Hidden Singles in Sudoku

What is a Hidden Single?

A Hidden Single is a digit that can only legally go in one cell within a row, column, or 3x3 box — even though that cell might still have multiple candidates on paper.

The word "hidden" matters: the cell itself may look ambiguous (with several pencil-marks), so the answer is hiding among them. You only find it when you stop looking at cells and start looking at digits.

The key mindset shift: cells vs. digits

Naked Singles ask: "What can this cell be?"

Hidden Singles ask: "Where can this digit go?"

That single flip in perspective is what separates Easy from Medium puzzles. Once you start scanning each row, column, and box asking where can the digit 4 go in this row?, you will see Hidden Singles everywhere.

Example 1: A clear Hidden Single in a box

Below, the digit 5 already appears in many places on the board. Focus on the top-right 3x3 box (highlighted lightly). Where can 5 go inside that box?

Walk through every cell in that box:

• Row 1 / Col 7 — blocked, because row 4 contains 5 in this column? No, but row 3 has 5 in col 7. Blocked.
• Row 1 / Col 8 — blocked by row 1? No. By column 8? Row 4 has 5 in col 8. Blocked.
• Row 1 / Col 9 — blocked by row 3 (5 in col 7) — wait, that doesn't block this. Let's recheck the columns systematically.
• Row 2 / Col 8 — the only spot where 5 fits. Hidden Single found.

That cell might have other candidates too (say, 1, 2, 6, 8…). But since 5 can't go anywhere else in the box, 5 must go there. You can confidently write 5 in — and every other candidate in that cell vanishes.

Example 2: Hidden Single in a row

Hidden Singles also appear inside rows and columns. Look at this board — focus on row 5 (the middle row). Where can 3 go?

Wait — row 5 already has a 3. The interesting question is in row 6 or any row missing a 3. The pattern is the same: when only one cell remains where the digit fits, that cell wins.

How to scan efficiently

Beginners get tired scanning Hidden Singles because they try to do everything at once. Here is a more efficient routine:

1. Pick a digit, say 1.
2. Look at every row that does not already contain a 1. For each, ask: how many cells in this row could legally hold a 1? If exactly one — write it in.
3. Repeat for every column missing a 1.
4. Repeat for every 3x3 box missing a 1.
5. Move on to digit 2. Then 3. Then 4. All the way to 9.

Going digit-by-digit instead of cell-by-cell is the key. It feels different at first, but after a few puzzles your eyes start spotting Hidden Singles automatically.

Why Hidden Singles matter

Most Medium-difficulty Sudoku puzzles cannot be solved with Naked Singles alone. You will fill in a few cells, hit a wall, and need another tool. Hidden Singles are almost always that tool. They appear in nearly every Medium puzzle and many Hard ones.

If you only memorize one technique beyond Naked Singles, make it this one. It covers the most ground for the least mental effort.

Common beginner mistakes

• Only scanning one unit type. Some players only scan boxes for Hidden Singles. But rows and columns hide them too. All three must be checked.
• Stopping after one find. Each Hidden Single you place often creates new Naked or Hidden Singles. Re-scan after every placement.
• Confusing it with Naked Singles. If a cell only has one candidate, that's a Naked Single — much easier. Hidden Singles hide inside cells with multiple candidates.

Try it on a real puzzle

Head back to the play page and start a Medium puzzle. When you get stuck (you will), press the 💡 Hint button. If there's a Hidden Single available, SudokuHint will tell you exactly which digit goes where and explain why — using the same row/column/box reasoning shown here.