Last Free Cell
A trivial win.
Last Free Cell is the simplest deduction in Sudoku, and the first one most players ever use. Every row, column, and 3×3 box must contain each digit from 1 to 9 exactly once — no repeats, no gaps. When only one cell in a given block, row, or column is still empty, that means eight of the nine required digits are already placed somewhere inside it.
Only one digit is left unaccounted for, and since the region has exactly one open cell left to hold it, there is nowhere else in the grid it could go. No candidates to track, no elimination logic, no ambiguity: identify the missing digit, place it, and the region is complete. Because it needs no notes and no scanning beyond a single region, Last Free Cell is where every other technique on this page ultimately leads — every elimination you make elsewhere exists to leave some region with exactly one gap remaining.
How It Works
When a 3×3 box, a row, or a column has exactly one empty cell, examine which digit from 1–9 is missing. That missing digit is the only number that can legally fill the gap.
- Locate any block, row, or column with exactly one empty cell.
- Identify which digit from 1–9 does not appear in that region.
- Place it.
No deduction required. No strategy. The digit simply ran out of places to hide.
Example
Below board presents three examples of the last free cell.
Group 1 contains all digits except 1, which means that digit 1 must be placed in the cell highlighted with green. The cell highlighted with purple must contain digit 5 since it's the only one missing in row 8. Column 7 is missing digit 9, which must be placed in cell highlighted with red.
On Evil Boards
An Evil board offers a free cell only deep in the endgame, after every placement leading up to it has been fought for. Every free cell you claim is an easy but a well deserved win. If a region isn't down to its last cell yet, check it with last remaining cell instead. You can also fall back to last possible number for a single stubborn cell anywhere on the board.