Last Remaining Cell

The Last Remaining Cell is a straightforward sudoku technique used when a row, column, or 3x3 box has exactly one empty cell left. Since the other eight cells already contain their digits, the missing number for that region must go in the final empty spot. It is simple, certain, and shows up at every difficulty level.

When To Look For It

A row, column, or box shows eight filled cells and only one blank.
You have just placed a number and want quick follow ups in the affected regions.
Your notes reveal that a region is missing a single digit and there is only one empty cell to take it.

How It Works

Choose a region: Pick a row, column, or 3x3 box with one empty cell.
List the digits present: Note which numbers from 1 to 9 are already placed there.
Find the missing digit: The number not present is the answer.
Fill the empty cell: Place the missing digit in the only remaining spot.

Quick Example

In a column you see 1, 2, 3, 4, 5, 6, 7, and 9 already placed with one cell empty. The only digit missing is 8, so the last cell must be 8.

Why It Works

Each row, column, and box must contain the digits 1 through 9 exactly once. If eight cells are already filled, the ninth cell is forced to be the missing digit. There is no guesswork involved.

Tips

Rescan after every move: New last-remaining situations often appear right after a placement.
Use with Last Possible Number: Alternate between these two basics to trigger fast cascades of safe placements.
Keep candidates updated: Clear notes make it easy to spot when a region is missing a single digit.

Common Mistakes To Avoid

Overlooking a filled cell: Double-check the region so you are truly dealing with one empty cell.
Mixing regions: Confirm that your missing digit belongs to the specific row, column, or box you are inspecting.

Where You Will Use It

Last Remaining Cell is common in easy puzzles and still essential in harder ones. Advanced deductions often reduce a region to one empty spot, letting you place a number with complete certainty.