About Classic Sudoku
Classic sudoku, free in your browser: fill the nine-by-nine grid so that every row, column and three-by-three box holds the digits 1 to 9 once each. Four levels from Easy to Expert, every puzzle with exactly one solution, and pencil marks, hints and undo when you want them. No download, no sign-up, and an unfinished grid is still there when you come back.
How to play sudoku
- Select an empty square, then choose a digit from 1 to 9.
- Each row, each column and each three-by-three box must end up holding every digit exactly once.
- The digits printed in bold are the givens and cannot be changed.
- A wrong digit turns red and counts as a mistake. It stays on the board until you change it.
- Turn on Notes to pencil in the digits a square could still take; a correct digit clears itself from the notes around it.
- Fill the last square correctly and the puzzle is solved.
Difficulty levels
| Level | What it takes |
|---|---|
| Easy | Plenty of givens; singles alone solve it: the only digit left for a square, or the only square left for a digit. |
| Medium | Fewer givens, still solvable with singles, but they are harder to find. |
| Hard | Needs pairs and pointing digits: two squares that share two candidates, or a digit confined to one line of a box. |
| Expert | Those run out; it needs longer chains of reasoning. |
Every puzzle is generated fresh and checked to have exactly one solution before it reaches the board.
How scoring works
| Level | Base | Speed bonus |
|---|---|---|
| Easy | 1,000 | up to 2,000, falling 4 points a second |
| Medium | 2,000 | up to 3,000, falling 3 points a second |
| Hard | 3,500 | up to 4,000, falling 2.5 points a second |
| Expert | 5,000 | up to 5,000, falling 2 points a second |
Each mistake takes 100 points off and each hint 250, but a solved puzzle always scores at least a tenth of its base. Only solved puzzles go to the leaderboard.
Controls and shortcuts
- Click or tap a square, then a digit on the number pad. The pad shows how many of each digit are still to place.
- Keys 1–9 enter a digit; Shift with a digit, or N to switch Notes on, pencils it in instead.
- Arrow keys move around the grid; Backspace or Delete clears a square.
- H fills a square for you: the selected one if it is empty or wrong, otherwise the easiest one left.
- U or Ctrl+Z undoes, as far back as you like.
Tips
- Start with the digit that is already on the board most often - it has the fewest places left.
- Scan each box for a digit it is missing, and cross out the rows and columns that already have it.
- Use notes in the squares with two or three candidates; they are where the next step usually is.
- When a digit is placed, check its row, column and box straight away: it often unlocks the next one.
The techniques behind each level are explained in how to solve sudoku.
