June 3, 2026 · 5 min read

Hidden Singles in Sudoku

A hidden single is a digit that can legally go in only one cell of a row, column, or box — even though that cell looks like it has several options. It's the most useful beginner technique after basic scanning, and it's the one that quietly solves most of a medium Sudoku. Learning to see hidden singles is the moment Sudoku starts to click.

The word "hidden" is the key. With a last remaining cell or a naked single, the cell itself obviously has one answer. A hidden single is disguised: the cell carries two or three candidates, so nothing about it stands out. The single only appears when you look at the whole unit and notice that one particular digit has nowhere else to go.

A worked example

• the unit in focus
• the pattern
• a placement
• an elimination

Look only at the top-right box. Three cells are empty, with candidates {4, 6}, {4, 5}, and {4, 6}. If you judged the middle cell on its own, you couldn't decide between 4 and 5. But scan the box for the digit 5 specifically: it appears in just one of the three cells. The box must contain a 5 somewhere, and there is only one cell that accepts it — so 5 is placed, even though that cell also listed a 4.

That's the whole technique. Instead of asking "what can go in this cell?", you ask "where in this unit can this digit go?" When the answer is "only here," you have a hidden single.

How to hunt for them

Work one digit at a time within one unit. Pick a box, then run through the digits it's still missing. For each missing digit, count how many of the box's empty cells could legally hold it — remembering that a digit is blocked if it already appears in that cell's row or column. If exactly one cell survives, place the digit.

Boxes are the easiest place to start because the 3×3 shape makes the elimination visual, but hidden singles live in rows and columns too. After you've swept the boxes, run the same check along any row or column that still has several gaps.

Why they're easy to miss

Beginners overlook hidden singles because they scan cell by cell, and a hidden single's cell never looks special — it has the same two or three candidates as its neighbours. The fix is to switch your unit of attention from the cell to the digit. Once you train yourself to ask "where can the 5 go in this box?" the hidden singles light up.

This is also why pencil marks help. With every cell's candidates written in, a hidden single is visible the moment you notice a digit that appears only once across a unit. On Sudoku Zen, turn on notes mode and the pattern becomes much easier to spot.

Next steps

Hidden singles pair naturally with cross-hatching, which is the visual scanning method for finding them inside a box. Once both feel automatic, move on to naked pairs — the first true candidate-elimination technique.