What to Do When You're Stuck on a Sudoku Puzzle

A reliable rescue sequence for when the grid stops moving — from re-scanning basics to smart backtracking.

Intro

It happens to everyone. You're halfway through a Sudoku puzzle, the grid is filling up nicely — and then you hit a wall. No obvious moves. No cells screaming for attention. Just a quiet, frustrating standstill.

Take a breath. Being stuck is a normal part of Sudoku, not a sign that you're doing it wrong. Every solver — from casual daily players to competitive speedrunners — reaches moments where the path forward isn't obvious. The difference between giving up and breaking through is knowing what to try next.

This guide walks you through a reliable rescue sequence. Follow these steps in order the next time you're stuck, and you'll almost always find a way forward.

Step 1: Pause and Reset Your Eyes

When you've been staring at the same grid for a while, your brain starts skipping over possibilities. You see what you expect to see rather than what's actually there.

What to do:

• Look away from the screen or paper for 10–15 seconds.
• When you come back, start from a completely different part of the grid than where you were focused.
• If you've been scanning left to right, try bottom to top. A small shift in perspective is often all it takes.

This isn't a technique — it's a reset. Your brain needs a fresh look at the same information.

Step 2: Re-Scan Every Row, Column, and Box

The most common reason solvers get stuck? They missed something simple earlier. A naked single hiding in plain sight, or a number that can only go in one place in a box they forgot to check.

What to do:

• Go through each of the 9 rows, one by one. For each row, check which numbers are missing and whether any of them can only go in one cell.
• Repeat for all 9 columns.
• Repeat for all 9 boxes.

Yes, this takes a couple of minutes. But it's the single most productive thing you can do when stuck. Experienced solvers call this a "full scan" — a deliberate, systematic sweep of the entire grid.

What you're looking for:

• Naked singles: cells where only one number is possible.
• Hidden singles: numbers that can only go in one cell within a row, column, or box.

These two techniques solve the vast majority of easy and medium puzzles. If you skipped a cell, a full re-scan will catch it.

Step 3: Update Your Pencil Marks

If you're not using pencil marks (also called "notes" or "candidates"), now is the time to start. If you are using them, they might be out of date.

What to do:

• For every empty cell, write down every number that could still go there based on the current state of its row, column, and box.
• If you already have pencil marks, go through each one and remove any that are no longer valid.

Cleaning up your pencil marks often reveals moves you couldn't see before. A cell that looked like it had three options might actually have one — you just hadn't eliminated the others yet.

Step 4: Look for Pairs and Triples

Once your pencil marks are clean, patterns start to emerge. The most common ones to look for when you're stuck:

Naked Pairs

Two cells in the same row, column, or box that share the exact same two candidates (for example, both contain only 3 and 7). When you find a naked pair, you can eliminate those two numbers from every other cell in that shared unit.

→ Learn the naked pairs technique

Hidden Pairs

Two numbers that only appear as candidates in the same two cells within a row, column, or box. Even if those cells have other candidates too, you can remove everything except the pair.

→ Learn the hidden pairs technique

Naked and Hidden Triples

The same idea, but with three cells and three numbers. These are harder to spot but follow the same logic.

→ Naked triples | Hidden triples

Pairs and triples don't always give you a direct placement — but they eliminate candidates, which opens the door for the next move.

Step 5: Try a Different Strategy

If scanning and pairs haven't broken the logjam, the puzzle is probably asking for a more advanced technique. Here are the most useful ones to try, roughly in order of difficulty:

Pointing Pairs

When a candidate in a box is confined to a single row or column, it can be eliminated from the rest of that row or column outside the box.

→ Pointing pairs explained

Box-Line Reduction

The reverse of pointing pairs: when a candidate in a row or column is confined to a single box, eliminate it from the rest of that box.

→ Box-line reduction

X-Wing

A pattern involving two rows and two columns where a candidate appears in exactly two cells per row (or column), forming a rectangle. Allows you to eliminate that candidate from other cells in the crossing columns (or rows).

→ X-Wing strategy

XY-Wing

Three cells forming an "L" shape where their candidates chain together to eliminate a shared candidate from cells that see all three.

→ XY-Wing technique

You don't need to master all of these at once. Try one at a time. If pointing pairs don't reveal anything, move to box-line reduction, and so on.

Step 6: Focus on the Most Constrained Cells

Instead of scanning the whole grid, zoom in on the cells that have the fewest candidates. A cell with two candidates is far more useful than one with five — it's closer to being solved, and any technique you apply is more likely to resolve it.

What to do:

• Find all cells with exactly 2 candidates.
• Check if any of them are part of a pair, triple, or pattern you recognise.
• Check if placing either candidate leads to an immediate contradiction (this is called a "what-if" test — see Step 7).

Step 7: When It's Okay to Use Trial and Error

Let's be honest: sometimes logic alone isn't enough — or at least, the technique you'd need is so advanced that trying a candidate and seeing where it leads is more practical.

This isn't "guessing." It's structured backtracking:

1. Pick a cell with only 2 candidates.
2. Choose one candidate and pencil it in as a trial.
3. Follow the chain of consequences. Does it lead to a valid grid, or does it create a contradiction?
4. If you hit a contradiction, you've proven that candidate is wrong. Erase your trial placements and fill in the other candidate with confidence.

Important guardrails:

• Only do this when you've exhausted the techniques above.
• Only pick cells with 2 candidates — not 3 or more.
• Mark your trial placements clearly so you can undo them cleanly.

→ More on trial and error (bifurcation)

Step 8: Take a Real Break

If you've worked through all of the steps above and you're still stuck, walk away. Close the app. Make tea. Come back in ten minutes — or an hour, or tomorrow.

This isn't quitting. Your brain continues processing problems in the background, and it's remarkably common to see the solution immediately when you return with fresh eyes. Puzzle solvers call this the "incubation effect," and it's real.

The Sudoku a Day app saves your progress automatically, so you can always pick up exactly where you left off.

Quick Reference: The Rescue Sequence

You're Not Bad at Sudoku — You're Just Learning

Getting stuck is how you get better. Every time you work through a stuck moment, you're training your brain to see patterns faster next time. The strategies that feel difficult today will become second nature with practice.

Frequently Asked Questions

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