Solving with logic
Find your next move
You do not need to guess at every empty cell. Start by asking what can go there—and where a missing digit can go.
1. Make a candidate list
A candidate is a digit that does not repeat an existing digit in the cell’s row, column or box. For X Sudoku, also check either long diagonal that the cell belongs to. Candidates are possibilities, not promises: a legal move can still lead to a contradiction later.
If a cell’s row rules out 1, 2 and 3, its column rules out 4 and 5, and its box rules out 6 and 7, the remaining candidates are 8 and 9. This alone does not tell you which one to enter.
Use Notes to keep your own candidate lists. With Smart keypad off you can write any mark, so check your reasoning carefully. Show all candidates calculates the legal possibilities for you.
2. Look for a naked single
A naked single is a cell with exactly one legal candidate. Look at the digits already present in its row, column and box together, rather than looking at only one group.
Suppose those groups rule out 1, 2, 3, 4, 5, 6, 7 and 9. Only 8 remains, so the cell must be 8.
After placing it, revisit the other empty cells in those groups. Removing 8 from their possibilities might leave another single. In Helpful Sudoku, One-click naked singles can enter this kind of forced move when you select its cell.
3. Find a hidden single
Now reverse the question: instead of asking which digits fit one cell, ask where one digit fits in a whole row, column or box.
A row is missing 2, 5 and 8. Its three empty cells have candidates {2, 5}, {2, 5, 8} and {2, 5}. Only the middle cell can take 8. It must be 8, even though that cell has three candidates.
The single is “hidden” among other candidates. This is why searching only for cells with one pencil mark misses useful moves. The same reasoning applies to an X Sudoku diagonal. Hint can explain both naked and hidden singles without placing the digit for you.
4. Know what a contradiction tells you
A repeated digit breaks a Sudoku rule. An empty cell with no legal candidates is also a contradiction: something earlier has made the position impossible. A board with no immediate conflicts is not necessarily correct.
Use Undo to revisit a recent entry. If you made a branching choice, Rollback can return to it. The solver also uses choices and backtracking when forced moves are not enough; a trial choice is different from a proven single.
Practise without giving the answer away
Try an Easy or Medium puzzle with singles highlighting off. Spend a moment checking the fullest groups, then ask for a Hint if you are stuck. Read its explanation and locate the relevant row, column or box before entering the digit yourself.
As you gain confidence, turn off automatic candidates and maintain your own notes. Puzzle difficulty and assistance are separate settings: a harder puzzle does not force you to use more or less help.
Try a puzzle →