A LITTLE FOCUS. A FRESH START.
Play Classic Sudoku
Play free Classic Sudoku online, from beginner to expert. Fill the familiar 9×9 grid with notes, hints, night mode, and saved progress.
Get the iPhone app
OneStop Sudoku, wherever you go.
Website saves stay in this browser and don’t transfer to the app.
Scan to downloador tap to open
The game loads here. Choose a difficulty to start or resume a puzzle.
LEARN THE TWIST
How to play Classic Sudoku
Classic uses only rows, columns, and regular 3×3 boxes. It is a useful starting point before trying variants with cages, lines, or extra regions.
The rules
Fill each row, column, and 3×3 box with 1–9, without repeats.
A useful first move
Start with nearly complete rows, columns, or boxes. Pencil in candidates when a cell has several possibilities, then remove candidates as nearby cells are solved.
If a row already contains 1, 2, 3, 4, 6, 7, 8, and 9, its last empty cell must be 5. With more empty cells, check each cell’s column and box to narrow the possibilities.
SOLVE IT STEP BY STEP
Turn two possibilities into one placement
A candidate is a digit that has not yet been ruled out. Writing two candidates in a cell does not mean either is a guess you should enter. Look for a row, column, or box that removes one of them. This small example shows how a placement in one unit can finish another.
- Third cell
- 5
- Fifth cell
- 3
List what the row is missing.
The seven placed digits leave only 3 and 5. Both empty cells begin with those two possibilities when you consider the row by itself.
Check the crossing column and box.
The third column already contains 3. The third cell in the top row therefore cannot be 3; it must be 5. Here the same given also shares its box, so either unit justifies the elimination.
Return to the row.
With 5 placed, the fifth cell must be 3. One solved cell often starts a chain, so revisit the units it touches before looking for a harder technique.
A single cell, or a single place?
A naked single is a cell with just one candidate, as in the third cell above. A hidden single is different: a digit has only one possible place in a row, column, or box, even if that cell still has several notes. Scan both ways. If neither works, look for two cells in the same unit whose complete candidate lists are the same pair; those two digits are reserved for those cells and can be removed from the rest of that unit.
Common mistakes to avoid
- Do not stop after checking a row. Every placement must also satisfy its column and box.
- Two cells sharing a candidate do not automatically form a pair. Each must have exactly the same two remaining candidates before the pair reserves those digits.
Try the reasoning yourself
A column is missing 2 and 8. One empty cell shares a box with an 8. What belongs in each empty cell?
Show the answer and why
The cell that sees the 8 must be 2. The other empty cell in the column is then 8. The box restriction chooses the order; the column alone did not.
Controls, difficulty, and saved games
Choose Beginner, Easy, Medium, Hard, or Expert above. Select an empty cell, then tap a number or use your keyboard. Notes let you pencil in candidates; three hints are available. Three mistakes end the puzzle.
Every mode and difficulty keeps a separate save in this browser. You can switch puzzles and return to your place. Clearing site data removes these saves; they do not sync with another device or the iPhone app. Settings include night mode and optional automatic cell advancement.
Correct entries earn points. High scores are stored by mode and difficulty and update only when beaten; completed-puzzle count and day streak are shared.
FIFTEEN WAYS TO PLAY
Explore all Sudoku modes
A familiar grid, a different kind of logic. Every mode is free to play in your browser.