Stuck on a Sudoku? What to Do Next (Step-by-Step Checklist)

Every Sudoku player knows the feeling. The first few minutes go beautifully, numbers drop into place, and then, suddenly, nothing. You stare at the grid, your eyes slide over the same empty cells, and the puzzle seems to have run out of moves. It hasn’t. A well-made Sudoku always has a next logical step; you just haven’t looked in the right place, or with the right tool, yet.

This guide is a practical checklist for exactly that moment. It doesn’t try to teach every technique in depth (we have separate guides for those, linked as we go). Instead it gives you an order to try things in, so that being stuck turns into a short routine rather than ten minutes of staring.

Why you get stuck (and why it’s normal)

When you’re stuck, it is almost always for one of four reasons:

• You missed an easy move. This is by far the most common one. Your eyes settle into a habit, say, checking boxes for the digits you’ve just placed, and skip whole parts of the board.
• Your notes are out of date or incomplete. A pencil mark that should have been erased hides a single, or a cell you never noted at all hides a pair.
• The puzzle needs a technique you haven’t used yet. Harder puzzles are built so that singles run out and a pattern like locked candidates or a naked pair is the only way forward.
• There’s a mistake on the board. One wrong digit, or one wrongly erased note, can make a perfectly good puzzle look impossible.

The checklist below deals with them in that order, because that is roughly how often each one is the culprit. Most of the time you will be unstuck by step 1 or 2.

The stuck-on-Sudoku checklist

Here is the whole ladder on one screen. The rule that makes it work is simple: whenever a step places a digit or removes a candidate, go back to step 1. A single new number often unlocks several easy ones, and there is no point hunting for an X-Wing when a hidden single is sitting in plain sight.

Step 1: Rescan for singles, systematically

Before anything clever, do a proper sweep for hidden singles: a digit that has only one possible place in a box, row or column. The key word is systematically. Random glancing is what got you stuck; a fixed routine is what gets you out.

A good routine is to take one digit at a time, from 1 to 9. For each digit, go through the nine boxes and ask, “Does this box still need this digit? If so, how many cells can take it?” Look along the rows and columns that cross the box for that digit. Each one rules out a line of cells. This is often called cross-hatching.

When the boxes are done, do the same for rows and columns. Lines are harder to scan than boxes, which is exactly why players miss singles there.

If you want to scan faster in general, our guide on how to solve Sudoku faster covers scanning routines in more detail.

Step 2: Bring your pencil marks up to date

If a full sweep finds nothing, it’s time for notes (also called pencil marks or candidates). Many stuck moments come from notes that are stale: you placed a 6 ten moves ago, but a few cells in its row still show a 6 as a candidate. That one leftover mark can hide a cell that really has only one option.

Go through the empty cells and, for each one, make sure its notes contain only digits that aren’t already in its row, column or box. Then look for any cell with a single candidate left. That is a naked single, and it must be that digit.

If you have only been writing notes in a few cells, now is the moment to fill them in everywhere, at least in the areas you’re working on. Pairs and other patterns are invisible unless every cell in a unit has its candidates. Our guide to Sudoku pencil marks shows a tidy way to do it.

Step 3: Look for locked candidates

Medium puzzles are where singles first run dry, and the usual way forward is locked candidates (you will also see the names pointing pairs and box/line reduction). The idea: if, inside one box, a digit can only go in cells that share a single row or column, then that digit must go on that line within that box. So you can remove it from the rest of the line, outside the box.

It also works the other way round. If, in a row or column, a digit can only go inside one box, you can remove it from the other cells of that box. Locked candidates rarely place a number by themselves, but they clear one or two notes, and that is often enough to reveal a single. Which is why, after every elimination, you go back to step 1.

A quick way to find them: for each box, look at your notes for one digit at a time. If all the notes for that digit sit in one row or one column of the box, check whether the same digit appears in that line outside the box. If it does, you have an elimination.

Step 4: Hunt for pairs and triples

On Hard puzzles the next rung is subsets. The most common is the naked pair: two cells in the same row, column or box whose notes are exactly the same two digits. Those two digits must go in those two cells, in one order or the other, so no other cell in that unit can hold them.

Look at the result: row 2 of that column had {4,5,6} and is left with just 5, and row 9 had {4,9} and is left with 9. The pair places nothing itself, but two naked singles appear straight after.

When you are hunting for subsets, these three checks cover most cases:

1. Naked pairs: in each unit, look for two cells with the same two notes and nothing else.
2. Hidden pairs: in each unit, look for two digits that can only go in the same two cells.
3. Triples: three cells whose notes use only three digits between them (each cell doesn’t need all three).

All of these are explained with worked examples in our guide to naked and hidden pairs.

Step 5: Advanced patterns

If you’ve worked through steps 1 to 4 carefully, with complete notes, and you are still stuck, the puzzle probably needs an advanced technique. On Sudoku.my that only happens at Expert level, and the patterns involved are:

• X-Wing: a digit that fits in exactly two cells in each of two rows, and those cells line up in the same two columns (or the same with rows and columns swapped). See our X-Wing guide.
• Swordfish: the same idea spread over three rows and three columns.
• XY-Wing: three cells with two candidates each that, together, force a digit out of any cell that sees both ends. Our Y-Wing guide walks through it.

For fish patterns, pick one digit and look only at its notes across the whole board. With the other eight digits ignored, rows or columns where that digit has just two spots stand out much more clearly. The full list of techniques, from singles to Swordfish, is in our Sudoku strategies guide.

Could there be a mistake on the board?

Sometimes you are stuck because the position in front of you is wrong. Warning signs include:

• A cell with no candidates left at all.
• A digit with nowhere to go in a row, column or box that still needs it.
• Two cells in the same unit that are each forced to the same digit.
• You keep “finding” moves that contradict each other.

On paper, the honest fix is to go back to the last point you are sure about. If you can’t find it, starting again is often quicker than hunting for one wrong digit among forty. A well-made puzzle has exactly one solution, so if the maths seems to break, the error is almost always yours rather than the puzzle’s. (Puzzles from unknown sources can be badly made, but that is the rare case, not the first thing to suspect.)

On Sudoku.my things are simpler. A wrong digit is flagged the moment you enter it and costs one of your three hearts, so you can’t carry a hidden wrong number for twenty moves. What the game doesn’t check is your notes. If you erased a candidate by mistake, the board can look stuck when it isn’t. If you suspect that, re-check the notes in the area you’re working on against the digits in each row, column and box, or use Undo to step back. Our sibling guide to common Sudoku mistakes lists the slips that cause this most often.

Take a break (it really can help)

If you’ve been staring at the same grid for a while, your eyes start to follow the same paths and skip the same cells. Walking away for a few minutes breaks that habit. Psychologists call the effect of setting a problem aside and coming back to it incubation. A 2009 review of 117 studies by Ut Na Sio and Thomas Ormerod in Psychological Bulletin found a positive incubation effect overall. It also found that the benefit was larger after longer work on the problem beforehand, and smaller when the break was filled with demanding mental tasks.

Those studies used puzzles and creative tasks in the lab, not Sudoku, so treat this as a sensible reason to try a break rather than a guarantee. In practice:

• Step away after a real effort, not after thirty seconds.
• Do something light: make a cup of tea, stretch, look out of the window. Don’t swap one hard puzzle for another.
• When you come back, start at step 1 of the checklist, in a part of the board you haven’t looked at recently.

On Sudoku.my you can tap pause at any time: the board is blurred and the timer stops. The game also pauses on its own when you switch to another app or tab, so a break never costs you time.

How to use hints well

A hint is not cheating; it is a lesson, if you use it like one. The problem with most Sudoku hints is that they just fill in a number, which unsticks you for one move but teaches you nothing. On Sudoku.my, a hint shows you the next step a person could find by logic, and tells you why:

• It places a correct digit and highlights the row, column or box (or the pattern cells) that the reasoning uses.
• For a simple step it explains the single, for example that a box needs a digit and every other cell there already sees one.
• For harder positions it first names the key technique that unlocks the move (such as locked candidates or a naked pair) and then the move itself.
• It works from the correct digits on the board, so a wrong entry of yours won’t lead it astray.

You get 3 hints per puzzle. To get the most out of them:

1. Work through the checklist first. A hint is most useful after you have really looked.
2. Read the explanation before you carry on. Then look at the highlighted cells and see the pattern yourself.
3. Ask “which step of the ladder was it?” If it was a single you missed, your scanning needs work. If it was a pair, practise pairs. Either way, you now know where your blind spot is.
4. Use one at a time. After a hint, go back to step 1 yourself before asking for another.

Your results still count when you use hints, and the “On my own” badge waits for the first puzzle you finish without any. Over time you will find you need fewer, which is the whole point. You can track that on your stats page.

If Medium feels comfortable, try Hard to practise pairs and triples, or the daily challenge, which starts easy on Monday and gets tougher towards the weekend. And if you’re brand new to the game, our beginner’s guide to playing Sudoku is the best place to start.

#Stuck#Solving tips#Hints