Naked Singles in Sudoku: The Complete Guide
What is a Naked Single?
A naked single is one of the simplest and most reliable techniques in Sudoku. It happens when a cell has only one candidate number left after eliminating all other possibilities based on its row, column, and 3×3 box.
Example:
If a cell "sees" 1, 2, 3, 5, 6, 7, 8, and 9 in its row, column, and box, then the only missing digit is 4. That cell must be a 4—no guessing involved.
Why Naked Singles Matter
- They're usually the first moves you can make in any Sudoku puzzle.
- They guarantee progress—no chance of being wrong.
- They act like "low-hanging fruit," clearing the way for harder techniques later.
- They build confidence for beginners while still being vital for experienced solvers.
How to Spot a Naked Single
- List candidates for each empty cell.
Write down the numbers 1–9 that could go there, eliminating those already in the row, column, or box. - Look for cells with only one candidate left.
If you see something like "{5}" as the only option, you've found a naked single. - Place the number and update the grid.
Every time you fill a naked single, update other cells' candidates—you'll often reveal new ones.
Naked Single vs Hidden Single
It's important not to confuse the two:
- Naked single: the cell itself has only one candidate left.
- Hidden single: a number can only appear in one specific cell within a row, column, or box—even if that cell has multiple candidates penciled in.
Think of it like this: naked singles are obvious once candidates are written down; hidden singles are "camouflaged" and require you to scan an entire region. Learn more about Hidden Singles.
Step-by-Step Example
Let's look at a sample situation:
Row 2: [ | 1 | | | . | | | 6 | ]
Focus on row 2, column 5.
- Row 2 already contains 1, 6
- Column 5 already contains 2, 7, 8, 9
- The 3×3 box already contains 4, 5
What's left? Only 3.
That makes row2-col5 a naked single. Write it in confidently.
Visual Example (Imagine This)
- You have a cell penciled with candidates {1, 4, 5, 9}.
- After placing a number elsewhere, you realize 1, 4, and 9 are no longer valid in that row.
- Now the candidates shrink to {5}.
- Boom—naked single! The cell must be 5.
Strategies for Spotting Naked Singles Quickly
- Cell-by-cell scan
Go through each empty cell in order and check its candidates. - Prioritize crowded areas
Rows, columns, or boxes with fewer blanks are more likely to reveal singles. - Digit-focused scanning
Instead of looking at cells, scan by digit. For example, ask "where can 7 go?" across a row. Sometimes you'll spot a cell where it's the only candidate left.
Common Pitfalls
- Forgetting to update candidates after placing a number. A missed update can hide naked singles later.
- Rushing without candidates: while advanced players can sometimes "see" naked singles by eye, beginners benefit from penciling everything in.
- Mixing up with hidden singles: if more than one candidate remains, it's not naked.
Practice: Find the Naked Singles
Try this small grid fragment:
Row 7: [ 2 | | | 8 | | | | 5 | ]
In the empty square at row7-col2:
- Row 7 already has 2, 8, 5
- Column 2 already has 1, 4, 9
- Box already has 3, 6
What's left? Only 7. Another naked single.
Why Naked Singles Set the Stage
Think of naked singles as the "on-ramp" to Sudoku solving. They clear the easy numbers first, which often exposes hidden singles, locked candidates, and even advanced moves like naked pairs or x-wings. Even expert solvers rarely ignore them—they're just too efficient.
Quick Recap
| Technique | How it Works | Difficulty |
|---|---|---|
| Naked Single | Cell has only one candidate left | Beginner |
| Hidden Single | A digit can only fit in one cell of a unit | Beginner |
| Locked Candidate | A digit's position locks across rows/cols | Intermediate |
Final Thought
Whenever you're stuck, ask yourself: have I missed any naked singles? More often than not, they're the key to unlocking progress.
Frequently Asked Questions
What is a Naked Single in Sudoku?
A naked single is one of the simplest and most reliable techniques in Sudoku. It happens when a cell has only one candidate number left after eliminating all other possibilities based on its row, column, and 3×3 box. If a cell "sees" 1, 2, 3, 5, 6, 7, 8, and 9 in its row, column, and box, then the only missing digit is 4.
How do I spot a Naked Single?
To spot a Naked Single: 1) List candidates for each empty cell by writing down numbers 1–9 that could go there, eliminating those already in the row, column, or box, 2) Look for cells with only one candidate left - if you see something like "{5}" as the only option, you've found a naked single, 3) Place the number and update the grid.
What's the difference between Naked Singles and Hidden Singles?
Naked singles are obvious once candidates are written down - the cell itself has only one candidate left. Hidden singles are "camouflaged" and require scanning an entire region - a number can only appear in one specific cell within a row, column, or box, even if that cell has multiple candidates penciled in.
Why are Naked Singles important?
Naked Singles are important because they're usually the first moves you can make in any Sudoku puzzle, they guarantee progress with no chance of being wrong, they act like "low-hanging fruit" that clears the way for harder techniques later, and they build confidence for beginners while remaining vital for experienced solvers.
How often should I look for Naked Singles?
You should look for Naked Singles after every placement. Every time you fill a naked single, update other cells' candidates - you'll often reveal new ones. They're the "on-ramp" to Sudoku solving and help clear easy numbers first, which often exposes hidden singles, locked candidates, and advanced moves.
Ready to advance? Check out our complete strategy guide for more techniques.
Practice Naked Singles
Next up: Try the Hidden Singles strategy.