How to Play Sudoku

Fill the 9x9 grid so that every row, every column and every one of the nine 3x3 boxes contains the numbers 1 to 9 exactly once. Some squares already contain numbers; these are your clues. Your job is to work out the rest.

1. Choose a difficulty. The game starts on Easy.
2. Select an empty square by clicking or tapping it. With a keyboard, press Tab to reach the grid and then use the arrow keys to move between squares.
3. Enter a number from 1 to 9 with the number pad, by tapping, or by pressing the number on your keyboard.
4. A wrong number stays on the board, marked in red with an underline and a small corner flag, and counts as a mistake. You can change or erase it, but the mistake stays counted.
5. The puzzle is complete when every square matches the solution. Your difficulty, time, mistakes and hints used are then shown.

Sudoku Rules

• Each row must contain the numbers 1 to 9, with no repeats.
• Each column must contain the numbers 1 to 9, with no repeats.
• Each of the nine 3x3 boxes must contain the numbers 1 to 9, with no repeats.
• The starting numbers already on the board (shown in darker, bolder text) can never be changed.
• Every puzzle has exactly one correct solution, and it can be found by logic alone.

What Each Button Does

• Easy, Medium, Hard: start a new puzzle at that difficulty. If you have made progress on the current puzzle, you are asked to confirm first because that progress will be lost.
• Number pad (1 to 9): enter that number in the selected square. On a keyboard, press the number key instead. Holding a key down counts as a single press.
• Notes: switches Notes mode on or off. In Notes mode, the numbers you choose become small pencil marks in the selected empty square, and choosing the same number again removes the mark. Pencil marks are never checked, never count as mistakes, and are cleared when you enter a number in that square. They are not updated automatically when you place numbers elsewhere, so tidy them up yourself.
• Erase: clears the number and any pencil marks from the selected square. Starting numbers cannot be erased. Backspace, Delete and 0 do the same on a keyboard. Erasing a wrong number does not take back its mistake.
• Hint: fills in one correct number, in a square chosen for you that is empty or holds a wrong number. There is no limit on hints. Each one adds 1 to the Hints counter, and a hint does not take back a mistake you have already made.
• Pause: hides the puzzle and stops the timer. While paused you cannot enter numbers; press Resume to carry on.
• Restart Puzzle: begins a fresh attempt on the same puzzle. All your entries, pencil marks, mistakes and hints are cleared and the timer returns to 00:00.
• New Game: creates a different, newly generated puzzle at the difficulty you are playing.

Mistakes, Hints and Game Over

A mistake is counted whenever you enter a number that does not match the puzzle solution, even if it does not clash with another number yet. Entering the same wrong number again counts again. The wrong number stays visible until you change or erase it, and correcting it does not lower the Mistakes counter. You can make two mistakes and keep playing.

On the third mistake the attempt ends. The timer stops, the board is locked, and a Game Over message appears with two choices. The number pad, Erase, Hint and Pause are switched off until you start again.

Restart Puzzle or New Game?

Restart Puzzle brings back the same puzzle with the same starting numbers, so you can try again with what you have learned; it is a fresh attempt, with mistakes back to 0 of 3, no hints used and the timer at 00:00. New Game gives you a different puzzle. Both are available after Game Over and after completing a puzzle.

Time, Completion and Player Statistics

The timer starts at 00:00 when a puzzle appears and counts up one second at a time. It stops when you press Pause, complete the puzzle or reach Game Over. It runs inside the page, so a browser may slow it if the page sits in a background tab for a long time; press Pause when you step away. Nothing is saved if you refresh or leave the page: you will get a new Easy puzzle.

You complete a puzzle when every square matches the solution, whether you entered the numbers or revealed them with hints. The completion panel shows the difficulty, your time, your mistakes and the hints you used. Sudoku has no points score: how well you did is simply your time, mistakes and hints.

Each Sudoku attempt that ends, either by completing the puzzle or by reaching Game Over, is added once to your Player Statistics. It raises Games Played by one, adds its time to Total Time Played, counts towards Favourite Tool, and extends your daily streak if you are playing on a new day. Because Sudoku has no points score, it does not change Highest Score, Total Score or Total Words Solved, and your hints and mistakes do not change what is recorded. The difficulty is not recorded. An attempt you abandon, by refreshing, leaving the page or starting a New Game before it ends, is not counted. Restarting after an attempt has ended begins a new attempt, which is counted separately when it ends. Your statistics are stored only in your own browser.

Easy vs Medium vs Hard Sudoku

The difficulty levels differ in how many starting numbers you are given: Easy starts with 38, Medium with 30 and Hard with 24 (occasionally a few more on Hard, when a clue cannot be removed without allowing a second solution). With fewer clues there are more squares to work out and fewer obvious first moves.

The number of clues is the only thing that sets the level, so a single puzzle can feel easier or harder than its label. As a rough guide, most Easy puzzles can be solved with the basic steps in the beginner tips below. Medium puzzles often also need the idea of finding the only place a number can go. Many Hard puzzles need pairs or other advanced techniques, which are described further down. Every puzzle at every level has exactly one solution.

Sudoku Tips for Beginners

• Start with rows, columns, or boxes that already have the most numbers filled in - they have fewer possibilities left to work out.
• Pick a number, such as 5, and look at every place it already appears. Each 5 rules out its whole row, column and box for any other 5.
• Use Notes mode to jot down every number that could still go in a square, then narrow those candidates down as you fill in more of the board. Remember that notes do not update themselves.
• If a number can only go in one square within a row, column, or box, it belongs there even if other numbers could also fit that square.
• Avoid guessing. Every wrong number counts as a mistake, and you only have three. Use Notes to test an idea instead of entering a number.
• Take your time. Sudoku has no time pressure built into the rules; the timer is there for you to see how long a puzzle took, and you can pause it whenever you need a break.

Worked Examples

Example 1: only one number fits

Look at an empty square. Its row already contains 1, 4 and 7, its column contains 2, 6 and 9, and its box contains 3 and 5. That is eight different numbers: 1, 2, 3, 4, 5, 6, 7 and 9. The only number missing is 8, so the square must be 8.

Example 2: only one place for a number

In the top-left box, four squares are empty: row 1 column 1, row 2 column 1, row 2 column 3, and row 3 column 2. You want to place a 5. A 5 elsewhere in row 1 rules out row 1 column 1. A 5 elsewhere in column 1 rules out row 2 column 1. A 5 elsewhere in row 3 rules out row 3 column 2. Only row 2 column 3 is left, so the 5 goes there, even though that square might also be able to hold other numbers.

Example 3: narrowing down with Notes

A square has the pencil marks 2 and 7. Another square in the same row is then solved as 7. Since a row cannot contain two 7s, remove the 7 from your marks. Only 2 is left, so enter 2.

Intermediate and Advanced Strategies

These techniques work on your pencil marks and are mainly needed for Medium and Hard puzzles. Use Notes mode to write the candidates down first.

Intermediate

• Naked pair: if two squares in the same row, column or box can each only hold the same two numbers, say 4 and 9, then those two numbers belong in those two squares. Remove 4 and 9 from the pencil marks of every other square in that row, column or box.
• Hidden pair: if two numbers can only go in the same two squares of a row, column or box, those squares must hold those two numbers. Remove every other pencil mark from those two squares.
• Pointing pair (or triple): if, within a box, a number can only go in squares that share one row or column, that number cannot appear anywhere else in that row or column outside the box. Remove it from those squares.
• Box and line reduction: the reverse idea. If in a row or column a number can only go inside one box, remove that number from the other squares of that box.

Advanced

• X-Wing: if a number can go in only two squares in each of two rows, and those squares fall in the same two columns, the number must fill one diagonal pair or the other. Remove it from all other squares in those two columns. The same works with rows and columns swapped.
• Y-Wing: find a square with exactly two candidates, XY, that sees two other two-candidate squares, XZ and YZ. Whichever way the first square resolves, one of the other two becomes Z, so any square that sees both of them cannot be Z.
• Swordfish: the three-row (or three-column) version of the X-Wing idea.

If you are stuck, the Hint button reveals one correct number. It does not cost a mistake, and it is counted so you can see how many you used.

Why Play Sudoku?

Sudoku is a relaxing, self-contained logic puzzle - there is no vocabulary or general knowledge required, just careful, step-by-step reasoning. It is a popular way to pass the time, unwind, or enjoy a quiet mental challenge, entirely at your own pace.

Frequently Asked Questions