Sudoku with a guarantee most generators skip: every puzzle is checked to have exactly one solution before you see it. Difficulty is measured by the techniques a puzzle needs, and hints say why a digit belongs where it does.
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Generating a Hard puzzle: how few clues can be left while the grid still has exactly one answer?
Generation
Target 30 clues · each removal kept only if one solution survives
Result
30 clues, exactly one solution, checked in 1.0 ms
A Sudoku with two completions is not a hard puzzle, it is a broken one: at some point two digits both fit and only a guess separates them, and a tool that keeps its own starting grid will then mark your legal answer wrong. The old generator blanked a fixed number of cells and checked nothing; measured over 60 puzzles each, 32 of 60 easy grids had more than one solution and every hard, expert and evil grid did. A cell is now removed only while a single completion survives, which costs about three milliseconds even at the hardest setting, and the page prints the clue count it actually reached rather than the one it was aiming for.
Place the digits 1 to 9 so every row, column and 3x3 box holds each of them exactly once. Every puzzle here has exactly ONE solution, which is checked before the puzzle is handed to you rather than assumed: a clue is only removed while the grid still has a single completion. That is what makes the game solvable by reasoning instead of by guessing, and it is the thing most generators quietly skip.
A puzzle a day keeps deduction sharp. Because every grid has one answer, time spent on it is time spent reasoning rather than guessing.
The line under the board names what the puzzle needs, so you can pick easy grids while naked singles are still new and move up when they stop being enough.
The concentration a grid demands makes it an effective way to practise sustained attention away from a feed.
Everything runs in the page and works offline once loaded, so a flight or a commute needs no connection.
36 clues, one solution, generated in about 3 ms. The panel reports whether naked and hidden singles alone will finish it, and the mistake budget is four.
Evil aims for 23 clues and usually stops at 24 or 25, because removing another would leave the grid with two completions. The page prints the number it reached.
The hint picks a cell whose digit follows from the board and says "Every other digit already appears in this row, column or box", rather than copying a value from the answer.
A Sudoku with two completions is not a hard Sudoku, it is a broken one. At some point in solving it you reach a position where two different digits are both consistent with everything you know, and nothing but a coin decides between them. Worse, a tool that keeps the grid it started from and calls that "the solution" will then mark your perfectly legal answer as a mistake on every cell where the two completions differ.
That is exactly what used to happen here. The old generator took a finished grid, blanked a fixed number of cells at random and handed over the result without checking anything. Measured over 60 puzzles per difficulty on 2026-09-07: 32 of 60 easy puzzles had more than one solution, 53 of 60 medium, and every single hard, expert and evil puzzle did. The evil setting removed 64 cells, leaving 17 clues, and although 17 is the proven minimum for a unique Sudoku (McGuire, Tugemann and Civario, Experimental Mathematics 23(2), 2014) almost no 17 clue arrangement is one of the unique ones, so that difficulty could essentially never produce a real puzzle.
Generation now removes a cell only when the grid still has exactly one completion, counting solutions with a search that stops the moment it finds a second. It costs about three milliseconds for the hardest setting, so nothing on screen waits for it, and the page prints the clue count it actually reached. Sometimes that is more than the target asked for, and it says so: below about 24 clues most arrangements stop being unique, and a generator that cannot get there has to tell you rather than hand over an ambiguous grid.
Difficulty is measured by solving rather than by counting blanks, because a 30 clue puzzle can be easier than a 40 clue one. Each generated puzzle is run through the two techniques people actually use first, naked singles (a cell with one candidate left) and hidden singles (a digit with only one home left in a row, column or box), and the page reports whether those two finish it and how many of each it needed. A puzzle they cannot finish needs a naked pair or something further, and the page says so instead of guessing.
The hint follows the same rule. It looks for a cell whose value FOLLOWS from what is already on the board and tells you the reason, and only falls back to reading the solution when no such cell exists, saying that it did. A hint that copies a digit out of the answer teaches nothing.
No. Nothing is added or multiplied. The digits are just nine distinguishable symbols; letters or colours would work exactly as well.
Not here. Every puzzle is checked to have exactly one solution before it is dealt, so every placement can in principle be justified. Harder settings may need a technique beyond singles, such as a naked pair, and the page tells you when a puzzle is in that class.
Because a Sudoku has to stay unique. The target is 23 clues, and the generator removes cells only while a single solution survives; on most grids it stops around 24 or 25. Seventeen is the proven minimum over all arrangements and is far out of reach for a generator that works this way, so aiming lower would just produce ambiguous grids.
A naked single is a cell with only one digit left that fits. A hidden single is a digit with only one place left in a row, column or box. Those two are the techniques nearly everyone uses first, and a puzzle they finish on their own is an easy one. The page runs both on your puzzle and reports what it found.
A digit that disagrees with the solution is flagged straight away and counts against your mistake budget, which is five on easy down to one on evil. Undo takes back the move.
It tries not to. It looks for a cell whose digit follows from the board as it stands, gives you that one and says why. Only when no such cell exists does it read from the solution, and it tells you that is what happened.
Your difficulty, theme, statistics and best times are kept in this browser. The puzzle itself is not, so finishing a grid in one sitting is the idea.
The game runs entirely in your browser. No account is needed and no gameplay data is collected.