How to Play Sudoku
Sudoku is a logic puzzle played on a 9 × 9 grid. The grid is split into nine smaller 3 × 3 boxes. Some numbers are already filled in. Your job is to fill the empty squares with the digits 1 through 9.
The rule is simple: every row, every column, and every 3 × 3 box must contain each digit from 1 to 9 exactly once. You do not need math. You only need to ask, square by square, which numbers are still possible.
- A row is one horizontal line of nine squares
- A column is one vertical line of nine squares
- A box is one of the nine 3 × 3 regions
- A candidate is a number that could still go in an empty square
A good Sudoku puzzle has one solution. You should be able to find it by logic, without guessing.
Sudoku Uno for Mac
Available from the Mac App Store.
Sudoku Uno for iPad
Not currently available.
Sudoku Duo for iPad
Not currently available.
Sudoku Uno for Mac
Sudoku Uno for Mac creates fresh Sudoku puzzles and explains its hints. The app solves puzzles with the same kind of logic a person would use, so a hint can show why a move works instead of just giving away an answer.
Difficulty is based on the solving techniques needed for a puzzle, not just on how many numbers are shown at the start. That matters because a puzzle with many given numbers can still be tricky, while a puzzle with fewer given numbers can be straightforward.
- Unlimited Sudoku puzzles on Mac
- Easy, medium, and hard difficulty levels
- Pencil notes, including optional automatic notes
- Hints that explain the next logical step
- No ads and no in-app purchases
How to Start a Sudoku Puzzle
Start with the numbers already on the grid. Pick one digit, such as 5, and look at where it already appears. Each 5 blocks the rest of its row, column, and box from also containing a 5. Often that leaves only one square where another 5 can go.
Then switch to another digit. Many beginners make the mistake of staring at one empty square for too long. It is usually easier to scan for a number across rows, columns, and boxes, then come back to stubborn squares later.
When the puzzle gets tighter, use pencil marks. Write small candidate numbers in empty squares. Cross them off when a number in the same row, column, or box proves they cannot fit. Good pencil marks turn a crowded puzzle into a set of smaller choices.
Sudoku Solving Techniques
The techniques below are the ones Sudoku Uno uses for its hints and difficulty levels. They are also the techniques most people use when solving by hand.
Easy: Single Candidate
A single candidate happens when an empty square has only one possible number left. For example, if a square shares a row, column, or box with 1, 2, 3, 4, 5, 6, 8, and 9, then the only missing digit is 7. Put 7 in that square.
This is the most direct Sudoku move. If you keep clean pencil marks, single candidates are easy to spot because one square has only one note left.
Easy: Single Position in a Row
Sometimes a row has several empty squares, but a certain digit can fit in only one of them. If the digit 4 is missing from a row, check each empty square in that row. Any square that already sees a 4 in its column or box cannot be 4. If only one square survives, that square must be 4.
Easy: Single Position in a Column
The same idea works in a column. Choose a missing digit and test the empty squares in that column. If every square but one is ruled out, place the digit in the remaining square.
Easy: Single Position in a Box
Look at one 3 × 3 box and choose a digit that is missing from it. The rows and columns crossing that box may eliminate most of the empty squares. If the digit has only one possible place inside the box, write it there.
Easy: Matching Pair
If two squares in the same row, column, or box contain the same two candidates, those two numbers are reserved for those two squares. For example, if two squares in a row can only be 2 or 8, then one of them is 2 and the other is 8. No other square in that row can be 2 or 8.
This does not always place a number right away. Its value is that it removes candidates from other squares, which often creates a single candidate somewhere else.
Medium: Pointing Pair or Triple
Look inside one 3 × 3 box. If all possible places for a digit sit in the same row, then that digit must be in that row within the box. You can remove the digit from the rest of the same row outside the box.
The column version works the same way. If all possible places for a digit inside a box sit in one column, remove that digit from the rest of the column outside the box.
Medium: Box-Line Reduction
Box-line reduction looks at the same pattern from the row or column side. If a row needs a 6 and all possible places for that 6 fall inside the same 3 × 3 box, then the 6 must be in that row inside that box. You can remove 6 from the other empty squares in the box.
This is a useful middle step because it narrows the puzzle without guessing. You are using the overlap between a row or column and a box.
Hard: Hidden Pair
A hidden pair appears when two digits can go only in the same two squares within a row, column, or box. Those two squares may have extra pencil marks, so the pair is easy to miss.
For example, suppose the digits 3 and 9 can appear only in the same two squares of a box. Those two squares must be 3 and 9 in some order. You can erase every other candidate from those two squares.
Hard: X-Wing
An X-Wing uses two rows and two columns. Pick one digit. If that digit can appear in exactly two squares in one row, and exactly two squares in another row, and those possible squares line up in the same two columns, you have an X-Wing.
Why does it work? The digit must occupy opposite corners of the rectangle formed by those rows and columns. Either way, the two columns are already taken by that digit. You can remove the digit from all other squares in those columns.
There is also a column version: if two columns have the same two possible rows for a digit, remove that digit from the rest of those rows.
Houdah Software Newsletter
Sign up to receive product news, tips & tricks, and special offers.