Stuck on a sudoku? Work through this in order
A fixed escalation order from cheapest to most expensive, why running it in order matters more than knowing more techniques, and how to tell being stuck from having already gone wrong.
Being stuck in sudoku usually feels like a knowledge problem — as though there is some technique you have not learned that would crack the grid. Occasionally that is true. Far more often the deduction available is one you already know perfectly well, and you did not find it because you were searching unsystematically.
The fix is an order. Always start at the cheapest technique, always run each stage over the whole grid before moving up, and always go back to the top after any placement. That last part is the one people skip, and it is the one that matters most: every digit placed changes twenty cells, so a naked single may now exist where there was none a moment ago.
Before anything: is the grid still valid?
If you have been stuck for a while, check for damage before hunting for technique. Scan each row, each column and each box for a repeated digit. It takes a minute, and it tells you whether you are solving a puzzle or trying to solve a broken one.
A wrong digit does not announce itself. It usually produces no contradiction at all for a long time, just a grid that quietly refuses to move because the constraint that would have driven the next step has been destroyed. If you find a duplicate, that is your answer and no amount of technique would have helped.
An app that highlights conflicts gives you this for free. Sudoku - Calm Number Puzzle also handles it in the hint: if the board already contains a wrong digit, the hint says so instead of pretending the position is sound and offering a deduction that cannot be right.
The escalation
1. Naked singles
Any cell with exactly one possible digit. Cheapest thing on the board and the first place to restart after every placement.
2. Hidden singles
Take each digit and each unit in turn: is there a digit that can only go in one cell of this row, column or box? Hidden singles are where the majority of all placements come from, in every puzzle at every level. Sweep digit by digit, not cell by cell.
3. Pointing and claiming
Both concern the three cells where a box overlaps a line. If a digit inside a box is confined to one row or column, erase it from the rest of that line. If a digit inside a row or column is confined to one box, erase it from the rest of that box. Neither needs pencil marks, so run both before you start writing.
4. Naked pairs and triples
Two cells in a unit with the same two candidates and nothing else means those two digits are spoken for; erase them from the rest of the unit. Triples work the same way with three cells drawn from three digits — and each cell needs only two of the three, which is the part that gets missed.
5. Hidden pairs and triples
Two digits in a unit that can only go in the same two cells. Those cells are taken, so every other candidate in them goes. This never places anything and looks like nothing happened — what it did was turn two crowded cells into a naked pair, which then cleans the unit for you.
6. X-Wing and Swordfish
Single-digit patterns across two or three lines. Pick a digit that is already well placed, list the rows where it has exactly two candidate cells, and check whether any two of those rows use the same two columns. Then repeat with rows and columns swapped — that transposed pass is where half of all missed X-Wings live.
7. XY-Wing
List every cell with exactly two candidates. Try each as a pivot, look among the ones it can see for two pincers sharing a third digit, and eliminate that digit from the cells both pincers see.
Three habits that unstick more grids than techniques do
- After every placement, go back to stage one. The grid you are looking at is not the grid you were analysing thirty seconds ago.
- Sweep by digit, not by cell. Almost every technique above naked singles is a statement about one digit at a time, and holding one digit in mind is far easier than holding nine.
- Scan in both orientations. Rows then columns, every time. A large share of missed deductions are simply the transpose of something you did look for.
If it is still stuck
Two possibilities remain. Either the puzzle needs something beyond your current repertoire, or it was never solvable by logic in the first place — some sources ship puzzles they never verified, and a grid with two solutions will strand you at a point where no reasoning can help.
You can rule the second one out by construction if the source verifies uniqueness. In Sudoku - Calm Number Puzzle every puzzle is checked to have exactly one solution before it is ever shown, and for every level except Expert it is also verified solvable within that level’s technique band. So on a Hard grid, a deduction genuinely exists and it is not harder than a hidden pair or a naked triple.
Which is also the honest case for taking a hint. A hint that names the technique and highlights the cells that prove it tells you what you failed to see, which is worth considerably more than the digit it fills in.
Frequently asked questions
How do I know if I made a mistake or the puzzle is just hard?
Check every row, column and box for a repeated digit before anything else. A duplicate means a mistake. If the grid is clean and you have run the full escalation without result, it is difficulty, not damage.
Is it worth restarting when I get stuck?
Only if you find an actual duplicate, or you cannot tell which of your digits were deduced and which were guessed. A clean grid never needs restarting — the deduction is there, and if the puzzle was verified unique it can be found.
What technique should I learn next?
Whichever one your escalation keeps ending at. If you routinely stall after naked pairs, learn hidden pairs. If you stall after those, learn X-Wing. Learning techniques far above where you actually get stuck does not help, because you will never reach them.
Does taking a hint mean I failed?
It means you did not spot one specific pattern. A hint that names the technique turns the block into something you can recognise next time, which is the only way anyone ever moves up a level.
Try it on a real puzzle
Sudoku - Calm Number Puzzle grades every puzzle by the hardest technique it actually needs, verifies a single solution before you ever see the grid, and its hints name the technique instead of just filling in a digit. Free, offline, no ads and no account.