About Sudoku
Fill a nine-by-nine grid so every row, column, and box contains 1 through 9.
Sudoku is a number-placement puzzle with a nine-by-nine board divided into nine three-by-three boxes. Fill the blank cells so each row, each column, and each box contains the digits 1 through 9 exactly once. The starting clues are fixed and cannot be changed.
Play 30 fixed puzzles, with 10 each in Easy, Medium, and Hard. Each has a checked unique solution. Our technique-based ratings distinguish naked singles, hidden singles, and puzzles that need branching in our checker; they are not based only on the number of clues.
Use pencil notes, correct duplicate entries marked with an exclamation point, or request a correct digit with Hint. Undo retains the last 10 entry or note changes. Pause hides the board and stops its clock. Your current entries, notes, completed puzzles, and best times are saved.
How to play
Select an empty cell and choose a digit. Check its row, column, and three-by-three box before committing. Turn Notes on to add or remove pencil candidates. Clear an editable cell, undo an entry, or request a hint. Fill the entire board correctly to finish.
Controls
Click or tap a cell, then use the on-screen number pad or keys 1–9. Arrow keys move the selected cell. Backspace, Delete, or 0 clears an editable cell. Notes toggles pencil marks; Undo reverses up to 10 entry or note changes. Hint supplies a correct digit. Pause covers the board and stops the clock; Resume continues. Choose a puzzle from the numbered difficulty menu. Starting clues stay locked.
Tips
Look for a row, column, or box with only one missing digit. In a more open area, write candidates and compare where a digit can fit. A duplicate-free board is not necessarily solved: all cells must satisfy the full puzzle.
Questions about Sudoku
Can I change the starting numbers?
No. Starting clues define the puzzle and stay locked. You can edit only the cells that began empty.
What are pencil notes for?
Notes record possible digits without filling a cell. They help you compare candidates as the rest of the board changes.
How are Easy, Medium, and Hard rated?
Easy puzzles can be completed using naked singles: cells with one possible digit. Medium puzzles also need hidden singles: a digit with only one possible position in a row, column, or box. Hard puzzles go beyond those two techniques, so our checker needs branching. These are ratings from our checker; advanced human techniques may solve a Hard puzzle without guessing.
