Naked Singles in Sudoku: How to Spot Them Every Time

What Is a Naked Single?

A naked single is the most fundamental Sudoku technique, and the one you will use more than any other. It happens when a cell has only one candidate number left after you eliminate everything that already appears in the same 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, the only missing digit is 4. That cell must be 4. No guessing, no advanced logic.

This is where every solver starts, from absolute beginners to experts. Master naked singles and you have a reliable foundation for everything that follows, from hidden singles to naked pairs and beyond.

Quick win: Practice on our free Sudoku printables (Easy through Master) to build speed and confidence.

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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 like pointing pairs later.
• They build confidence for beginners while still being vital for experienced solvers.
• They trigger chain reactions: each placement often reveals new hidden singles, locked candidates, and even advanced moves like naked pairs or X-Wing patterns.

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How to Spot a Naked Single

1. 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.
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.
   Every time you fill a naked single, update other cells' candidates - you'll often reveal new ones.

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Beginner Tips for Finding Naked Singles

• Start with the most crowded regions. Rows, columns, or boxes with only 1–2 empty cells almost always contain naked singles. Look there first.
• Pencil in candidates religiously. Naked singles hide when candidates aren't written down. Even if it feels slow, penciling in pays off fast.
• Check after every placement. Each number you place changes the candidate lists of 20 other cells (the row, column, and box it belongs to). New naked singles often appear immediately.
• Use the app's highlighting. In the Sudoku a Day app, tap a digit to highlight all instances. Empty cells in regions with 8 of 9 digits filled are instant naked singles.
• Don't overthink it. If a cell has one candidate, fill it. Naked singles are never wrong — they're pure logic with zero ambiguity.

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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.

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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.

This same logic applies every time you place a number. After each fill, recheck nearby cells. Candidates often shrink to one, revealing new naked singles in a chain reaction.

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Another Example: Chain Reaction

Naked singles often trigger each other. Watch how one placement creates the next:

Starting point: Row 4 has 1, 3, 4, 6, 7, 8, 9 filled. Two cells remain: R4C2 and R4C5.

• R4C2: Column 2 has 5. Box has 2. → candidates: {2, 5} — not a naked single yet.
• R4C5: Column 5 has 2. Box has 5. → candidates: {2, 5} — also not yet.

But wait — look at column 2 more carefully. If 2 already appears in column 2 (from another row), then R4C2 can only be 5. That's a naked single.

Chain reaction: Once R4C2 = 5, the only digit left for R4C5 is 2. Another naked single, triggered by the first.

This is why naked singles are the engine of Sudoku solving. Each one can unlock the next.

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Strategies for Spotting Naked Singles Quickly

1. Cell-by-cell scan
   Go through each empty cell in order and check its candidates.
2. Prioritize crowded areas
   Rows, columns, or boxes with fewer blanks are more likely to reveal singles.
3. 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.

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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.

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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.

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Quick Recap

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Wrap-Up

Naked singles are the technique you'll use from your first puzzle to your thousandth. They're fast, they're certain, and they open the door to everything else in Sudoku. When you feel stuck, ask: have I missed a naked single? More often than not, the answer unblocks the grid.

Your next steps:

• Practice with today's free daily Sudoku
• Learn Hidden Singles, the natural companion technique
• Try Naked Pairs when you're ready for the next level
• Download printable puzzles for offline practice

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.

Can I solve an entire Sudoku using only naked singles?

Easy puzzles — often yes. Naked singles alone can carry you through most beginner grids. As difficulty increases, you'll need to combine naked singles with hidden singles, naked pairs, and other techniques. But naked singles remain your first move at every difficulty level.

Ready to advance? Check out our complete strategy guide for more techniques.

Related Strategies

Once you've mastered naked singles, these techniques build naturally:

• Hidden Singles - The next logical step after naked singles
• Naked Pairs - When two cells share the same two candidates
• X-Wing - Advanced pattern recognition technique
• Swordfish - Expert-level fish technique
• Beginner Tips - More basic techniques to master first

Practice Naked Singles

• Printable Sudoku Puzzles (Easy–Master)

Next up: Try the Hidden Singles strategy.

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