What makes Evil Sudoku so hard?
It comes down to one number: 23. That's how many digits an Evil puzzle starts with, against 36 for Easy - nearly two fifths fewer. Every empty cell you don't fill in leaves more room for the others to stay uncertain, so the whole grid resists you for longer.
With so few givens, the obvious moves dry up almost immediately. On an Easy grid you can place number after number just by scanning. On Evil, you'll often stare at a full board of pencil marks with nothing forced anywhere - and that's normal. The puzzle is still perfectly logical; the logic is just buried deeper.
One thing never changes, though: there is always exactly one solution, and you never have to guess. However thin the start, an Evil puzzle can be solved by pure deduction. If you're guessing, there's a technique you haven't spotted yet.
Techniques you'll actually need
The beginner tools - naked singles and hidden singles - still matter; they're how you turn each breakthrough into a run of placements. But on their own they'll stall fast. Evil is where the advanced patterns earn their keep:
- X-Wing Sudoku Technique Explained - One digit confined to the same two columns across two rows (or vice versa) - eliminate it from the rest of those lines.
- Forcing Chains - Follow both values of a bi-value cell; if both branches force the same digit somewhere, that conclusion is certain.
Don't worry if these take practice to spot - they're supposed to. The reward is that moment when an X-Wing you almost missed suddenly clears three cells at once and the grid starts moving again. If you want to build up to it, the Sudoku School walks through every technique with worked examples.
How to solve an Evil Sudoku without guessing
When an Evil grid stops moving, the answer is almost never "guess and see" - it is to change what you are looking at. Complete your pencil marks, then stop asking "what fits in this cell?" and start asking "where must this digit go?" Sweeping one digit at a time across the boxes is what breaks most stalls, and it is where most Evil grids give way. Beyond that, every Evil board here is checked before it is dealt: it only ships if finishing it needs at least box/line elimination, so a stall on one of these is a technique waiting to be spotted rather than a hard board that happens to be easy. Pairs, X-Wings and chains come after that, in that order.
And if nothing moves, take a break rather than a guess. Fresh eyes find hidden singles that tired ones walk straight past - and unlike a guess, a break cannot corrupt your grid.
The long version, worked through on a real Evil board: How to Solve an Evil Sudoku Without Guessing.
What is a 17-clue Sudoku?
Seventeen is a famous number in Sudoku. It is the fewest givens a puzzle can have and still leave exactly one solution - proven in 2012 by an exhaustive computer search, which confirmed that no valid 16-clue puzzle exists. Thousands of distinct 17-clue grids are known.
Our Evil puzzles start with 23, and the gap to seventeen is worth explaining. Puzzles that sparse are found by long, targeted searches - not produced on demand, which is how puzzles are normally made: take a finished grid and keep removing digits for as long as the solution stays unique. That process runs out in the low twenties, and 23 is where ours stops while still landing on the same number every single time. The example grid at the top of this page is a real 17-clue puzzle, if you want to see how bare the theoretical minimum looks.
Common mistakes on Evil grids
- Guessing when you're stuck. It feels faster, but one wrong guess can send you ten moves down a dead end. There's always a logical next step - look for it.
- Skipping pencil marks. On Easy you can hold candidates in your head. On Evil you can't - mark every candidate, because the advanced patterns only become visible once they're all on the board.
- Chasing one number. When a region won't budge, switch digits or switch units. A hidden single for a 7 is often sitting right next to the 3 you've been fighting.
Evil, Extreme, Diabolical - is it all the same level?
Roughly, but you cannot rely on the name. There is no standards body for Sudoku difficulty, so every site invents its own ladder: the hardest tier is Evil on one and Expert, Extreme, Master or Diabolical on another. Compare the clue count, not the adjective - ours is 23.
What is Evil Sudoku? goes into where the name came from and how the levels line up.
Prefer it on paper?
Every level here is also available as a printable sheet. The printable evil Sudoku page builds PDF pages at this exact difficulty - handy for a commute, a classroom, or anywhere a screen is in the way.
All five difficulty levels
Difficulty on SudokuStreak starts with how many digits the puzzle gives you, but the clue count is not the whole answer - and we can show that rather than assert it. On 31 August 2026 we generated 40 boards at every level and had our own solver finish each one, recording the hardest technique it was forced to use. The result was uncomfortable: 28 of the 40 Expert boards needed nothing harder than a hidden single, which is what Medium asks for. A 25-clue grid can still be a scan.
So Expert and Evil are no longer dealt straight from the generator. Boards for those two levels are measured first, away from the site, and only the ones that genuinely need the harder techniques are kept and handed out. That is what the last column below means: always is a floor the puzzle cannot fall below, usually is what the measurement found without one. The counts are for a standard 9×9 board:
| Level | Givens | Hardest technique needed (measured) |
|---|---|---|
| Easy Sudoku | 36 | usually naked single |
| Medium Sudoku | 30 | usually hidden single |
| Hard Sudoku | 26 | usually hidden single |
| Expert Sudoku | 25 | always pointing or harder |
| Evil - you are here | 23 | always box/line or harder |
The names are not standardised, though: "Expert" is the fourth rung of five here and the fourth of six elsewhere, and the hardest tier is called Evil on some sites and Extreme on others. Sudoku difficulty levels explained compares the common ladders and says what each level here actually asks of you.
Frequently asked questions
How many numbers does an Evil Sudoku puzzle start with?
Every SudokuStreak Evil puzzle on a standard 9×9 board is generated with 23 given digits, out of the 81 cells in the grid. Variants built on a different board set their own count for the same level - Samurai, for instance, is five overlapping 9×9 grids and fills each of them separately.
Do the puzzles always have exactly one solution?
Yes. However few givens a puzzle starts with, it has exactly one solution, and that solution is reachable by logic alone.
Do I ever have to guess?
No. Every puzzle can be finished by pure deduction. If you feel you have to guess, there is a technique on the board you have not spotted yet.
Is Evil Sudoku free to play?
Yes - every puzzle on SudokuStreak is free to play in your browser, with no account required. Signing in only adds streaks, scores and the leaderboard.
How do you solve an Evil Sudoku without guessing?
By working on candidates instead of digits. Pencil in the possibilities with a disciplined notation such as Snyder notation, then look for the patterns that eliminate candidates rather than place them - X-Wing, Swordfish, XY-Wing and forcing chains. Every Evil puzzle here has exactly one solution reachable by pure deduction, so if you feel forced to guess, there is a pattern on the board you have not spotted yet.
Where can I play Evil Sudoku online for free without downloading an app?
Right here, in the browser. Evil Sudoku on SudokuStreak is free, needs no account and no download, and runs on the phone as well as the desktop. Signing in only adds streaks, scores and the leaderboard.
Is Evil the same as Extreme, Master or Diabolical?
Roughly, but the names are not standardised - every site invents its own ladder, so the hardest tier is called Evil on one and Expert, Extreme, Master or Diabolical on another. The number to compare is the clue count, not the adjective: Evil here means 23 givens. That is close to the practical floor for a generated puzzle - keep removing digits and a board eventually stops having a single solution, which would make it broken rather than hard.
Can I print Evil Sudoku puzzles?
Yes. The printable evil Sudoku page generates PDF sheets at this exact difficulty, ready to print. You can start from the printable evil page.