Guide · Rules
How to Play Sudoku
The rule (there's really just one)
Fill every row, every column, and every 3×3 box with the digits 1 through 9. No digit may repeat inside a row, column, or box. A well-formed puzzle has exactly one solution.
Step 1 — Scan for forced digits
Pick a digit like 5. Find every row, column, and box that already contains a 5. Any cell that can't be any other digit — because 5 is the only one left — must be a 5.
Step 2 — Use notes (candidates)
When scanning stops finding forced digits, pencil in every possible digit for each empty cell. Look for cells with only one candidate ("naked singles") and units where a digit appears in only one cell ("hidden singles").
Step 3 — Pairs, pointing, and beyond
Two cells in the same unit that share the exact same two candidates form a naked pair — remove those two digits from every other cell in the unit. This is the gateway to intermediate techniques covered in our advanced strategies guide.
Common mistakes
Guessing (there's always a logical next step), forgetting to update notes after placing a digit, and skipping scans on the digits you find hardest — usually 4, 6, or 8.
Frequently asked questions
What are the rules of Sudoku?
Fill every row, every column, and every 3×3 box with the digits 1 through 9 without repeating any digit inside a row, column, or box.
Is guessing allowed in Sudoku?
A well-formed Sudoku puzzle can always be solved by pure logic — no guessing required. If you feel like guessing, there's a technique you haven't tried yet.
How many clues does a valid Sudoku need?
A Sudoku puzzle must have at least 17 given digits to have a unique solution. Most easy puzzles start with 35–45 clues.
What are Sudoku notes / candidates?
Notes are small pencil-marks showing the possible digits for a cell. They're essential from Medium upward. Enable Notes mode on the toolbar or turn on Auto-notes per game.