How to play Sudoku

Sudoku is a logic puzzle on a 9×9 grid. Fill every empty cell with a digit from 1 to 9 so that each row, each column and each 3×3 box contains every digit exactly once. Some digits are given at the start; you deduce the rest, one certain cell at a time, and you never have to guess. This guide explains the board, the rules, the single-solution principle, notes and a simple strategy for your first moves. You can practise everything in Bare Sudoku, which is free and has hints that explain the logic.

What are the rules of Sudoku?

There are only three rules, and they are the same rule seen from three directions: every row must contain the digits 1 to 9, every column must contain the digits 1 to 9, and every 3×3 box must contain the digits 1 to 9. A digit can therefore never appear twice in the same row, column or box.

The given digits, printed in bold, are fixed. Your job is to fill the empty cells. There is no arithmetic and no ordering: the digits are just nine different symbols. You could play with letters or shapes and nothing would change.

How is the board organised?

The board has 81 cells in 9 rows and 9 columns. Thick lines divide it into nine 3×3 boxes. Each cell belongs to exactly one row, one column and one box, and these three groups are its units. The 20 other cells that share a unit with a cell are called its peers; a digit placed in a cell rules that digit out in all 20 peers.

Cells are written as r3c7: row 3, column 7. Rows are counted from the top, columns from the left, and boxes 1 to 9 from the top left, row by row. This notation is used in every technique guide and is worth learning: r5c5 is the centre of the board.

Why does every Sudoku have exactly one solution?

A proper Sudoku has one solution, and that is what makes logic work. Whenever you can show that a digit cannot go anywhere else in a unit, it must go in the remaining cell, because the solution exists and is unique. If a puzzle had two solutions, no amount of reasoning could tell them apart and you would be forced to guess at some point.

Bare Sudoku generates every puzzle on your device and checks that it has exactly one solution and that its logical solver can finish it with the techniques of the chosen level. So if you are stuck, there is always a next step to find, and the hint shows it.

How do you start a Sudoku puzzle?

Start with what is most constrained. This order works at every level:

1. Pick a digit that already appears many times, say six or seven, and find the boxes that still miss it. Often only one cell in a box is left for it: that is a hidden single.
2. Look for crowded cells. A cell whose row, column and box together contain eight different digits has only one candidate left: a naked single. Cells in nearly full rows or boxes are the best place to look.
3. Every digit you place removes a candidate from 20 peers, so check those peers straight away. Singles tend to come in chains.
4. When the easy placements dry up, write notes: the candidates for each empty cell. Then look for a digit that is confined to one line inside a box (locked candidates) or two cells that share the same two candidates (a naked pair).
5. Return to singles after every elimination. Even Expert puzzles are solved mostly by singles; the advanced technique just unlocks the next one.

Bold digits are given, blue digits were placed while solving, small digits are candidates. Highlighted cells form the pattern, crossed-out candidates are removed, and a circled candidate is the digit to place.

What are notes and when should you use them?

Notes, also called pencil marks or candidates, are the small digits you write in an empty cell to record which digits are still possible there. In easy puzzles you rarely need them; from Hard upward they are essential, because techniques like pairs, X-Wing and Y-Wing are about candidates, not placed digits.

Keep notes accurate: when you place a digit, remove it from the notes of all 20 peers. In Bare Sudoku this happens automatically, and Fill notes writes every candidate into every empty cell with one tap, so you can start from a complete, correct set of candidates. A note is a fact, not a guess: it means that nothing rules this digit out yet.

What should you do when you are stuck?

First recheck the basics: every nearly full row, column and box is a source of singles. Then make sure the notes are complete, because a missing candidate hides patterns and a stale one creates false ones. Then work through the techniques in order of difficulty: locked candidates, pairs and triples, X-Wing, Y-Wing, Swordfish and XYZ-Wing. Each has its own page with a real example.

If nothing works, take a hint. In Bare Sudoku the hint first checks whether a placed digit is wrong, then finds the next cell that can be filled by logic and names the technique, so you learn something even when you use it. Tap the hint again to place the digit.

How do you enter digits in Bare Sudoku?

Tap a cell, then a digit. Or tap a digit first: it locks, and every cell you tap receives it, which is the fastest way to place the sixth, seventh and eighth copy of a digit. Tap the digit again to release it. The Notes button switches to pencil marks, Erase clears a cell, Undo takes back any move, and mistakes are marked in red unless you turn that off in the menu.

On a keyboard, use the arrow keys to move, 1 to 9 to enter a digit, Backspace to erase, N for notes, U for undo, H for a hint, F to fill notes and Escape for the menu. The game saves itself, works offline and has no ads and no account.

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Made by Alparslan Selçuk Develioğlu · Updated 2026-09-30