What is Sudoku?
A classic Sudoku puzzle uses a 9x9 grid divided into nine 3x3 boxes. Some cells start with fixed numbers called givens. Your job is to complete the empty cells using the digits 1 to 9.
A well-made Sudoku has exactly one solution. You never need arithmetic, guessing or trial and error. The puzzle is solved by excluding impossible numbers and spotting where a number must go.
- Each row must contain the digits 1 to 9 once.
- Each column must contain the digits 1 to 9 once.
- Each 3x3 box must contain the digits 1 to 9 once.
How to play Sudoku
Start by scanning rows, columns and boxes that already contain many numbers. If a row is missing only one digit, the missing value is forced. Then look at each empty cell and ask which digits are ruled out by its row, column and box.
Use notes when a cell has several possible values. Notes are small candidate digits that help you track what may still fit. On this Sudoku board, the notes button lets you add candidates manually, and auto notes can fill the current candidates for every empty cell.
- Select an empty cell.
- Tap a number or use the keyboard to place it.
- Turn on notes to add candidate numbers instead of final answers.
- Use hint if you are stuck and want one correct cell revealed.
- Use undo, redo or erase when you want to adjust your work.
Sudoku difficulty levels
Difficulty is not only about how many starting numbers appear in the grid. A puzzle can have many givens and still be tricky if the next logical step is hidden. The best Sudoku difficulty combines the number of clues with the techniques required to finish the solve.
This online Sudoku starts on Medium, which is a good everyday challenge. Easy is best for warm-ups and new solvers, Hard introduces more candidate work, and Expert is for players who enjoy slow, careful deductions.
- Easy: more starting clues and mostly direct placements.
- Medium: a balanced challenge with singles and candidate elimination.
- Hard: fewer clues and more pressure to use notes well.
- Expert: a tougher grid for experienced solvers.
Core Sudoku strategies
The strongest solvers do not jump around at random. They use repeatable checks. First, look for naked singles: cells with only one possible candidate. Then look for hidden singles: a digit that has only one possible position inside a row, column or box.
When singles dry up, compare candidate sets. If two cells in the same row, column or box can only contain the same two digits, those digits can be removed from the rest of that unit. This is the basic idea behind pairs, triples and many advanced techniques.
- Scan nearly complete rows, columns and boxes first.
- Track candidates with notes instead of guessing.
- Remove candidates from related cells after placing a number.
- Search one digit at a time when the whole grid feels busy.
- If stuck, move to another region and return with fresh constraints.
Why play Sudoku online?
Online Sudoku is quick to start and easy to pause. The timer tracks your solve, the board highlights related cells, and your progress is saved locally in your browser so you can come back later.
This page is designed as the evergreen Sudoku home for Logic Puzzles Online. Future printable Sudoku, Sudoku helper and Sudoku solver pages will link from here once those tools are ready, while generated puzzles stay inside the game instead of becoming thin puzzle-instance URLs.