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Naked Singles in Sudoku

The simplest, most fundamental Sudoku technique. Master this and you can solve most Easy puzzles end-to-end.

By SudokuHint TeamLast updated

What is a Naked Single?

A Naked Single is a Sudoku cell that has only one possible value remaining after you eliminate every digit already present in its row, column, and 3x3 box. The word "naked" just means the candidate is visible to everyone — you don't need any clever pencil-mark tricks to see it. It is the most direct kind of deduction in Sudoku.

Every Sudoku puzzle, no matter how hard, eventually relies on Naked Singles to fill in the final cells. They are the bedrock of every solving sequence.

How to spot them

Pick any empty cell. Walk through the digits 1 to 9 and ask, for each digit:

  1. Does this digit already appear in the same row?
  2. Does it appear in the same column?
  3. Does it appear in the same 3x3 box?

If the answer is yes for any of those, eliminate the digit. If you end up with exactly one digit not eliminated — that's a Naked Single. Fill it in.

Example 1: An easy spot

Below is the classic "world's first Sudoku" puzzle. Look at the highlighted cell. What digit can go there?

5
3
7
6
1
9
5
9
8
6
8
6
3
4
8
3
1
7
2
6
6
2
8
4
1
9
5
8
7
9
The highlighted cell (Row 1, Column 3). What digit must it be?

Walk through the logic:

  • Row 1 already has 5, 3, and 7 — so 5, 3, 7 are out.
  • Column 3 already has 8 — so 8 is out.
  • The top-left 3x3 box has 5, 3, 6, 9, 8 — so 5, 3, 6, 8, 9 are out.
  • Combine all the eliminations: 5, 3, 7, 8, 6, 9 are blocked.
  • That leaves 1, 2, 4 as possibilities. Still three — not a Naked Single yet for this cell.

That's an important lesson: not every empty cell is a Naked Single right now. As you fill in other cells, more become singles. The key is to keep scanning, because new singles appear every time you place a digit.

Example 2: The very last cell

The most obvious case of a Naked Single: when only one cell in the puzzle is empty.

1
2
3
4
5
6
7
8
4
5
6
7
8
9
1
2
3
7
8
9
1
2
3
4
5
6
2
3
4
5
6
7
8
9
1
5
6
7
8
9
1
2
3
4
8
9
1
2
3
4
5
6
7
3
4
5
6
7
8
9
1
2
6
7
8
9
1
2
3
4
5
9
1
2
3
4
5
6
7
8
Only one cell empty. Row 1 contains 1-8, so the missing digit must be 9.

Row 1 contains 1 through 8 — so the missing cell must be 9. This kind of "last cell standing" is the easiest version of a Naked Single, and you will see it constantly toward the end of a solve.

The thinking process

Naked Singles teach you the most important habit in Sudoku: scan three constraints at once (row, column, box). Once that becomes muscle memory, you stop thinking about it consciously — you just look at a cell and know what cannot go there.

A pro tip many beginners miss: every time you fill in a digit, immediately re-scan the cells around it (same row, same column, same box). Filling a digit often unlocks 2 or 3 more Naked Singles in its neighborhood. This chain reaction is what makes solving feel fast.

When Naked Singles run out

Easy Sudoku puzzles are solvable using only Naked Singles. Medium and Hard puzzles usually need at least one of:

  • Hidden Singles — the next step up, where you look at where a digit can go (not what a cell can hold).
  • Naked Pairs, Hidden Pairs, Pointing Pairs — when no single cell can be solved outright, but you can still eliminate candidates.
  • Advanced patterns like X-Wing and Y-Wing for the hardest puzzles.

See the full list of techniques on the Techniques overview page.

Try it on a real puzzle

Head back to the play page and start an Easy puzzle. Press the 💡 Hint button when you get stuck — if there's a Naked Single available, SudokuHint will highlight it and explain the reasoning the same way this article does. That feedback loop is the fastest way to internalize the technique.