Naked Single sudoku technique

A naked single is a cell with exactly one candidate left. Every other digit from 1 to 9 already appears somewhere in its row, column, or box, so there is nothing to decide: the last remaining digit must go in. Most easy puzzles can be finished with naked singles alone, and even the hardest grids come down to a chain of them once the heavy lifting is done.

How to find a naked single

1. Pick an empty cell in a crowded area of the grid, where its row, column and box already hold plenty of digits.
2. Run through 1 to 9 and cross off every digit visible from that cell.
3. If a single digit survives, write it in. If two or more survive, move on and come back later.
4. Each placed digit removes candidates around it, so re-check the neighbours you just changed.

The pattern on the grid

Worked example

Look at r5c5. Its row already holds 1 (r5c1), 2 (r5c3) and 3 (r5c8); its column carries 5 (r1c5), 6 (r3c5) and 9 (r8c5); and the centre box adds 7 (r4c4) and 8 (r6c6). Eight digits crossed off, one survivor: r5c5 is a 4. No pencil marks needed — just the discipline of checking all three units.

When it fails

The technique itself cannot fail — but hunting naked singles cell by cell across an empty-ish grid wastes minutes. If two sweeps find nothing, the puzzle wants hidden singles instead; switch the question around.

Puzzles that need it

These grids come from rated games on this site — our solver confirmed that naked single was part of the shortest solving path for each one.

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Related techniques

• Hidden Single
• Naked Pair
• Pointing Pair

All solving techniques