Hidden Single Sudoku Technique

Updated 11 August 2026

A hidden single is a digit that can legally go in only one cell of a row, column or box, so that cell must take the digit even though it still shows other candidates.

When a hidden single applies

Each of the nine digits has to appear exactly once in every row, column and box. That gives you a second way to reason. Instead of asking “what can go in this cell?”, you ask “where can this digit go in this unit?”. When the answer is a single cell, the placement is forced regardless of how many other candidates that cell carries.

Hidden singles are the most common non-trivial move in Sudoku and the technique most beginners skip, because pencil marks make a cell with four candidates look unsolvable. They are especially frequent inside boxes, where the three surrounding rows and three surrounding columns cut a digit down quickly.

A hidden single never needs an elimination step first, which is why it sits directly after full houses and naked singles in the Suuudokuuu solving order. Only when the board runs out of all three does the engine reach for intersections and subsets.

Worked example

Column 5 already contains 7, 1, 9 and 8, so it still needs 2, 3, 4, 5 and 6 across its five empty cells: r1c5, r2c5, r3c5, r4c5 and r9c5. Now follow the digit 3. The top-middle box already has a 3 at r3c6, which blocks r1c5, r2c5 and r3c5. Row 9 already has a 3 at r9c3, which blocks r9c5. Only r4c5 is left, so 3 goes there.

The point worth absorbing is that r4c5 was never a naked single: it still had 2 as a candidate at the moment of the deduction. Reading the cell alone would have told you nothing. Reading the digit across the unit solved it in one pass.

How to spot a hidden single

1. Pick a digit, not a cellChoose a digit that already appears several times on the board. The more copies exist, the more units it is blocked from.
2. Sweep the box with rows and columnsFor a box that still needs your digit, trace the three rows and three columns crossing it. Every line that already contains the digit rules out three cells of the box.
3. Count the surviving cellsIf exactly one cell in the unit can still hold the digit, you have a hidden single. Two or more survivors may still be useful later as a pointing pair or a hidden pair.
4. Place it and repeat for the same digitA newly placed digit blocks a fresh row, column and box, so re-sweep the same digit across the rest of the grid before switching to another one.

Common mistakes

• Only scanning boxes. Hidden singles hide in rows and columns just as often, and those are the ones most solvers miss.
• Assuming a cell with several pencil marks cannot be solved. The whole point of the technique is that the winning argument comes from the unit, not from the cell.
• Forgetting that a hidden single also removes every other candidate from the cell. Update those pencil marks or you will trip over stale notes later.
• Sweeping one digit and moving on. Placing it changes the picture for that same digit elsewhere on the board.