Naked Single Sudoku Technique

Updated 11 August 2026

A naked single is an empty cell whose row, column and box between them already contain eight of the nine digits, so exactly one candidate survives and that candidate is the answer.

When a naked single applies

Every cell in a Sudoku grid sits at the intersection of three units: one row, one column and one 3×3 box. A digit is a candidate for that cell only if it is absent from all three. A naked single is the case where those three units, taken together, account for eight distinct digits and leave a single survivor.

The word “naked” refers to how the deduction is read. You look at the cell, see one candidate and place it. Nothing about the other cells in the unit matters. That is the opposite of a hidden single, where the cell may show several candidates and the proof comes from the fact that no other cell in the unit can take a particular digit.

Naked singles are the workhorse of every easy and medium puzzle. They also appear constantly as a follow-up: almost every elimination technique on this site is worth using precisely because the candidates it removes turn some nearby cell into a naked single.

Worked example

Look only at r1c8. Row 1 already holds 9, 3, 6, 8 and 2. Column 8 already holds 5, 3, 2, 9, 6 and 7. The top-right box already holds 2, 5, 1, 8, 3 and 6. Put those three lists together and they cover 1, 2, 3, 5, 6, 7, 8 and 9. Only 4 is left, so 4 goes in r1c8.

Note how little of the grid was needed: twenty cells at most, all of them visible from the target. That locality is why naked singles are cheap to scan for and why a solver tries them immediately after full houses.

How to spot a naked single

1. Choose a promising empty cellPrefer cells sitting in a crowded row, column or box. The more digits already surround a cell, the fewer candidates it can have.
2. Eliminate along the row and the columnRead the row across and the column down, crossing off each digit you meet. Most cells fall apart at this stage.
3. Eliminate inside the boxAdd the digits in the surrounding 3×3 box. This third pass is the one people skip, and it is often the pass that removes the last competing candidate.
4. Place the survivor and re-scan its neighboursIf exactly one digit survives, write it and immediately re-check the twenty cells that can see it, because a naked single very often creates another one.

Common mistakes

• Checking the row and column but forgetting the box, which leaves a phantom second candidate and stalls the solve.
• Confusing a naked single with a hidden single. If the cell shows two or more candidates and you are arguing about where a digit can go in the unit, you are using a hidden single instead.
• Trusting stale pencil marks. A naked single found from candidate notes you forgot to update after your last placement is simply a guess in disguise.
• Hunting for exotic patterns while naked singles are still on the board. They are always the cheaper move.