Sudoku

Sudoku guide

Sudoku strategy

Good Sudoku strategy is mostly disciplined scanning: find what must be true before making a move.

Try the strategy in a puzzle No install, no timer pressure
Example Sudoku board with the next useful cell highlighted

Start with singles

A naked single appears when only one number can fit in a cell. A hidden single appears when a number has only one possible place in a row, column, or box.

Use notes carefully

Notes help on harder puzzles, but too many notes can slow you down. Update them whenever a new number removes a candidate.

Avoid guessing

Guessing can create hidden mistakes. If you are stuck, rescan boxes and look for candidate pairs before placing a risky number.

Strategy examples

What to look for

Last possible number

The selected square already sees 1, 2, 4, 5, 6, 7, 8, and 9 in its row, column, and box. Only 3 remains.

Pick one empty cell and remove every digit already present in its row, column, or box. If one digit remains, place it.
Last possible number example: blockers leave only 3

What to look for

Last remaining cell

In the highlighted row, 7 has only one legal cell left. The selected square is the last remaining place for that number.

Scan one number at a time across a row, column, or box. If the number has only one home, place it before moving on.
Last remaining cell example: only one row cell can contain 7

What to look for

Pointing pair

The two 9-candidates in the upper-right box sit on the same row, so 9 can be removed from the rest of that row.

When a candidate is locked to one row or column inside a box, use that line to remove matching candidates outside the box.
Pointing pair example: two 9-candidates point across the row

What to look for

Obvious pairs

The two selected cells in the column contain only 2 and 6. Those numbers are reserved there, so 2 and 6 can be removed from the other cells in the column.

When the same two candidates appear in exactly two cells of a unit, remove those candidates from the rest of that unit.
Obvious pairs example: two cells reserve 2 and 6

What to look for

Obvious pairs in a box

The two selected cells in the lower-left box contain only 4 and 5. Those numbers are reserved there, so 4 and 5 can be removed from the other cells in that box.

Check each 3x3 box for two cells with the same exact pair of notes. The pair controls the whole box, not just a row or column.
Obvious pairs box example: two cells reserve 4 and 5

What to look for

Hidden pair

In the highlighted row, 1 and 9 appear only in the two selected cells. Those cells must take 1 and 9, so the other notes in them can be removed.

Hidden pairs are the inverse of naked pairs: find two digits that share exactly two homes, then clean the extra notes from those cells.
Hidden pair example: 1 and 9 are locked to two cells

What to look for

Hidden pairs in a box

Only the two selected cells in the box contain 5 and 9. They must take those two numbers, so the extra notes in those cells can be removed.

For hidden pairs, scan the homes of two digits inside one unit. If both digits can only live in the same two cells, keep only those digits in those cells.
Hidden pairs box example: 5 and 9 are locked to two cells

Practice

Play the pattern on a real board

More Sudoku guides