Paper Logic

How to Mark Sudoku Candidates, Step by Step

Learn how to find the possible numbers for a cell, when to erase a candidate, and why a possibility is not yet an answer.

Nexo de Papel · 25 September 2026 · Updated: 7 October 2026 · 5 min read

To use candidates, note the possibilities for a cell and remove only those ruled out by a certain answer; do not turn a note into an answer until you have deduced it is the only option. A candidate is a number that could still fit in an empty cell. Writing it small, often in pencil, helps you compare possibilities without having to remember them all.

Work out the list by checking the three areas that cross the cell: its row, column, and box. Rule out numbers already placed in any of them. If you are still learning the rules, start with our small worked sudoku example.

Example: finding the candidates for cell A

This original 9×9 grid marks A in row 1, column 5. The thicker lines outline the 3×3 boxes. Before reading on, see which numbers you would keep.

A: row 1, column 5. B: row 1, column 3. Count coordinates from the top and left. · 9 × 9; boxes of 3 × 3
Column 1Column 2Column 3Column 4Column 5Column 6Column 7Column 8Column 9
Row 1BlankBlankB · blank4A · blank6789
Row 24BlankBlankBlank89BlankBlankBlank
Row 3Blank8Blank1Blank3456
Row 4BlankBlankBlank5BlankBlank89Blank
Row 55BlankBlankBlankBlankBlank234
Row 6Blank912BlankBlank56Blank
Row 7BlankBlank5Blank7BlankBlankBlankBlank
Row 86BlankBlank91Blank345
Row 9BlankBlank234567Blank
A: row 1, column 5. B: row 1, column 3. Count coordinates from the top and left.
  1. Row 1: It already contains 4, 6, 7, 8, and 9. The missing numbers are 1, 2, 3, and 5.
  2. Column 5: It already contains 8, 7, 1, and 4. Cross out 1 from the row's list. That leaves 2, 3, and 5.
  3. The top-middle box: It contains 4, 6, 8, 9, 1, and 3. Cross out 3. The candidates for A are 2 and 5.

Write a small 2 and 5 in A. Neither conflicts with a number already in the cell's row, column, or box. That does not tell us which one is the answer; we need another deduction.

The key difference: “It could be 2” does not mean “it is 2.” A candidate is a possibility, not a number to try and see what happens.

Now try cell B

B is in row 1, column 3. Start with the numbers missing from the row, then check the column and box. How many candidates remain?

Show B's candidates and deduction

The row allows 1, 2, 3, and 5. Column 3 already contains 1, 5, and 2, so all three are ruled out. Only 3 remains. Nothing in the top-left box rules it out. B must be 3, so you can write it as a full-size answer.

Keep your notes up to date

After placing a certain number, remove that candidate from other cells in the same row, column, and box. Do not erase it from the whole grid: it may still be possible elsewhere.

Once you place 3 in B, A's notes of 2 and 5 stay as they are. Sharing a row does not make a candidate disappear with every move; remove only the number you just placed, if it was noted in an affected cell.

After deducing B = 3. The asterisk marks the added number; A remains blank with candidates 2 and 5. · 9 × 9; boxes of 3 × 3
Column 1Column 2Column 3Column 4Column 5Column 6Column 7Column 8Column 9
Row 1BlankBlank3 added by the reader4A · blank6789
Row 24BlankBlankBlank89BlankBlankBlank
Row 3Blank8Blank1Blank3456
Row 4BlankBlankBlank5BlankBlank89Blank
Row 55BlankBlankBlankBlankBlank234
Row 6Blank912BlankBlank56Blank
Row 7BlankBlank5Blank7BlankBlankBlankBlank
Row 86BlankBlank91Blank345
Row 9BlankBlank234567Blank
After deducing B = 3. The asterisk marks the added number; A remains blank with candidates 2 and 5.

Do you need to write every candidate?

You can begin with one area that is holding you up. Just distinguish a complete candidate list for a cell from a partial note about one number you are tracking. If you wrote down only 2 for convenience, seeing it alone does not prove it is the only possibility.

A consistent layout makes notes easier to read. Imagine a tiny 3×3 grid inside each cell, with 1 at the top left and 9 at the bottom right. That is not a sudoku rule; it is simply a way to put each candidate in a predictable spot.

If your notes contradict one another, pause and check whether you missed a possibility or erased one that belonged to a different area. Our guide to common beginner sudoku mistakes offers a way to find the error without wiping the whole grid.

Reading path: Start with sudoku

Keep exploring this topic