When only one digit can fit in a cell, place it with confidence.
Also known as: Sole Candidate, Full House
A Naked Single occurs when a cell has only one possible digit remaining. Every other digit from 1 to 9 has been eliminated by the row, column, or box that cell belongs to. Since exactly one digit fits, you can place it immediately.
This is the most fundamental solving technique in Sudoku. If you can identify Naked Singles, you can solve every Easy puzzle and most Medium puzzles. Every other technique in Sudoku ultimately creates Naked Singles. They strip away candidates until only one remains.
Naked Singles appear frequently in Easy and Medium puzzles. In harder puzzles, you'll still find them, but usually only after applying more advanced elimination techniques first.
Every cell in Sudoku belongs to three units: a row, a column, and a 3×3 box. The rules state that each unit must contain the digits 1 through 9 exactly once. This means any digit already placed in a cell's row, column, or box is eliminated as a candidate for that cell.
A Naked Single forms when eight of the nine digits are eliminated. Suppose a cell is in row 3, column 5, and box 2. If the digit 1 appears in row 3, digit 2 appears in column 5, digits 3, 4, and 5 appear in box 2, digits 6 and 7 appear elsewhere in row 3, and digit 9 appears in column 5, that leaves only digit 8. The cell at R3C5 must contain 8.
The key insight is that you don't need to see all eight eliminations explicitly. If you're tracking candidates (pencil marks), a Naked Single is simply any cell with exactly one pencil mark. Without pencil marks, you scan the row, column, and box to mentally count which digits are already placed.
Consider the cell at R9C6 (row 9, column 6). Let's check what digits are already present in its three units.
Step 1: Scan row 9. The digits 1, 3, 4, 6, 7, 8, and 9 are already placed. That eliminates seven candidates.
Step 2: Scan column 6. The digits 2, 3, 4, 7, and 9 are placed in other rows. Digit 2 is the only new elimination beyond what row 9 already covered.
Step 3: Scan box 8 (bottom-center). It contains digits 1, 3, 4, 6, 7, 8, and 9. No new eliminations beyond what the row and column already provided.
Step 4: Count what's left. Digits 1, 2, 3, 4, 6, 7, 8, 9 are all eliminated. Only digit 5 remains. Place 5 in R9C6.
After placing this digit, check whether the new 5 creates Naked Singles in adjacent cells. Often, one placement triggers a chain of others. This cascade is what makes Easy puzzles flow quickly.
Explore all 28 solving techniques in our complete technique guide.