A digit has only one place to go in a row, column, or box, even if the cell has other candidates.
Also known as: Last Remaining Cell, Unique Candidate
Pick a digit and one unit — a row, column or box — and ask where that digit can still go. If exactly one cell is left, place it there, whatever other candidates that cell shows. Hidden Singles are the most frequently used technique in Sudoku.
A Hidden Single occurs when a particular digit can only go in one cell within a row, column, or box. The cell itself might have several candidates, but for that specific unit, this is the only cell where that digit fits. Since the digit must appear somewhere in the unit, it must go in that cell.
Hidden Singles are the workhorse technique of Sudoku solving. You'll use them more than any other method. While Naked Singles are easier to understand, Hidden Singles are actually more common in practice because they let you place digits even when cells look "busy" with multiple candidates.
The name "Hidden" comes from the fact that the placement isn't obvious from looking at the cell alone. The cell might show candidates {2, 5, 7}, but if 5 can't go anywhere else in that row, then this cell must be 5. The answer is "hidden" among the other candidates.
Choose a digit, say 7, and look at a single unit (a row, column, or box). Ask yourself: "In which cells of this unit could 7 go?" If the answer is exactly one cell, you've found a Hidden Single. Place 7 in that cell.
The logic is straightforward. Every row, column, and box must contain every digit from 1 to 9 exactly once. If digit 7 is eliminated from all cells in a row except R3C8, then R3C8 must contain 7. It doesn't matter that R3C8 might also be a candidate for digits 2 and 5. The fact that 7 has no other option in that row is enough.
When you place the digit, remove it as a candidate from all cells in the same row, column, and box. This often creates new Naked Singles or Hidden Singles elsewhere. Each placement ripples outward.
The most reliable way to find Hidden Singles is systematic scanning: pick a digit and sweep across all rows, then all columns, then all boxes. Experienced solvers do this quickly by looking at where a digit is already placed and mentally blocking out rows, columns, and boxes.
Let's find where digit 1 goes in row 3. Currently, row 3 has six digits placed (8, 6, 9, 5, 7, 4) and 1 is not yet in the row.
Step 1: Identify the empty cells in row 3. Three cells are empty: R3C1, R3C5, and R3C7.
Step 2: Can 1 go in R3C1? Check column 1. R1C1 already contains 1. So R3C1 is eliminated.
Step 3: Can 1 go in R3C5? Check column 5. R7C5 already contains 1. So R3C5 is eliminated.
Step 4: Can 1 go in R3C7? Column 7 has no 1. Box 3 (top-right) has no 1. R3C7 is the only cell in row 3 where digit 1 can go. Place 1 in R3C7.
Notice that R3C7 also has candidate 3. That's fine. The Hidden Single logic doesn't require the cell to have only one candidate. It requires the digit to have only one possible cell in the unit.
Example 2: Hidden Single in a Column
Look at column 3. Where can digit 7 go? Scanning down: R1C3 has 0 (empty, check candidates), R3C3 already has 3, R5C3 has 8, R6C3 has 0 (empty), R7C3 has 5, R8C3 has 1, R9C3 has 0 (empty). After checking row/box constraints, only R4C3 can hold 7. Place 7 there.
Example 3: Hidden Single in a Box
Look at box 9 (bottom-right). Where can digit 6 go? The box contains cells in rows 7-9, columns 7-9. Checking each empty cell of box 9: R7C9 has candidates {2,5}, R8C8 has {5,7}, R9C7 has {2,6,7}, and R9C9 has {2,5,7} (cells like R7C7=4 and R8C7=3 are already filled). Digit 6 appears as a candidate only in R9C7, so 6 must go there.
Take one digit at a time and sweep the grid with it: all nine rows, then all nine columns, then all nine boxes, asking each time where that digit could still go. A unit with exactly one available cell is a Hidden Single. Boxes are usually quickest, since a 3×3 area scans at a glance.
The digit is hidden among the cell's other candidates, so the placement is not obvious from the cell alone. A cell showing candidates {2, 5, 7} looks undecided, but if 5 has nowhere else to go in its row, that cell must be 5. The unit reveals what the cell hides.
Scan for Naked Singles first, since a cell holding one candidate and nothing else is instant to confirm, then sweep digit by digit for Hidden Singles. Naked Single is the prerequisite technique and the quicker check, but the digit sweep is where most placements in a real solve come from.
No. You can find them by looking at where a digit is already placed and mentally blocking out the rows, columns and boxes it rules out, then seeing which cell of the unit survives. Pencil marks make the same search easier to verify, and a candidate that appears once in a unit is a Hidden Single.
Remove that digit as a candidate from every cell in the same row, column and box. Those removals frequently produce another Naked or Hidden Single nearby, so re-scan the intersecting units before starting a fresh sweep. Placements tend to ripple outward, and following the ripple is faster than beginning again.
Because a Hidden Single does not require the cell to be nearly resolved. A Naked Single needs eight of the nine digits eliminated from one cell, whereas a Hidden Single only needs one digit ruled out of the unit's other cells, so it works while cells still look busy with multiple candidates.
Explore all 28 solving techniques in our complete technique guide.