Hidden Single in Sudoku: How to Spot It
Learn the Hidden Single Sudoku technique: when a number has only one possible position in a row, column, or box.
Guided example
See the Hidden Single on this board
Read this as one teaching unit: first identify the active unit, then the marked pattern cells, then the candidates removed or maintained.
Quick answer
A Hidden Single in Sudoku is when a number can go in only one cell within a row, column, or box. The cell may have several candidates, but that number has only one possible position in the unit, so it must be placed there.
How to spot it
Follow the board in order
- 1 Choose one row, column, or box that has several empty cells.
- 2 Pick a missing number from that unit.
- 3 Check every empty cell where that number could go.
- 4 If there is only one possible cell for that number, place it there.
What is a Hidden Single?
A Hidden Single happens when a number has only one possible position inside a row, column, or box. The cell may still show multiple candidates, but that number is forced.
When to look for it
Look for Hidden Singles after scanning for obvious Single Candidates. Pick a unit and ask where each missing number can still go.
For beginners, rows and boxes are often easiest to scan first because you can visually compare all empty cells in one area.
Example walkthrough
This walkthrough uses a real bundled app puzzle tagged Hidden Single. In row 2, candidate 2 appears only in r2c8. Other open cells in that row have notes like 3589, 35, 159, 139, or 1359, so none of them can take 2. Therefore r2c8 = 2.
Why the placement works
If a row still needs a 5 and only one empty cell in that row can accept 5, then 5 must go there. Other candidates in that cell do not matter.
Hidden Single vs. Single Candidate
A Single Candidate is found by looking at one cell and asking, “What numbers can this cell still be?”
A Hidden Single is found by looking at one number in one unit and asking, “Where can this number still go?”
Common mistake
Do not ignore a cell just because it has several candidates. A Hidden Single can live inside a cell with multiple candidates if one of those candidates has no other place in the unit.
FAQ
What is a Hidden Single in Sudoku?
A Hidden Single is a number that has only one possible position in a row, column, or box.
How is Hidden Single different from Single Candidate?
Single Candidate focuses on one cell with one candidate. Hidden Single focuses on one number with one possible position in a unit.
Why is it called hidden?
It is hidden because the target cell may still have multiple candidates, even though one number is forced there.
Is Hidden Single beginner-friendly?
Yes. It is one of the most important beginner techniques after Single Candidate.
In this lesson
- Board example: see the pattern first.
- Walkthrough: connect each highlight to the rule.
- FAQ: check edge cases after the move is clear.
Practice this technique in Sudoku Coach
Read the pattern, then practice it step by step with guided hints that explain why the move works.
Related Sudoku techniques
Single Candidate in Sudoku: How to Spot It
Learn the Single Candidate Sudoku technique: check one cell's row, column, and box until only one legal number remains.
Candidate Notes in Sudoku: How to Use Them
Learn how to write and maintain Sudoku candidate notes so every later technique has reliable evidence.
Hidden Pair in Sudoku: How to Spot It
Learn how Hidden Pairs work in Sudoku, why they are harder to see than Naked Pairs, and how to clean up candidates.