NoCharge

Candidates, not answers

Sudoku pencil-mark helper

Enter a 6×6 or 9×9 grid, inspect one cell's candidates, and learn the elimination rule. An optional full solve is clearly labeled; hidden singles are not automatically detected.

Category: Learning

Bottom line: Type a 6×6 or 9×9 grid and select an empty cell to list the digits not already used in its row, column, or box. Candidates are not answers. Reveal is an explicit optional spoiler and does not prove the puzzle has only one solution.

One grid, your notes

Pencil marks without spoilers

Select a cell to see its candidates.

Click or tap to select a cell, then enter digits with the keyboard. The helper computes candidates from what you typed — if a digit is wrong, the candidates reflect your mistake, not a hidden answer.

How it works

Select an empty square and enter board digits with the keyboard. Its candidates are the allowed digits, 1–6 or 1–9, minus every digit already present in the same row, column, or box. Boxes are three columns wide and either two or three rows high, depending on the grid size.

Worked example

On a 6×6 grid, suppose the selected cell's row uses 1 and 4, its column uses 2, and its box uses 1 and 3. The combined exclusions are 1, 2, 3, and 4, leaving candidates 5 and 6. If another square in that box is then filled with 5, only 6 remains for the selected square.

One remaining candidate is a naked single. A hidden single is different: a digit has only one possible location across a whole row, column, or box. This helper lists one cell's candidates; it does not automatically search all units for hidden singles. Reveal is an explicit spoiler that tries backtracking to fill a complete grid consistent with the entered digits.

What it cannot do

  • Candidates describe your entries, not a hidden authoritative answer. A wrong but locally consistent entry can still lead to an incorrect or impossible puzzle.
  • Reveal checks for contradictory digits and searches for a completion, but it does not prove uniqueness, grade difficulty, or explain each solving step.
  • This is separate from the playable Sudoku games. It neither imports their boards nor changes their saved progress.
  • Clicking selects a square; digit entry in this helper uses the keyboard. Clear, changing size, or reloading discards the typed grid; it is not saved.

FAQ

Does the helper solve the puzzle?

By default it lists candidates for the selected cell without filling answers. The explicitly labeled Reveal button can search for one complete solution to a consistent entered grid; it does not establish uniqueness.

What is "hidden single"?

A digit that can only go in one cell of a row, column, or box even though other cells show it as a candidate. The worked explanation distinguishes it from a naked single; the helper does not automatically detect hidden singles.

Why does it default to non-spoiler?

The helper shows candidates from the grid you typed, not from its own solution. You can opt into a full solve with the clearly-labeled Reveal button; NoCharge does not hide that button behind a second feature.

Can it handle a 6×6 grid?

Yes. Choose the size; box shape changes between 3×2 (6×6) and 3×3 (9×9). The candidate logic is identical.

Embed this tool

<iframe src="https://nocharge.net/tools/sudoku-helper/" width="400" height="640" title="NoCharge sudoku pencil-mark helper"></iframe>
<p>Source: <a href="https://nocharge.net/tools/sudoku-helper/">NoCharge tool</a></p>

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