Volodymyr Sakhan  ·  March 29, 2026

A hidden single is a digit that can be placed in only one cell within a row, column, or 3×3 box — even though that cell may appear to have multiple candidates at first glance.

Unlike a naked single, where a cell has only one possible digit, a hidden single is discovered by scanning the house rather than the cell. When every other empty cell in a row, column, or box is blocked for a given digit, the remaining cell must hold it.

What is a hidden single?

Every digit from 1 to 9 must appear exactly once in each row, column, and 3×3 box. If you scan a row, column, or box and find that a particular digit can only fit in one of its empty cells, that cell is a hidden single.

The cell may look like it has several candidates — that is why the single is "hidden". The uniqueness comes from scanning the group, not from looking at the cell alone.

When to use this technique

Apply the hidden singles technique whenever naked singles and Last Free Cell leave the board stuck. Scan each row, column, and box and ask: for each digit 1–9, is there only one empty cell where it can go? Look for these conditions:

• A row, column, or box with several filled cells, so most positions for a digit are already blocked.
• A digit that appears in multiple intersecting rows or columns, blocking all but one empty cell in the target group.
• Any position where a digit has only one remaining valid cell — even if that cell still shows other candidates.

Worked examples

Below are three examples — one for each group type: box, row, and column. In every case the target cell is highlighted in green, cells that cannot hold the digit are orange, and the blocking digits are blue.

Hidden single in a box

Box 2 (rows 1–3, columns D–F) has five empty cells. Pick digit 7 and check which empty cells can hold it:

1. Pick digit 7 and scan box 2. Which empty cells can hold 7?
2. D2: check column D — already contains 7 at D8. Eliminated.
3. E2: check column E — already contains 7 at E6. Eliminated.
4. F2 is the only remaining empty cell where 7 is not blocked.
5. Place 7 at F2.

When a digit can only go in one cell of a 3×3 box, that cell is a hidden single — place the digit immediately.

Hidden single in a row

Row 5 has three empty cells: A5, B5, and D5. Pick digit 7 and check which empty cell can hold it:

1. Pick digit 7 and scan row 5. Which empty cells can hold 7?
2. A5: check column A — already contains 7 at A7. Eliminated.
3. D5: check column D — already contains 7 at D8. Eliminated.
4. B5 is the only remaining option.
5. Place 7 at B5.

Scanning a row for where a specific digit can still fit reveals hidden singles even when multiple cells appear empty.

Hidden single in a column

Column I has four empty cells: I3, I4, I6, and I9. Pick digit 7 and check which empty cell can hold it:

1. Pick digit 7 and scan column I. Which empty cells can hold 7?
2. I3: check row 3 — already contains 7 at G3. Eliminated.
3. I4: check row 4 — already contains 7 at H4. Eliminated.
4. I6: check row 6 — already contains 7 at E6. Eliminated.
5. I9 is the only remaining option. Place 7 at I9.

The same scanning logic works for columns — systematically check each empty cell until only one position remains for the digit.

Practice and next steps

The best way to master hidden singles is to play sudoku and actively scan rows, columns, and boxes for digits with only one available position. Start with easy puzzles and work up from there.

Together with naked singles and Last Possible Number, hidden singles form a powerful beginner toolkit. Once you can spot them quickly, you will solve easy and medium puzzles without guessing.