Sudoku Solver & Generator

About the Sudoku Solver & Generator

Type or paste a Sudoku and press Solve for the answer at once — or press Step by step to be walked through it the way a person solves it, one named technique at a time: full house, naked and hidden singles, pointing and claiming, naked and hidden pairs, triples and quads, X-Wing, Swordfish, Jellyfish, XY-Wing and XYZ-Wing. Every step names the cells and houses it uses and explains why the candidates it removes cannot be right.

It handles 4×4, 6×6, 9×9 and 16×16 grids. Check tells you whether the clues are valid and whether the puzzle has exactly one solution — if it has more, the cells where two solutions differ are outlined — and grades it Easy, Medium, Hard or Expert by the hardest technique it needs.

Make & print puzzles creates new puzzles of the size and difficulty you choose, each with exactly one solution, and saves them as a print-ready PDF with an answer key. Everything runs in your browser.

Common uses

• Checking your answer — or finding the one cell you got wrong — in a newspaper or app puzzle.
• Getting unstuck without spoiling the whole puzzle: one hint or one step at a time.
• Learning techniques such as pointing pairs, X-Wings and XY-Wings on puzzles you are actually solving.
• Testing whether a puzzle you made has exactly one solution before you share it.
• Printing puzzles with answer keys for a classroom, a club, a newsletter or a long journey — 4×4 for young children up to 16×16 for experts.

How the difficulty is graded

The solver always takes the easiest step available, and a puzzle’s grade is the hardest technique that path needs:

• Easy: full houses, naked singles and hidden singles only.
• Medium: also pointing, claiming, and naked or hidden pairs.
• Hard: also naked or hidden triples, X-Wings and XY-Wings.
• Expert: also naked or hidden quads, Swordfish, Jellyfish and XYZ-Wings.
• Beyond these techniques: the puzzle needs something the tool does not teach — chains, forcing arguments or trial and error. Solve still finds its answer.

There is no universal difficulty scale: books, newspapers and apps each grade in their own way, so a puzzle called “hard” elsewhere may be Medium here, or the other way round.

The techniques in brief

• Full house: the last empty cell of a row, column or box.
• Naked single: a cell that only one value fits.
• Hidden single: a value that fits only one cell of a row, column or box.
• Pointing: inside a box, a value fits only in one row (or column), so it can be removed from the rest of that row.
• Claiming: inside a row (or column), a value fits only in one box, so it can be removed from the rest of that box.
• Naked pair, triple, quad: two, three or four cells of a house that between them can only hold the same two, three or four values; those values go nowhere else in the house.
• Hidden pair, triple, quad: two, three or four values that fit only the same two, three or four cells of a house; those cells hold nothing else.
• X-Wing, Swordfish, Jellyfish: a value that fits only in the same two, three or four columns of two, three or four rows (or the other way round); it can be removed from the rest of those columns.
• XY-Wing: a two-candidate cell XY that sees cells XZ and YZ; whichever value it takes, one of them is Z, so any cell that sees both cannot be Z.
• XYZ-Wing: like an XY-Wing with a three-candidate pivot XYZ; cells that see all three cannot be Z.

None of these techniques assumes the puzzle has only one solution, so every step is a safe deduction.

How the puzzles are made

Each puzzle starts from a random, completely filled grid. Clues are then taken away one at a time — in matching pairs on opposite sides of the centre when Symmetric clue pattern is ticked — for as long as the puzzle can still be solved with the techniques of the chosen difficulty. A puzzle that logic alone solves has exactly one solution, so every puzzle printed has one. It is kept only if it really needs a technique of its level; otherwise the generator starts again with a new grid.

Easy puzzles keep more clues (at least 37 in a 9×9 grid); Medium puzzles stop at about 29 once they need a Medium technique; Hard and Expert puzzles keep as few clues as the method allows. Puzzles are made fresh each time — press Make puzzles again for a new set.

Typing and pasting puzzles

Rows run from the top and columns from the left, so R4C7 is the cell in row 4, column 7. Boxes are numbered left to right and top to bottom; in a 9×9 grid box 5 is the centre one. 6×6 grids have boxes of 2 rows × 3 columns.

A pasted puzzle can be one line of 81 characters, a grid drawn with |, -, +, * or box-drawing lines (such as *-----------* borders and ---+---+--- rules), or numbers separated by commas or spaces (16×16 puzzles may use 1–16). Empty cells can be written as 0, ., -, _, *, ? or x. 16×16 puzzles use 1–9 then A–G, or 0–9 then A–F; the tool recognises a G or a mix of 0s and dots and switches for you. Copy solution and Copy as grid give the answer back in the same styles.

Limitations

• The step-by-step solver knows the techniques listed above. Some very hard puzzles need more advanced ones (chains, forcing nets, uniqueness patterns); there it stops and offers to reveal one cell or the whole solution. Solve still finds the answer of every valid puzzle.
• Difficulty follows this tool’s own scale — the hardest technique needed — and may not match other books or apps.
• 4×4 puzzles are made in Easy only and 6×6 puzzles up to Hard: such small grids leave little room for the harder techniques.
• An Expert puzzle can take a few seconds to make, longer on an older phone; if one is not found within 25 seconds, making stops there and the page says how many were made.
• Standard Sudoku only: killer, jigsaw, diagonal and other variants are not supported.
• A 16×16 grid is wider than a phone screen, so it scrolls sideways there.

Privacy

Puzzles are solved, checked and made in your browser, in a background worker; nothing is uploaded. The last puzzle you entered and your print settings are kept in this browser’s storage, so they are still there next time.