How to play Sudoku

What is the goal?

Fill the whole 9x9 grid so that every row, every column and every 3x3 box holds the digits 1 to 9 exactly once. Some digits are given at the start, and you find the rest by logic.

How a game starts

• You see a 9x9 grid made of nine 3x3 boxes, with some cells already filled in. These starting digits are fixed and you cannot change them.
• The empty cells are yours to fill. A harder puzzle starts with fewer given digits.
• A number pad sits under the board. In Tidoku you tap a cell and then a digit, or pick a digit first, as you like.
• Tools such as Undo, Erase, Notes and Hint sit between the board and the pad.

How to play, step by step

1. Look at the grid and find a row, column or box that already has many digits.
2. Pick an empty cell in it and ask: which digits from 1 to 9 are still missing from its row, its column and its box?
3. If only one digit can go in the cell, write it in. This is called a naked single.
4. If a digit has only one possible cell left in a row, column or box, write it there. This is called a hidden single.
5. When a cell has two or three possible digits, write them as small pencil notes and come back later.
6. Each new digit removes that digit from the notes of the other cells in its row, column and box. Keep your notes up to date.
7. Scan the grid again after every digit. New singles appear all the time.
8. Keep going until all 81 cells are filled. If a digit repeats in a row, column or box, undo and look for the mistake.

What are the rules of sudoku?

Sudoku has three rules. Each row must hold the digits 1 to 9 once. Each column must hold the digits 1 to 9 once. Each of the nine 3x3 boxes must hold the digits 1 to 9 once. The digits that are given at the start never change, and there is no maths to do.

Do you need to guess in sudoku?

No, a well-made sudoku never needs guessing. Every puzzle has exactly one solution, and you can reach it step by step with logic. If you feel stuck, you have probably missed a single or a pair somewhere. In Tidoku every puzzle, even Extreme, is solved by logic and never by trial and error.

What are pencil notes in sudoku?

Pencil notes are small digits you write in a cell to remember which numbers could still go there. They help you spot singles and pairs. In Tidoku you hold a cell or tap Notes to write them. The Notes setting can be Manual, Auto-clear or Auto-fill.

What are the rules of Killer sudoku?

Killer sudoku uses the normal sudoku rules plus cages. A cage is a group of cells outlined with dashes, with a small sum in its corner. The digits in a cage must add up to that sum, and no digit can repeat inside one cage. Most Killer boards start with few or no given digits.

What are the rules of Jigsaw sudoku?

In Jigsaw sudoku the 3x3 boxes are replaced by nine outlined shapes of irregular form. Each row, each column and each shape must hold the digits 1 to 9 exactly once. The rows and columns work as in normal sudoku, so only the regions change shape.

What are the rules of Diagonal sudoku?

Diagonal sudoku, also called X sudoku, adds one rule to normal sudoku. Both long diagonals, from corner to corner, must also hold the digits 1 to 9 exactly once. Rows, columns and 3x3 boxes still follow the usual rule, so the two extra lines give you more clues.

What does a mistake mean in sudoku?

A mistake is a digit that does not match the one solution of the puzzle. In Tidoku, Classic mode allows three mistakes and shows a wrong digit at once. Relaxed has no limit and hides wrong digits until you tap Check. Strict ends the puzzle on the first mistake.

How do you win and score?

In Tidoku you win by filling the board correctly. The game tracks your time and mistakes and shows a score when you finish. For the Weekly Challenge, the leaderboard time adds 30 seconds for each mistake, and only runs with no hints count. In Journey levels you earn up to three stars.

Common beginner mistakes

• Guessing a digit when no single is available. This usually leads to a wrong digit many moves later.
• Forgetting to update the pencil notes after placing a digit, so old notes stay on the board.
• Checking only the row and the column and forgetting the 3x3 box.
• Only looking for empty cells, not for digits. Asking where a digit can go often finds a hidden single faster.
• Rushing and tapping the wrong cell. Look at the highlighted cell before you place a digit.

Tips and strategy

• Start with the digit that appears most often on the board. Ask where its missing copies can go.
• Scan rows, columns and boxes that are nearly full first. They have the fewest options.
• Write pencil notes once the easy singles run out, then look for cells with only one note left.
• Look for naked pairs: two cells in the same row, column or box that hold the same two notes. No other cell there can use those digits.
• When a digit can only go in one row inside a box, it cannot go anywhere else in that row. This is a pointing pair.
• Take a short break if you are stuck. A fresh look often shows a single you missed.
• Use the hint as a teacher. In Tidoku, Nudge shows where to look, and Explain names the technique so you can learn it.
• Practise one technique at a time. Easy puzzles are the best place to build speed.

Words to know

Row

A horizontal line of nine cells.

Column

A vertical line of nine cells.

Box

One of the nine 3x3 squares of the grid.

Candidate

A digit that could still go in a cell. Pencil notes show candidates.

Naked single

A cell where only one digit is left, because every other digit is already in its row, column or box.

Hidden single

A digit that has only one possible cell left in a row, column or box.

Naked pair

Two cells in the same house that hold only the same two candidates, so those digits can be removed from the other cells there.

Cage

In Killer sudoku, an outlined group of cells whose digits must add up to a given sum.

Playing Sudoku in Tidoku

Tidoku is a sudoku game that teaches you to solve. Play Classic, Killer, Jigsaw and Diagonal boards in six tiers from Easy to Extreme. Follow the 600-level Journey, or play the Daily Puzzle, which is the same for everyone. Nudge shows where to look for free. Explain names the technique, and the Tide Chart has one-minute lessons. Choose Classic, Relaxed or Strict mistakes. You can play it on a plane or anywhere else without a connection, and it costs nothing to install.