Hidden Singles is a powerful beginner technique that approaches Sudoku solving from the opposite direction of Naked Singles. Instead of asking "what digit can go in this cell?", you ask "where can this digit go in this unit?" A Hidden Single occurs when a particular digit can only be placed in one cell within a row, column, or 3x3 box, even though that cell might have multiple candidates overall.
To find Hidden Singles, pick a digit from 1 to 9 and examine a row, column, or box. Check which cells in that unit could contain the chosen digit by eliminating cells that already have a value and cells where the digit is blocked by appearances in crossing rows, columns, or boxes. If the digit can only fit in one cell within the unit, that cell must contain the digit — it is a Hidden Single.
Hidden Singles are called "hidden" because the cell itself may appear to have several candidates, masking the fact that for one specific digit, it is the only valid location in its unit. This technique is extremely effective and can solve a large portion of easy and medium puzzles on its own. Learning to scan for Hidden Singles trains you to think about digit placement from the perspective of entire units rather than individual cells, which is a critical mindset shift for advancing your Sudoku skills.
Try It Yourself
Walk through each step of the hidden singles technique on a real puzzle. Follow the instructions and try entering the correct value when prompted.
Let us search for the digit 1 in the top-middle 3x3 box (rows 0-2, columns 3-5). This box currently contains 4, 6, 9, 5, 3, 9, 8, 2, 5 — but no 1. Where can 1 go?
Step-by-Step Guide
Choose a digit from 1 to 9 to analyze.
Select a row, column, or 3x3 box that does not yet contain that digit.
For each empty cell in the unit, check if the chosen digit is blocked by the same digit in a crossing row, column, or box.
Eliminate cells where the digit cannot be placed.
If only one cell remains where the digit can go, it is a Hidden Single.
Place the digit in that cell even if the cell has other candidates.
Repeat for other digits and units across the entire grid.
Imagine assigning lockers to students. Even though a particular locker could fit several students, if one student has no other locker available, that student must get this one — regardless of who else could have used it.
Every unit must contain each digit exactly once, so every digit must appear somewhere in the unit. If, after applying all row, column, and box exclusions, a digit has only one legal cell remaining within a unit, that cell is the digit's only possible home. Placing it anywhere else would leave the unit with no valid cell for that digit, violating the completeness constraint. The placement is therefore not just possible but mandatory.
When to use: After scanning for Naked Singles, switch your perspective: instead of asking which digit fits a cell, ask where a specific digit can go within a row, column, or box. If a digit has only one legal cell in any unit, place it there.
Common Mistakes to Avoid
Overlooking a Hidden Single because the target cell has multiple candidates, making it look unsolvable.
Remember that Hidden Singles are about the digit, not the cell. Even if a cell has five candidates, if one of those digits can only go in that cell within the unit, it is solved.
Only checking boxes for Hidden Singles and neglecting rows and columns, which also contain them frequently.
Systematically check all three unit types. Hidden Singles in rows and columns are just as common as in boxes.
More Examples
See hidden singles applied in different puzzle configurations to strengthen your pattern recognition.
Highlighted cells show the hidden singles pattern
Practice Puzzles
Apply the hidden singles technique on these mini challenges. Tap a highlighted cell and enter the correct digit.
Quick Reference
- Pattern:
- A candidate appears in only one cell within a row, column, or box
- Action:
- Place that candidate in the cell where it uniquely appears
- Look for:
- A digit that can only go in one place within a unit