1. Scanning for hidden singles
Choose a digit, say 5. For each box that doesn't have a 5 yet, look along the rows and columns that pass through it. Every row or column that already contains a 5 rules out its cells in that box. If only one cell in the box survives, that's where the 5 goes. It's called a hidden single because the cell may have several candidates, but it is the only home for that digit. Easy and Medium puzzles on PuzzleLab can be solved with this alone.
2. Naked singles
Flip the question: pick an empty cell and list the digits its row, column and box already use. If eight are taken, the ninth is the answer. This is slower to spot by eye, which is why candidates (pencil marks) help.
3. Pencil marks, done properly
Once singles dry up, write every possible digit into every empty cell. Update them every time you place a digit. Candidates turn hard puzzles from staring into looking for patterns. The Fill notes button on the Sudoku game does the tedious part.
4. Naked pairs
Two cells in one unit that share exactly the same two candidates, say {3, 7}, claim those digits between them. Neither 3 nor 7 can appear anywhere else in that unit, so cross them out. The same logic works for three cells sharing three candidates (a naked triple).
5. Pointing pairs and box-line reduction
If every candidate for 4 inside a box lies in a single row, the box's 4 is in that row, and no other cell in that row, outside the box, can be 4. It works the other way too: if a row's only candidates for 4 fall inside one box, remove 4 from the rest of that box.
Watch it work
This Hard puzzle needs every technique above. Press Next step: the cells that prove each move are shaded, placed digits turn green and eliminated candidates are struck through in red.
Enjoy the moment a stubborn grid suddenly gives way? ahaboo offers that same click for the night sky, with a narrated, hands-on look at why the Moon goes through phases.
When you're really stuck
Harder techniques exist (X-wing, Swordfish, XY-wing and chains), but you won't need them for the Easy to Hard levels here. For any puzzle, from a newspaper or a book, the Sudoku solver will show you the next logical step rather than the whole answer.
Ask the puzzle coach
Stuck on a rule or a tactic? Ask in plain words. The coach only talks puzzles, and the answer is written for you on the spot.
Questions players ask
What is the best Sudoku strategy for beginners?
Scan one digit at a time. Pick a digit, look at every box missing it, and cross out the rows and columns where it already appears. Where only one cell is left, place it. Repeat for each digit.
Should I guess in Sudoku?
A well-made Sudoku never needs guessing: every cell can be deduced. If you are stuck, fill in all candidates and look for pairs.
What is a naked pair?
Two cells in the same row, column or box that can each only be the same two digits. Those two digits must go in those two cells, so they can be removed from every other cell in that unit.
What is a pointing pair?
When all the possible places for a digit inside a box lie in one row (or column), the digit must be in that row within the box, so it can be removed from the rest of that row outside the box.