Sudoku

Play one here, or print a sheet of six. Four levels, and two guarantees worth more than the puzzles: exactly one solution, and no guessing required.

Every puzzle has exactly one solution

A Sudoku with two solutions is not a Sudoku. Solve it far enough and you reach a point where two squares could each hold either of two digits, with nothing left to reason from — the only way forward is to pick one and hope. On paper, in pen, that is a wasted sheet.

They are produced by a specific mistake. A generator starts from a completed grid and removes clues; if it removes several before checking, or checks only at the end, it will produce a grid with more than one valid completion. The puzzle looks perfectly normal. Checking after every removal is the textbook fix and most serious generators do it — but not all: of the five most-starred open-source Sudoku generators, two ship no uniqueness check at all, and one of those promises unique puzzles in its own description.

Here the check runs after every single removal. A clue is taken out, the grid is tested for having exactly one solution, and it goes straight back if it does not. That is slower to generate and it is the whole reason to use this site rather than another.

The difficulty label means something

Some sites grade by clue count — sudoku.com says so in its own words, that "the level of difficulty depends on how many numbers are already shown". It is easy to compute and it is a poor proxy: a puzzle with more starting digits can require a harder technique than one with fewer, so the labels drift and "hard" comes to mean "looks sparse". Others do grade by technique, and grade it well. What almost nobody does is say which — so a difficulty label tells you nothing you can compare between sites.

Difficulty here is measured by solving the puzzle the way a person does and recording the hardest technique it forced. The four levels are Gentle (singles only — nothing to spot beyond one digit fitting one place), Moderate (needs a naked pair: two cells sharing the same two candidates), Tough (needs a pointing pair — a digit confined to one line inside a box), Stern (needs box/line reduction as well, the hardest technique on this site).

That is a smaller and more honest claim than a numeric rating out of ten. It also means the level tells you what to learn next, which a number never does.

No puzzle here needs a guess

The generator runs a solver limited to the techniques described on the level pages. Anything that solver could not finish was thrown away rather than published as a harder puzzle.

That matters most on paper. A wrong guess on a screen is one undo; a wrong guess in pen halfway down a printed grid means starting again, and finding out that the puzzle itself was at fault is worse still.

It also puts a real ceiling on this site: the hardest level published is the hardest one that stays solvable by a technique somebody can learn from a paragraph. Sites advertising "evil" or "diabolical" are frequently shipping puzzles that cannot be solved by reasoning at all.

Playing it here, and the pencil marks

The board at the top is a real puzzle, in the page before any script runs. Type into it, or use the digit pad — and a given clue is not a box you can type in, so the commonest annoyance in browser Sudoku, overwriting a clue by accident and not noticing for ten minutes, cannot happen.

Check tells you whether anything on the board contradicts itself: a digit that clashes with another in its row, its column or its box is marked. It deliberately does not tell you whether a digit disagrees with the hidden answer, because that is just handing over the solution one square at a time. It is the same information you would have on paper, and no more.

Pencil marks go in the corner of a square rather than in a separate mode you have to switch out of, because the entire point of them is to be visible while you are deciding. Entering a real digit clears them: they were a question, and that is the answer.

Nothing is saved to a server and there is no account, so nothing tracks a streak or a time. That is a deliberate omission — the sites that do it are optimising for return visits, not for the puzzle.

What comes out of the printer

Six puzzles to a sheet by default, in a grid large enough to write pencil marks in — which the harder levels genuinely require, and which most printable Sudoku is too small for.

The answers print on a separate sheet rather than the same one, so a puzzle cannot be spoiled by the page it is on. Print both and keep the second one face down.

Everything is generated in your own browser. Nothing is uploaded, there is no account, and there is no limit on how many you print — the puzzles are made on your machine, so there is nothing for anybody to meter.

Common questions

The other pages

• Easy SudokuSingles only
• Medium SudokuNaked pairs
• Hard SudokuPointing pairs
• Expert SudokuBox/line reduction
• Shared gameSame grid, two people
• Sudoku solverSolver
• Blank gridsPrintable