Sudoku
Sudoku asks you to fill a 9 × 9 grid so that every row, every column and every 3 × 3 box contains the digits 1 to 9 exactly once. Every puzzle here is generated fresh and checked to have exactly one solution — the uniqueness test runs on each clue removed, which is what separates a sudoku from a grid with several answers.
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This level
40 clues. Solvable throughout with naked and hidden singles — no guessing, no chains.
Keyboard
- Arrows
- Move the selected cell
- 1 – 9
- Enter a digit
- Backspace
- Clear the cell
- P
- Pencil marks on or off
- F
- Full screen
- Grid
- 9 × 9, nine boxes
- Easy
- 40 clues
- Expert
- 24 clues
- Proven minimum
- 17 clues
What are the rules of sudoku?
- Each row contains 1–9, each digit once.
- Each column contains 1–9, each digit once.
- Each 3 × 3 box contains 1–9, each digit once.
- The clues printed at the start cannot be changed.
- A correct puzzle has exactly one solution and never requires guessing.
How many clues does a sudoku need?
Seventeen. In 2012 Gary McGuire, Bastian Tugemann and Gilles Civario completed an exhaustive computer search proving that no valid sudoku with a unique solution has fewer than 17 clues. The Expert puzzles here stop at 24, which is hard without being pathological. For scale, there are 6,670,903,752,021,072,936,960 valid completed grids — a figure Bertram Felgenhauer and Frazer Jarvis computed in 2005.
| Level | Clues | Techniques needed |
|---|---|---|
| Easy | 40 | Naked and hidden singles only |
| Medium | 32 | Pointing pairs, box-line reduction |
| Hard | 27 | Naked and hidden pairs and triples |
| Expert | 24 | Multiple interacting techniques |
What is a naked single and a hidden single?
A naked single is a cell where eight of the nine digits are already taken by its row, column or box — only one candidate is left, so it must go there. A hidden single is the mirror image: a digit that has only one possible home left in a row, column or box, even though that cell still has several candidates of its own. Almost every easy and medium sudoku is nothing but these two, applied repeatedly. The hint button here names which one it used.
Do you ever have to guess in sudoku?
Not in a properly constructed puzzle. Because the solution is unique, every cell is determined by the clues — the question is only whether you know the technique that reveals it. If you find yourself guessing, the deduction exists and you have not spotted it. Pencil marks are how you find it: write the candidates into the cells and the patterns become visible.
How do you get better at sudoku?
- Scan for hidden singles by digit, not by cellPick a digit, look at each box, and cross off the rows and columns where it already appears. Very often exactly one cell survives — that is faster than examining cells one at a time.
- Pencil in a whole box at a timeCandidates are only useful when they are complete. Half-filled pencil marks lead to false deductions.
- Look for pairs before triplesTwo cells in a unit with the same two candidates lock those digits out of every other cell in that unit. It is the cheapest technique beyond singles and it unlocks most hard puzzles.
- Work the most-constrained unitThe row, column or box with the fewest empty cells is where the next deduction almost always is.
Sudoku questions people ask
- Is every sudoku on this site solvable without guessing?
- Yes. Each puzzle is generated by removing clues from a complete grid and testing after every removal that exactly one solution remains. A clue whose removal creates a second solution is put back.
- What is the minimum number of clues in a sudoku?
- Seventeen, proven by exhaustive search in 2012. No 16-clue puzzle with a unique solution exists.
- What do pencil marks do?
- They record the candidate digits for a cell without committing to one. Turn on pencil mode and your entries are stored as small marks; the auto-fill button writes every legal candidate for you.
- How do I check whether I have made a mistake?
- The board highlights any digit that conflicts with another in its row, column or box as you type. Turn on Check to also flag entries that disagree with the puzzle's unique solution.
The rules in six lines
- 1Fill every cell with 1 to 9The grid has 81 cells. When it is full and legal, the puzzle is solved.
- 2No repeats in a rowEach of the nine rows must contain every digit from 1 to 9 exactly once.
- 3No repeats in a columnThe same applies down each of the nine columns.
- 4No repeats in a boxEach of the nine 3 × 3 boxes must also contain 1 to 9 exactly once.
- 5Clues are fixedThe digits present at the start are part of the puzzle and cannot be changed.
The long version, with worked examples and the questions beginners actually ask, is on how to play sudoku.
Put Sudoku on your own site
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