Sudoku – rules

Sudoku is a logic game for one. You get a 9 by 9 grid with some digits already in place. You fill in the rest so that each digit from 1 to 9 stands once in every row, every column and every 3 by 3 box.

How to play, step by step

Step 1

The grid

The grid has 81 cells: 9 rows, 9 columns and 9 boxes of 3 by 3, set apart by a thicker line. The dark digits are given at the start and cannot change. You fill the empty cells. Under the grid, or beside it, are the buttons with the digits 1 to 9.

Step 2

Each digit once

In every row, every column and every 3 by 3 box, each digit from 1 to 9 stands exactly once. Each puzzle has only one solution, and you can reach it by logic alone, without guessing.

Step 3

Enter a digit

Tap an empty cell, then a digit under the grid. You can also do it the other way round: tap a digit first, then each cell it should go into. The picked cell's row, column and box are shaded, and the same digit is lit across the grid. Tapping the same digit again clears the cell.

Step 4

Mistakes

A wrong digit turns red at once. You can turn this off in Options ("Sudoku: show mistakes at once"). Mistakes do not end the game and have no limit, but their number shows at the end. The "Erase" button clears the picked cell.

Step 5

Notes

When you are not sure which digit fits, turn on "Notes": digits then go in small, as a try, several in one cell. A digit you enter leaves the notes in its row, column and box by itself. On the harder levels notes help a lot.

Step 6

Hint and undo

"Hint" puts the right digit into one cell: the picked one if it is empty or wrong, otherwise the cell that is easiest to fill now. Each hint adds 30 seconds to your time. "Undo" at the top of the screen takes back moves without a limit, but mistakes and hints stay counted.

Step 7

Level and style

In the Games window, under Sudoku, you pick the "Level": easy, medium, hard or very hard. Easy gives more digits at the start; the harder ones need longer thinking. The "Style" button by the grid changes its look: paper, tiles, glass or frame.

Step 8

End of the game

When every cell holds the right digit, the "Sudoku solved" window opens with your time, the number of mistakes and hints, and your best time at this level. "New game" gives a new puzzle, and "Retry game" the same one again.

The rules in this game

Sudoku is played in different ways. Here the rules above apply.
The classic 9 by 9 grid with the digits 1 to 9.
Each puzzle has one solution that logic alone leads to; the level depends on how hard the steps it needs are.
Mistakes do not end the game and have no limit.
The clock stops while the game is paused ("Pause") or the browser tab is hidden.

Scoring

Your score is your time: the shorter, the better. Each hint adds 30 seconds to it; mistakes add nothing. For example, you solve a puzzle in 9 minutes and 40 seconds and take 2 hints, so your time is 10:40. A solved puzzle goes to your statistics and to the Ranking, separately for each level; a puzzle played again counts separately.

Tips

Start with the rows, columns and boxes that miss only a few digits.
Take one digit and check each box: often it fits only one cell there.
On the harder levels, put notes in the cells where only 2 or 3 digits fit.
Tap a digit on the grid: the highlight shows at once where it already stands.

Play Sudoku