How to play Extreme Sudoku
- The Extreme level is already selected; you can change it above whenever you like (Easy, Medium, Hard, Expert or Extreme).
- Fill the 9×9 grid so that every row, every column and every 3×3 box contains the numbers 1 to 9 without repeats.
- Write down all the candidates with Notes and make progress with logic until nothing is certain any more.
- When you get stuck, pick a cell with only two candidates, try one and keep going until you hit a contradiction: if one appears, the other candidate is the right one. Undo takes you back to the starting point.
What an extreme (or diabolical) sudoku is
An Extreme puzzle has a single solution, but neither singles, nor pairs and triples, nor intersections, nor X-Wing, Swordfish or Y-Wing are enough to solve it completely: at some point the site's logic solver gets stuck and the only way out is a hypothesis (try and backtrack) or a long chain of implications.
That is why it is not measured by the number of starting digits: a puzzle with few clues can be easy and one with many can be hell. Here the level comes from the technique required, and every puzzle is generated on your device with a verified unique solution.
If you want to train first, go through the Hard and Expert levels, or enter a puzzle in the Sudoku Solver to see which technique each step needs.
Things to keep in mind
- Building an extreme puzzle can take a few seconds on slow phones; it is done in the background.
- Your best time only counts if you do not use hints, and it is saved only in this browser.
- Trying hypotheses by hand is allowed at this level; if you prefer to solve with pure logic every time, play the Expert level.
Frequently asked questions
What does it mean for a sudoku to be extreme or diabolical?
That the usual techniques are not enough and you have to try a hypothesis and follow it to a contradiction, or chain together several long deductions.
Do all extreme sudokus have a unique solution?
Yes. This is checked when they are generated; if one had more than one solution it would not be a valid sudoku.
Is it fair to guess and backtrack?
At this level, yes, it is exactly what it calls for. With Notes and Undo you can try a candidate and go back without losing the rest.
How many clues does an extreme sudoku have?
It usually has between 21 and 29 numbers, but that is not what makes it hard: the required technique decides.
What do I do if I get stuck?
Use Hint to fill in a cell (that puzzle no longer counts for your best time) or enter it in the Sudoku Solver to see the reasoning step by step.
Is it free?
Yes, it is free, needs no sign-up, and everything is calculated in your browser without sending anything to a server.