Guide
How to play Sudoku
Sudoku is a logic puzzle played on a 9x9 grid. You do not need any maths, only the digits 1 to 9 and a bit of patience. This guide covers the rules and the first techniques to learn.
The rules
The grid has 81 cells, arranged in nine rows, nine columns and nine 3x3 boxes. Some cells already contain a digit. You fill in the rest so that:
- every row contains each digit from 1 to 9 once,
- every column contains each digit from 1 to 9 once,
- every 3x3 box contains each digit from 1 to 9 once.
A proper Sudoku has exactly one solution, and it can be reached by logic alone. Every puzzle on PuzzleGrids is checked for this before it is shown.
How to start a puzzle
- Find the most common digit. If there are already six or seven 5s on the grid, the last few have very few places to go.
- Scan the boxes. For that digit, look at each box that does not have it yet. Rule out the rows and columns where it already appears. If one cell is left, that is where it goes.
- Check the nearly full units. A row, column or box with only one or two gaps is quick to finish by counting what is missing.
- Repeat. Every digit you place removes options elsewhere, so go back over the digits you have already tried.
Basic techniques
Hidden single
A digit that can only go in one cell of a row, column or box, even though that cell may look like it has several options. This is the technique from the steps above, and it solves most easy puzzles on its own.
Naked single
A cell where eight of the nine digits are already used in its row, column and box, leaving one possibility. These are easier to spot once you start writing notes.
Pencil notes
When a cell has two or three possible digits, write them in as small notes. On PuzzleGrids, turn on Notes (or press N) and tap the digits. Notes turn guesswork into bookkeeping: as you fill cells, cross candidates off until one remains.
Naked pair
If two cells in the same row, column or box can each only hold the same two digits, those two digits must go in those two cells. You can remove both digits from the notes of every other cell in that unit. This is the first technique you need for hard puzzles.
Pointing pair
If all the possible cells for a digit inside a box sit in a single row or column, the digit must be in that line within that box. Remove it from the rest of the line outside the box.
Common mistakes
- Guessing early. A wrong guess may not show up for twenty moves. Look for a certain move first.
- Forgetting the box. Beginners often check the row and column and miss that the digit is already in the 3x3 box.
- Leaving stale notes. After placing a digit, clear it from the notes in the same row, column and box.
Which level should you play?
If this is your first puzzle, try 6x6 Sudoku or Easy Sudoku. When easy puzzles take you under ten minutes, move to Medium and start using notes. Hard and Expert are for when pairs feel routine. The Daily Sudoku is medium difficulty and a good habit to build.