Beginner
Last Free Cell
A core Sudoku solving technique for beginner players
What is the Last Free Cell technique?
The Last Free Cell is the simplest Sudoku solving technique. When a row, column, or 3×3 box has only one empty cell remaining, you know exactly which number belongs there - It must be the single digit from 1–9 that is not yet present in that unit.
No candidate tracking is needed. You do not need to write pencil marks. You simply look for a unit (row, column, or box) that is almost full and fill in the gap.
Step-by-step guide
- Scan every row from top to bottom. Count empty cells in each row.
- When a row has exactly one empty cell, note which numbers 1–9 are already in that row.
- The missing number is the answer - Write it in the empty cell.
- Repeat the scan for all nine columns.
- Finally, scan all nine 3×3 boxes for any that have only one empty cell.
How it compares to other beginner techniques
| Technique | What you scan | Candidate marks needed? | Typical puzzle level |
|---|---|---|---|
| Last Free Cell | One unit with 1 empty cell | No | Easy |
| Last Remaining Cell | One digit with 1 open spot in a unit | No | Easy |
| Obvious Singles | Cells with only one candidate | Yes | Medium |
Common mistakes
| Mistake | Why it happens | How to avoid it |
|---|---|---|
| Counting wrong - Missing a number already in the row | Rushing through the scan | Count filled cells from left to right before deciding |
| Skipping boxes - Only scanning rows and columns | Habit | Always run a box scan after every row/column pass |
| Writing the number in the wrong empty cell of a near-full box | Two empty cells look like one | Count empties carefully before writing |
Pro tip: Apply Last Free Cell on every scan pass before trying harder techniques. As you fill in cells, new Last Free Cells appear - One fill-in often unlocks a chain of several more.