Sudoku Rules - How to Play
Place digits so every house is complete and unique. Clues start filled in; the rest is deduction.
The board and the goal
Classic sudoku uses a 9×9 grid divided into nine 3×3 boxes. Some cells already show given digits. Your goal is to fill every empty cell with a digit from 1 to 9 so that each row, each column, and each box contains every digit exactly once. No other symbols are used, and a finished board has no blanks.
On this site, puzzles are generated with a single solution. You may enter digits with the on-screen pad or the keyboard. Given clues cannot be overwritten. When the grid is complete and valid, the puzzle is solved and the timer stops for that run.
Houses, conflicts, and legal fills
Think in houses: a row, a column, or a 3×3 box. Within any house, a digit may appear only once. If you place a second 7 in the same row, column, or box, that placement conflicts with the rules. Conflict highlighting marks those repeats so you can correct them before chasing speed.
A legal fill is any digit that does not repeat inside its three houses. Guessing is allowed in the interface, but strong play prefers placements forced by scanning and candidates. Clearing a conflict restores a legal partial board; the engine still expects the unique final solution.
Pencil notes, hints, and difficulty
Pencil notes let you store candidate digits inside a cell while you work. Turning notes on writes or toggles candidates; placing a final digit clears that cell’s notes and removes the digit from peers in the same row, column, and box. Hints reveal one correct empty cell when you are stuck. The timer keeps running until the board is finished.
Easy, Medium, and Hard change how many clues remain and how deep the solving path usually goes. Starting a new difficulty generates a fresh puzzle. Best finish times for each difficulty stay in this browser’s local storage, along with an in-progress board if you leave mid-solve.
Frequently asked questions
- Must every row, column, and box use 1–9 exactly once?
- Yes. A finished sudoku has each digit from 1 to 9 once in every row, every column, and every 3×3 box.
- Are the puzzles here uniquely solvable?
- Yes. Each generated grid has one solution. Conflict highlighting flags illegal repeats while you work.
- What do pencil notes do?
- Notes let you mark candidate digits in a cell. Placing a final digit clears that cell’s notes and removes the digit from related houses.
- Does a hint pause the timer?
- No. The timer keeps running until the board is complete, even if you reveal a correct empty cell with a hint.
Put the rules into practice
Open a solo board, then use the strategy guide when you want a repeatable scanning plan.