Last Possible Number
The Last Possible Number technique focuses on a single empty cell in the sudoku grid and asks: which digits are still available? By eliminating all digits that already appear in the cell's row, column and 3×3 box, the solver narrows down the candidates. When only one possible digit remains, that digit must be the answer.
Focus on one cell
Select any empty cell in your sudoku puzzle. Look at the digits already present in its row — those cannot appear in this cell. Look at the digits already present in its column — those are also forbidden. Finally, look at the digits in its 3×3 box — those too are excluded. Whatever digit from 1 to 9 is not among those three groups is a candidate. If only one candidate remains, you have found the last possible number for that cell. This is sometimes called the "cross-hatching" technique in sudoku literature because you mentally cross out digits across three intersecting houses.
The only candidate
This technique is most powerful in the early stages of a sudoku game, especially on easy and medium difficulties. Cells in the center of the grid, where rows, columns and boxes overlap with many placed digits, are prime candidates. The method also pairs well with pencil notes: after auto-filling all candidates on your sudoku board, any cell showing a single candidate is a last possible number. Even when multiple candidates remain, narrowing them down prepares you for more advanced sudoku strategies like hidden singles, naked pairs and pointing claims.
Practice makes it fast
With consistent practice, scanning a cell's three houses becomes a split-second mental operation. Expert sudoku players use this technique hundreds of times in a single puzzle without consciously counting — they simply "see" the missing digit. To train this skill, play medium-level sudoku puzzles on Sudoked.com and avoid the hint button for the first ten minutes. Challenge yourself to fill every cell you can using only Last Possible Number and Last Free Cell before switching to notes. You will develop the pattern recognition that makes hard and expert sudoku feel manageable.