Play evil sudoku online with a new daily grid at three levels: Hard, Expert and Evil. Every puzzle has exactly one solution and is rated by the hardest technique you need to finish it, so an Evil grid always calls for more than pairs and pointing.
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How to play
The rules are the same as any classic sudoku. Fill the empty squares so that every row, every column and every 3 by 3 box holds the digits 1 to 9 exactly once. The numbers already on the board are fixed clues.
A small example: say row 4 already holds 1, 2, 3, 5, 6, 7, 8 and 9. Its one empty square has to be the 4. Now say a box is missing a 7, and the 7s elsewhere on the board block every empty square in that box but one. That last square is the 7, even if it could take other digits too. Most of a puzzle is built from moves like these.
On an evil grid, those simple moves run dry early. That is where pencil marks come in. Turn on Notes and write the possible digits into each square, then use the techniques below to cross them off until a square is left with one choice.
Using the board
- Tap a square, then a number on the pad. Tap the same number again to clear it.
- Notes switches the pad to pencil marks. Placing a digit removes that digit from the notes in its row, column and box.
- Check marks any square that does not match the solution. A digit that clashes with another in its row, column or box turns red straight away.
- Hint fills one square and tells you which technique finds it, for example "X-wing on 4s" followed by "hidden single in box 6".
- On a keyboard, type 1 to 9, hold Shift (or press N) for notes, use the arrow keys to move, Delete to erase and Ctrl+Z to undo.
Your progress and timer are saved on your device, so you can close the page and come back later. Press New puzzle for as many extra grids as you like at the same level.
What makes a sudoku evil
Clue count on its own is a poor guide. A grid with 24 clues can fall apart with nothing but singles, while one with 27 can stop you cold. So each puzzle here is rated by solving it step by step the way a person would, always trying the simplest technique first, and noting the hardest one it needed.
| Level | What you will need |
|---|---|
| Hard | Singles plus pointing, box/line reduction and naked or hidden pairs |
| Expert | At least one triple, quad or X-wing |
| Evil | At least one swordfish, XY-wing, XYZ-wing, skyscraper, two-string kite, W-wing or colouring chain |
Our Expert and Evil grids usually have between 22 and 26 clues. None of them need guessing: every one can be finished with logic alone.
Techniques for evil sudoku
Pointing and box/line reduction
If every possible spot for a 5 inside a box sits in one row, the box will put its 5 in that row. So 5 can come out of the rest of that row, outside the box. The reverse works too: if a row's only spots for a 5 sit inside one box, 5 comes out of the rest of that box.
Naked and hidden pairs
Two squares in a row that can only be 3 or 8 will take the 3 and the 8 between them, so no other square in that row can be 3 or 8. A hidden pair is the mirror image: if 3 and 8 can only go in the same two squares of a row, those squares cannot hold anything else. Triples and quads follow the same idea with three or four squares.
X-wing and swordfish
Pick one digit. If it has exactly two possible spots in each of two rows, and those spots line up in the same two columns, the digit must sit on opposite corners of that rectangle. Either way, both columns are covered, so the digit comes out of every other square in those two columns. A swordfish is the same pattern across three rows and three columns.
XY-wing
Find a square with only two notes, say 2 and 7. This is the pivot. Now find two squares it can see: one with 2 and 9, and one with 7 and 9. Whatever the pivot turns out to be, one of those wings must be 9. Any square that sees both wings cannot be 9.
Skyscraper and two-string kite
These use "strong links", where a digit has only two spots left in a row, column or box, so one of the two must be it. When one end of a strong link sees one end of another strong link for the same digit, at least one of the two far ends has to hold that digit. Squares that see both far ends can lose it.
When you are truly stuck
Check your notes first: a single missing pencil mark is the most common reason an evil puzzle seems to have no next move. If that is not it, press Hint. It reads the board as it stands now, so it explains the next step from where you are rather than from the start.
Want something lighter between evil grids? Canoku is our Canadian emoji sudoku with 4x4, 6x6 and 9x9 grids. For a twist on the rules, try killer sudoku with cage sums, take on the five linked grids of samurai sudoku, or make a printable sudoku sheet to solve on paper.