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 Extreme Sudoku here?
Extreme is the hardest level on this site: classic sudoku dealt from a bank of 5,000 puzzles, one at random each time, each with exactly one answer. 4,980 of them come from the list of the hardest sudoku puzzles kept by players on the Enjoy Sudoku forum. They start from 21 to 27 given digits, 24 on a typical board, but few givens is not what makes them hard. On every one of those 4,980 there is no single to find when it is dealt, and none of the eighteen techniques our sudoku helper knows can make a single move.
What does an Extreme puzzle ask of you?
Every candidate is on the board above, and none of them is alone. We found the same on all 4,980 forum puzzles: no square with one digit left, no digit with one place left in a row, column or box, and no locked candidate, pair, triple, fish, wing or colouring that removes anything. The other 20 came from the same random collection as Evil, and they are milder: the helper finishes 14 of them.
What does it take, then?
Chains, and usually chains inside chains. The forum grades puzzles with Sudoku Explainer, a program that solves a puzzle with human techniques in order of difficulty and rates it by the hardest step it needed. Every one of our 4,980 is rated above 11.0 on that scale. For comparison:
| Technique | Sudoku Explainer rating |
|---|---|
| Hidden single | 1.2–1.5 |
| Naked single | 2.3 |
| Naked pair | 3.0 |
| X-Wing | 3.2 |
| Swordfish | 3.8 |
| XY-Wing | 4.2 |
| XYZ-Wing | 4.4 |
| Unique rectangle | 4.5–5.0 |
| X-cycles | 6.5–6.9 |
| Forcing chains | 7.1–7.5 |
| Dynamic forcing chains | 8.8–9.6 |
| Our Extreme puzzles | above 11.0 |
Ratings from Sudoku Explainer 1.2.1’s own technique table; the ratings above these belong to forcing chains nested inside one another. The 4,980 come from the part of the forum’s May 2019 list rated above 11.0.
A forcing chain takes one candidate and follows what it forces: if this square were 4, that one would have to be 7, which would leave a third square with no digit at all. A contradiction like that proves the starting candidate false. At this level a chain often needs a second chain inside it before anything breaks, and the bookkeeping is the whole difficulty. Whether that is logic or organised guessing is argued about among solvers; either way, it is how these puzzles are solved by hand.
How do you work one?
- Put in every candidate with Start Notes and keep them exact: on these boards a missing candidate makes a false chain look sound.
- Pick a square with two candidates, or a digit with two places in some house. Those are the strong links a chain is built from.
- Follow one candidate: assume it, strike what it rules out square by square, and watch for a square left with nothing or a house left without a digit.
- If it breaks, the candidate is false: remove it and look for singles. If nothing breaks, the chain proved nothing; try the other candidate.
- Undo takes back every step, so you can try a candidate on the grid itself; keep a trial digit only once its alternative has led to a contradiction.
The page itself finds each puzzle’s answer when it deals it, by search, branching on the square with the fewest candidates. On all 5,000 that takes at most 44 milliseconds. By hand there is no shortcut: these puzzles were collected because people found them hard.
Has this page always dealt puzzles like these?
Yes, every one with exactly one answer. The page went up on 7 May 2026 with 250 puzzles, grew to 2,500 the next day and to 5,000 on 9 May, and every puzzle it has ever dealt is still in the bank; we counted all 5,000 again for this page. Until 11 October 2026 it worked out each answer with a slower search, which could hold the page for over half a second when an Extreme puzzle was dealt.
What has changed is what the page said. It told you the first pass “often gives very little. You may remove some candidates, find one or two placements”: on 4,980 of the 5,000 it gives nothing at all. It said “Extreme sudoku often needs these quiet eliminations” of fish such as the X-Wing and Swordfish, which find nothing on those 4,980 either. And it promised the puzzles could be solved without guessing, which depends on what you call the nested chains above.
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 Extreme 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?
If Extreme is more than you want today, Evil is the step down: hard random puzzles on which named techniques still carry three in four. The sudoku trainer drills those techniques on real boards; it stops before chains, as our helper does. The hardest sudoku ever page has the famous puzzles by name, Arto Inkala’s among them, and 17-clue sudoku deals puzzles with the fewest givens any sudoku can have.