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

Take r1c5 in the position below. Cross off every digit visible from it — its row, column and box between them account for all digits except 4. One candidate left means one legal move: r1c5=4. Load the position into any solver and it will agree.

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.

Every string is the standard 81-character format (0 = empty) — paste it into any solver to verify the rating and the solving path yourself. Want to try one under the clock? Play a free puzzle — no account needed — or create an account to train specific difficulty tiers.

Related techniques

• Hidden Single
• Naked Pair
• Pointing Pair

All solving techniques