Strategy
Sudoku techniques, from singles to the X-wing
Every technique you need to solve Sudoku without guessing, in the order the puzzles ask for them, with a plain explanation of why each one works.
4 min read
Sudoku
Fill the grid so every row, column and box holds 1 to 9. Four levels, pencil notes, hints and your progress saved as you go.
Level
Time0:00
Click a square, then type a number. Hold Shift to write a note.Tap a square, then tap a number.
Move between squares with the arrow keys. Type 1 to 9 to fill a square, or hold Shift to add a pencil note. N turns notes mode on or off. Backspace erases, U undoes, H gives a hint and P pauses.
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About Sudoku
Sudoku is a number puzzle on a 9 × 9 grid divided into nine 3 × 3 boxes. Some squares start with numbers. Fill in the rest so that every row, every column and every box contains each number from 1 to 9 exactly once.
No arithmetic is involved, only logic. Every puzzle here has exactly one solution, which you can always reach by reasoning rather than guessing. Pick a level, use pencil notes to track the options, and pick up where you left off whenever you come back.
How to play Sudoku
- Pick a level. A new puzzle appears with some numbers already given.
- Select an empty square, then type a number or tap it on the number pad.
- Each number can appear only once in each row, column and 3 × 3 box. Clashing numbers turn red.
- Not sure yet? Turn on Notes (or hold Shift) to write small pencil marks of the numbers that could go there.
- Fill every square correctly to solve the puzzle. Hints are there if you get stuck.
Sudoku controls
| Select a square | Click it, or move with the arrow keys |
|---|---|
| Write a number | 1 to 9 (type the same number again to clear it) |
| Write a pencil note | Shift + 1 to 9, or turn on Notes with N |
| Erase | Backspace, Delete or 0 |
| Undo | U, or Ctrl+Z (Cmd+Z on a Mac) |
| Hint | H |
| Pause and resume | P (Esc also pauses) |
| Sound on or off | M |
| Select a square | Tap it |
|---|---|
| Write a number | Tap it on the number pad |
| Write a pencil note | Turn on Notes, then tap numbers |
| Erase, undo or get a hint | The buttons above the number pad |
Sudoku rules
The rules
Every row, every column and every 3 × 3 box must contain the numbers 1 to 9, each exactly once. The numbers printed at the start are part of the puzzle and cannot be changed.
One solution
Every puzzle is checked by a solver to have exactly one solution, and a second solver that only uses human techniques confirms it can be finished by logic alone, without guessing.
Levels
Levels are set by the hardest technique a puzzle needs, not just by how many numbers are given. Easy starts with 36 to 40 numbers and only needs singles. Medium starts with 26 to 31 and still only needs singles, but they are harder to find. Hard needs pointing pairs, box-line reductions or naked and hidden pairs. Expert needs triples or an X-wing.
Mistakes and hints
A number that repeats in a row, column or box turns red straight away. A wrong number that does not clash yet is only pointed out when you ask for a hint. A hint first points out any wrong number, otherwise it fills in the easiest next square and explains why it must be that number.
Notes
Pencil notes are small numbers that remind you what could go in a square. When you place a number, the same note disappears from that square’s row, column and box.
Time and saving
The clock starts with your first move and stops while the game is paused. Your puzzle is saved in this browser as you play, so you can close the page and continue later. Best times per level only count puzzles solved without hints.
Tips and strategy
- Start with the numbers that already appear most often. If seven 5s are placed, the last two are usually easy to pin down.
- Scan each box for a number it is missing, ruling out squares whose row or column already has it. The one square left is a hidden single.
- Write notes once you run out of easy placements. Patterns that are invisible in your head become obvious on paper.
- Naked pair: if two squares in a row hold only the same two notes, those two numbers can be removed from every other square in that row.
- Pointing pair: if a number can only go in one row within a box, it cannot go anywhere else in that row outside the box.
- X-wing: if a number fits only two squares in each of two rows, and those squares share the same two columns, remove it from the rest of those columns.
Frequently asked questions
Does every Sudoku puzzle have only one solution?
Yes. Every puzzle is checked by a solver to have exactly one solution, and each can be finished by logic without guessing.
How are the difficulty levels decided?
By the hardest solving technique a puzzle needs. Easy and Medium need only singles (Easy gives you more numbers to start with), Hard needs pairs or pointing techniques, and Expert needs triples or an X-wing. Needs triples or an X-wing on top of everything else.
Can I stop and finish a puzzle later?
Yes. Your puzzle, notes and time are saved in this browser as you play. Come back to this page on the same device and it will be waiting.
What does a hint do?
If you have a wrong number, a hint points it out. Otherwise it fills in the easiest next square and tells you why that number must go there. Hard and Expert puzzles sometimes have no simple step left, in which case a hint fills in a square from the solution.
Is Medium harder than Easy even though both use singles?
Yes. With fewer starting numbers, the squares that can be filled are harder to spot, so you need to scan more carefully.
Guides and tips
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