzzle to stop without completing it. Afterward, Replay same seed repeats the puzzle, New seed keeps the settings but changes the seed, and Change setup returns to the setup choices.

Interpreting Results:

Filled counts occupied cells, including original clues, reveals and your entries. It does not measure correctness. Conflicts counts distinct cells involved in a repeated digit, not the number of wrong answers. Two matching digits in one row therefore flag both cells, even when one is an original clue.

A red ! means a digit repeats in its row, column or box. A fixed clue or reveal may be marked because an editable entry repeats it; correct the editable entry. With Conflicts at 0, continue checking deductions rather than assuming every entry is right. Ordinary digit entry is not compared with the solution.

Sudoku solved confirms completion. Puzzle ended does not, even if its summary says that no cells remain empty: a filled grid can still contain repetitions. The final summary reports Digit entries and Reveal hints used, not a time or competitive rating.

An unassisted best requires a solved grid with Reveal hints used equal to 0. Notes are allowed, and unused reveal availability does not disqualify the attempt. Best counts are compared only for the same seed, clue density and hint allowance under the same rules. A lower count means fewer changed digit placements, not a stronger measure of solving ability.

Technical Details:

Sudoku uses an all-different constraint: each nine-cell row, column and box must contain nine distinct digits chosen from 1 to 9. Original clues remain fixed, and pencil marks do not count as filled cells. Completion depends on the whole grid, not on whether the most recent entry caused a warning.

When the grid is complete

For a starting puzzle with exactly one solution, a completed grid satisfying these rules must be that solution. An unfinished grid can pass the repetition check yet still need earlier entries changed before it can be completed.

Pencil marks and corrections

Pencil marks are entered manually and are not checked for legality. Placing or revealing a digit clears every note in that cell and removes the same digit’s notes from its peers. This also happens after a mistaken placement. Erasing or replacing a digit does not restore previously removed notes, so corrections may require rebuilding candidates by hand. Erase clears both the selected digit and that cell’s notes.

Counting digit entries

A new digit in an empty cell, or a replacement with a different digit, adds one to the entry count. Re-entering the same digit, selecting cells, changing notes, erasing and using reveals add no digit entries. Erasing does not subtract an earlier entry, so filling that cell again adds another.

The minimum possible digit-entry count for an unassisted completion is the number of cells that were initially empty.

Here, C is the starting clue count and E_min is the minimum unassisted Digit entries. Both are whole-number counts, with no rounding. The benchmark assumes every blank is filled once without a reveal.

These levels describe clue density, not a formal rating of the deductions required. Entry totals from different densities are not directly comparable because the number of blanks differs.

Seeded puzzle variations

The puzzle set contains nine original starting boards, three per clue density, each with exactly one solution. A seed selects a board and rearranges it by relabeling digits, exchanging rows within three-row groups, exchanging columns within three-column groups, moving whole groups and optionally transposing the grid so rows become columns. Each transformation is reversible and preserves the Sudoku rules, clue count and number of solutions.

Within the current rules, the same seed and clue density recreate the same grid. Changing the hint allowance does not change that grid. Different seeds can produce equivalent puzzles, so a new seed is not a guarantee of a fundamentally different challenge. Clue layouts do not have to be symmetric.

Privacy Notes:

Sudoku play and rule checking take place in your browser. Leaving or reloading the page discards the current grid and notes, even when the puzzle is paused. Reusing its seed recreates the starting puzzle, not your previous progress.

Only local best records are kept between visits, for up to 24 puzzle-and-setup combinations. Each record contains the seed, clue density, hint allowance, rules version and best unassisted entry count. A new record or a strictly lower qualifying count is saved; an equal or higher count does not replace the best.

Use Clear local records under About these puzzles in setup to remove them. Unreadable records do not prevent play and can be cleared to reset them. When browser storage is unavailable, a best may remain available only for the current visit. These are personal practice records with no monetary value.

Worked Examples:

References:

• Sudoku rules, Nikoli.