Sudoku Made and maintained by Brian Hamilton and Karan Hamilton.
Brian builds the playable puzzle tools, while Karan helps test clarity, difficulty feel, and the player experience.
Playable puzzles and controls are tested in the browser for rule accuracy, usability, and a clear solving experience. See how we make and test puzzles, our editorial standards, or report a problem.
What is Evil Sudoku here?
Evil is classic sudoku at the second-hardest level on this site. Easy to Expert are dug fresh for every game by our generator; Evil deals from a fixed bank of 5,000 puzzles instead, one at random each time, and every one of them has exactly one answer. They start from 17 to 30 given digits, 26 on a typical board, but what they share is not a clue count. It is a stall: on every one, singles, locked candidates and naked pairs run out long before the grid is full, and the next step has to come from somewhere else.
What does an Evil puzzle ask of you?
We walked all 5,000 with the eighteen techniques our sudoku helper knows, always taking the simplest move left. Singles start 4,999 of them and fill 14 squares on a typical board before they stall. Locked candidates and naked pairs carry some further, then stop with a median of 39 squares still open. Not one of the 5,000 can be finished with those steps alone. What the helper reaches for next:
| First step past singles, locked candidates and naked pairs | Puzzles | Share |
|---|---|---|
| Hidden pair | 1,530 | 31% |
| Almost locked set | 800 | 16% |
| XY-Wing | 670 | 13% |
| W-Wing | 583 | 12% |
| Naked triple | 269 | 5% |
| Skyscraper | 257 | 5% |
| X-Wing | 253 | 5% |
| XYZ-Wing | 244 | 5% |
| Simple colouring | 147 | 3% |
| Hidden triple, unique rectangle, Swordfish or Jellyfish | 146 | 3% |
| None of the eighteen | 101 | 2% |
All 5,000 Evil puzzles, walked by our helper’s eighteen detectors taking the simplest move each time; the row is the first technique a puzzle needed beyond naked and hidden singles, pointing pairs, box/line reduction and naked pairs. Measured in October 2026.
Keep going and the eighteen finish 3,655 of the 5,000, almost three in four. The other 1,345 reach a point where no named pattern applies at all. They still have one answer each, so a chain of reasoning exists; it is the kind that follows a candidate’s consequences across the grid, which our helper does not search for.
What is a hidden pair?
A hidden pair is two digits that, within one row, column or box, can each go in only the same two squares. Those two squares have to hold both digits between them, so anything else written in them is impossible, however many other candidates they show. It is the most common first step past the stall on Evil, in 1,530 of the 5,000, and the easiest to walk past, because the pair hides among other candidates instead of standing alone. Take one house at a time and ask, for each digit it is missing, how many places that digit has: two digits with the same two places are a pair.
What is a W-Wing?
A W-Wing joins two squares that hold the same two candidates and do not see each other. If some row, column or box has only two places for one of those digits, and one place sees each square, the two squares cannot both take that digit: the row, column or box would be left without it. So one of them holds the other digit, and that digit comes out of every square that sees both. Both ends of the link must be squares other than the two in the wing; a link that ends on one of them proves nothing. It was the first step past the stall in 583 Evil puzzles.
How should you work an Evil puzzle?
- Run the singles to the end first: hidden singles digit by digit, naked singles wherever a house is nearly full. On a typical Evil board they fill a dozen or more squares before stopping.
- Put every candidate in — Start Notes does it in one go — and keep the notes current. On a board this open, a stale candidate hides every pattern behind it.
- Clear what locked candidates and naked pairs can clear, and look for singles again after each.
- Then search each house for hidden pairs, the squares with two candidates for XY-Wings and W-Wings, and almost locked sets, the second most common step in the table.
- Fish come later and rarely: an X-Wing was the first step past the stall in only 253 of the 5,000.
One puzzle in four gets there. Pick a square with two candidates and follow one of them: what does it force in its row, column and box, and what does that force in turn? If the chain of forced squares contradicts itself, the candidate you started from is false. That is the reasoning the last quarter of Evil needs, and the point where our helper stops.
Where do the puzzles come from?
From a public collection of a million puzzles that solver programs are tested on, drawn at random from every minimal sudoku — minimal meaning that no given can be taken away without the puzzle gaining a second answer. Random puzzles are mostly gentle: of the first 47,773 in that collection, singles alone finish 16,818, and 24,499 still stall once singles, locked candidates and naked pairs are spent. Our bank builder scored each of those by how hard a computer search found it, kept 4,998 for Evil and set the deepest aside, then added two puzzles with only 17 givens, the fewest any sudoku can have.
For a person the difference is smaller than “deepest” suggests: our helper finishes 73% of the Evil bank and 68% of a sample of the puzzles set aside. Evil is a fair picture of what a hard random sudoku looks like, not a hand-picked set of monsters. Those are on Extreme.
Has this page always dealt puzzles like these?
Yes, and every one has had exactly one answer. The page went up on 7 May 2026 with 250 puzzles; the bank grew to 2,500 the next day and to 5,000 on 9 May, and every puzzle it has ever dealt is still in it. We counted the answers of all 5,000 again for this page. What has changed is what the page said about them. It promised a “ranked puzzle bank”: the bank is stored in order of the builder’s score, but the page has always dealt at random. It told you to “Look at the kind of move the hint suggests”; Hint has never suggested a move, it fills one square with its answer. And it said that “Expert Sudoku often introduces advanced techniques”, which stopped being true on 10 October 2026, when Expert was rebuilt to stop at naked pairs.
What do the buttons do?
- Notes switches the keypad to pencil marks, and Start Notes fills every empty square with the digits it can still hold, in one go.
- Hint picks a square at random from the ones that are empty or wrong and puts its answer in. It does not explain the step and it is an ordinary move: Undo takes it back.
- Undo and Redo step back and forward through your moves (Ctrl+Z and Ctrl+Y, or Cmd on a Mac); Erase clears the selected square.
- A wrong digit turns red and adds one to the mistake count. There is no limit: the count never ends the game.
- Solve asks before it fills the grid. New Game deals another Evil puzzle, picked at random from the 5,000, so one you have played can come round again. The level buttons above the grid switch level without leaving the page.
- The timer starts when the puzzle is dealt and runs until it is finished. Your board, the clock and your notes are saved as you go, apart from the other level pages.
Where next?
Extreme is the step up: puzzles on which none of the eighteen techniques makes a move at all. The sudoku trainer drills hidden pairs, wings and the rest of the eighteen on real boards, one pattern at a time. And Expert is there when you want a puzzle that singles, locked candidates and naked pairs always finish.