If you're new to Sudoku and wondering, "Why can't this cell be X?"—this post is for you.
Why is this 8 wrong?
Let’s break it down so you can understand the logic behind solving Sudoku puzzles and avoid one of the most common beginner mistakes.
The Two Times You Should Place a Digit in Sudoku
There are only two situations where you should place a digit in a cell:
When it’s the ONLY PLACE that digit can go in the row, column, or box.
Even if other digits could technically fit in that cell, if a digit has no other valid spot in its row, column, or box, it must go there.
When it’s the ONLY DIGIT that can go in that cell.
If no other digit is valid for a particular cell—even if this digit could potentially fit elsewhere—it must be placed there.
Why Guessing Doesn’t (always) Work
Good Sudoku puzzles are designed to have one unique solution. That means every number you place must be based on logical reasoning, not guesses. A common beginner mistake is thinking, "If there’s no immediate contradiction, I can just place this number here." But that’s not how Sudoku works!
If you can’t logically prove why a number must (or must not) go in a specific cell - or why it can’t go anywhere else - then you’re not ready to place it yet. Keep looking for clues and deductions elsewhere.
Advanced Techniques and Complex Proofs
As puzzles get harder, you’ll encounter situations where more complex reasoning is required to rule out candidates. These advanced techniques (like X-Wing, XY-Wing, or Skyscraper) help you prove why certain numbers can’t go in specific cells. Mastering these methods will make solving medium and advanced puzzles much easier!
TL;DR: Use Logic, Not Luck, Not Assumptions!
To sum up:
• Only place a number when you’ve logically proven it’s the only option for that cell or location.
• Avoid guessing—it leads to errors and frustration.
• Use beginner techniques like Naked Singles and Hidden Singles first, then move on to advanced strategies as needed.
SOME EXAMPLES
Recall the rules: no repeats in every row, column and box
In box 9 (the right bottom box), there's only one spot for 8 so 8 has to go there.
No repeats
No repeats in every row and column so there's only one 8 in row 7 AND column 8.
Therefore, green cell has to be 8.
Row and Column
This one is trickier:
Trickier
There are 9 digits.
If a cell 'sees' all but one digit, that cell has to be that digit.
This green cell sees 14678 in row 2 and 235 in column 1. That leaves 9 as the only option for that cell.
If you're still confused, try thinking if there's any other digits you could place in the green cell apart from 9.
Eventual Impossible State
Even if the contradiction is not readily apparent, making a mistake will inevitably lead to a contradictory/impossible state later on.
If you're still stuck or want examples of how to solve without guessing, ask a question! The members here are willing to help you out. Happy solving! 😊
Special thanks to u/Special-Round-3815 who wrote this original guide, and the other members of r/sudoku who commented and who make this sub a pleasure to be involved with.
I got a sudoku book for christmas and I loved it - I spent so much less time on my phone, and it felt nice to do something both tactile and think-y. But I have an issue, I hate most sudoku books. I'm hoping y'all can help with that.
My main gripes are books that are too small/put too many boards on one page. I'd like a bigger book that has one board that takes up the whole page, with ample room in each square to write down & erase the guessed numbers. It would also help if the pages were off-white, as bright pages are difficult to look at for extended periods of time. Bonus bonus points if it's spiral-bound.
Anyone know any publishers that fit this bill? Thank you in advance! Also in advance, I know I can print out pages I find online & make a binder; I would prefer not to do this, and buy a book instead.
Hi there, I’m new to advanced sudoku and ran into a bit of a head scratcher. I have the skyscraper denoted in pink and by logic, both roof cells see the green cell. So that should eliminate the seven and result in a 4. However, the 4 flags as the wrong number when I put it in. Can anyone help?
Is this supposed to be an extreme 8-star sudoku? I managed to solve it in fifteen minutes without using any advanced techniques.
I usually use an app where I can't solve extreme sudokus without advanced techniques, so I wondered if these are actually 'easy' or if my app's extreme level is just really hard.
Every time I've played on sudoku.com today, regardless of device, a big Norton Antivirus screen pops up a few minutes in. I don't even have Norton Antivirus, so I've been closing the tab quickly just to be cautious.
Puzzle is from Logic-Wiz. They offered a hint about the deadly unique triangle formed by the 89s in the top. I understand that one of the squares must break from the 89 pattern to avoid two solutions, but i do not see how to reason which one out. Thanks for your patience and help!
First time playing sudoku on my laptop and thought that phone medium level was easy and I'm not as good as I thought. I was about to leave the puzzle and try another one then I noticed something was wrong. The puzzle is impossible, so weird, kinda funny I would say. Just wanted to chare it. Have you ever encountered something like that?
I think I’ve found everything I can place with certainty. Is the next step just guessing a number somewhere and seeing how far I get? Are hard sudokus distinguished by the fact that you can’t know a number for certain?
I think I have been staring at this so long that my brain has lost all logic… can anyone advise what next step i should take here? I can’t seem to find anything to eliminate
