Hidden Singles

What is Hidden Singles?

A Hidden Single occurs when a particular digit has only one valid placement within a row, column, or 3×3 box — even though that cell might appear to have multiple candidates. It is "hidden" because the cell visually contains other candidates too, which can make it easy to overlook. The digit is not the only candidate in that cell, but it is the only candidate for that digit within the entire unit (row, column, or box). Hidden Singles are the single most common logical technique used to solve Easy, Medium, and many Hard Sudoku puzzles. In fact, many puzzles rated "Easy" can be solved entirely through Hidden Singles alone, making it the absolute backbone of logical Sudoku solving. Mastering this technique not only accelerates your solve times but also trains your eyes to scan each unit systematically — a habit that makes every subsequent technique more intuitive. Unlike Naked Singles (where only one digit fits in a cell), Hidden Singles require you to think about where a digit fits in a unit, rather than what digit fits in a cell. This subtle shift in perspective is a major step toward advanced solving.

When to Use Hidden Singles

Use this technique throughout every puzzle — especially in the early and middle stages when many cells are still empty. Scan each unit (row, column, 3×3 box) for any digit that can only reside in one position. This is most effective when you have partially-filled rows, columns, or boxes with several placed digits already narrowing down candidate cells. As a rule of thumb: every time you place a digit anywhere on the board, immediately check all units that the placed digit affects for new Hidden Singles. After completing Naked Singles (if any), move directly to scanning for Hidden Singles before attempting more complex techniques. The technique is also particularly powerful in boxes where 5 or more digits have already been placed — with fewer empty cells, the hidden single in the remaining positions becomes much easier to spot.

How to Apply Hidden Singles — Step by Step

1. 1

   Choose a digit to track

   Pick a digit from 1–9 that does not yet appear in a particular row, column, or 3×3 box. Focus on digits that already appear many times on the board overall — the more a digit is already placed, the fewer empty cells remain for it, making hidden singles easier to find. For example, if the digit 7 appears in 7 of the 9 rows, it can only appear in 2 more rows — those rows become your focus.

2. 2

   Mark candidate cells within the unit

   Within the target unit (say, a specific 3×3 box), mark every empty cell where the chosen digit could possibly appear. A cell is a valid candidate if and only if the digit does not already exist in that cell's row, column, or box. Use pencil marks or mentally cross out invalid cells. Be thorough — missing a constraint will lead to false conclusions.

3. 3

   Identify the single remaining cell

   If only one cell in the unit remains as a valid candidate for your digit, that cell must contain it. This is the hidden single. The digit has nowhere else to go within that unit, regardless of what other candidates that cell may show. The "hidden" aspect is that the cell might show [2, 5, 7] as candidates, but since 2 and 5 can appear elsewhere in the unit and only 7 is confined to this one cell, 7 is the hidden single.

4. 4

   Place the digit and propagate

   Write the confirmed digit in its cell and immediately update all related candidates. Remove that digit from every other cell in the same row, column, and 3×3 box. After each placement, re-scan all three units (the row, column, and box of the placed cell) for newly revealed hidden singles, naked singles, or other patterns. This cascade of updates is where much of the solving progress happens.

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