Sudoku solver and generator

Type in the grid from your newspaper and it will check it, fill one cell, or solve it outright. Or ask for a fresh grid at the difficulty you want, or play today’s — the same one for everybody. Free, no account, no app, nothing stored.

What it does, and why it is here

Most sudoku sites do one of two things badly. A solver hands back the finished grid, which ends the puzzle rather than helping with it. A generator hands back a grid with no promise that it has a single solution — and a puzzle with two solutions is not a puzzle, it is a trap that punishes correct reasoning.

So this one does three separate jobs and keeps them apart. It solves, when you want the answer. It hints, which is the interesting one: instead of revealing a cell, it names the deduction that fills it — “row 4, column 7 has a single candidate left”, “this 5 has only one place left in its box” — so you learn the move rather than losing the puzzle. And it generates, with uniqueness checked cell by cell as the grid is carved out, so a drawn grid always has exactly one solution.

It is also honest about the moments when it cannot reason. When a hint has no deduction behind it, it says so in those words: this one comes from the solution, not from a deduction.

What you fill in

• Board — 9×9, the classic; 6×6, with boxes of 2 by 3 and digits 1 to 6, which is the size for children and for a short break; or 16×16, digits 1 to 9 then A to G, for an evening. A grid you paste says its own size by its length, so this only matters when drawing or starting empty.
• Difficulty — how many clues a drawn grid keeps: 40 for easy, 32 for medium, 28 for hard on a 9×9. Used only when drawing.
• Then one of three ways in: draw a grid, I have a grid to type for the one in front of you, or today’s grid — the same puzzle for everyone on that date, so times are comparable.

What you get back, and what you can do on the board

• A playable board in the page — type digits straight into it, no app and no account.
• Notes — pencil marks for the candidates in a cell, the way the puzzle is actually solved on paper.
• Check my grid — tells you whether what you have so far holds up, before you have filled forty more cells on top of a mistake.
• Fill one cell — one hint, with its reasoning: the cell, the digit, and why. A naked single, a hidden single in a row, a column or a box, or a cell forced once the constraints are propagated. When candidates had to be crossed out first, it names the technique that did it — pointing pair, box-line reduction, naked pair.
• Solve it for me — the finished grid, when you would rather know than continue.
• The clue count and the uniqueness guarantee on a generated grid: how many clues it kept, and the statement that it has exactly one solution, verified as the grid was carved out.

What it does not do

It does not read a photograph. A puzzle from a newspaper has to be typed in — which is a minute of work, and the reason the board accepts digits as fast as you can press them.

It keeps no history: no account, no saved games, no times leaderboard. Reload and the grid is gone, so copy a grid you want to keep.

And it does not rate a grid you bring by how hard it feels. Difficulty here is the clue count of a drawn grid, not a judgement about the techniques a pasted puzzle will demand.

The techniques a hint will name

Almost every grid short of the cruellest falls to these, roughly in this order.

The two on the bottom are the step most players are missing, and they are also where a hint earns its keep: it does not just fill a cell, it points at the elimination you did not see.

Questions

Is it free?

Yes. No account, no app, no advert-gated hints.

Do you keep the grids I type in?

No. A grid is solved or hinted and then forgotten.

What is today’s grid?

One puzzle a day, the same for everyone who opens it on that date — so a time is worth comparing with somebody else’s.

Does every sudoku have exactly one solution?

A proper one does, and every grid drawn here is checked for it as it is made. Grids from other sources sometimes are not, which is worth knowing when a puzzle refuses to resolve.

Is guessing ever necessary?

Not on a well-made grid. If you are guessing, there is an elimination available — ask for a hint and it will name it.

What is the minimum number of clues?

Seventeen on a 9×9, proved by exhaustive search in 2012. Grids that low are rare and very hard; the hard setting here keeps 28.

Can I photograph a puzzle instead of typing it?

Not here — type it in. The board is built for that: click a cell and type.

My newspaper puzzle will not solve. What did I go wrong?

Almost always a transcription error. Use check my grid right after typing it in, before you have built forty cells of reasoning on a mistyped clue.

Why do some easy-rated puzzles feel impossible?

Because ratings usually count clues rather than techniques. A grid with plenty of clues can still hide its next move behind a pointing pair, and a grid with few clues can fall to singles all the way down.

Sources and further reading

• Sudoku — the rules, and where the puzzle came from.
• Mathematics of sudoku — the grid count and the seventeen-clue minimum.
• Constraint satisfaction — what the solver is actually doing.