Sudoku Solving Checklist: A Systematic Approach to Every Puzzle

The difference between a frustrated Sudoku solver and a confident one often isn’t knowledge of techniques — it’s systematic approach. Knowing how to find a naked pair is valuable, but knowing when to look for it, and what to do before and after, is what transforms isolated technique knowledge into consistent solving ability.

This checklist gives you a repeatable, step-by-step process for approaching any Sudoku puzzle, from the easiest to the most diabolical. Follow it in order, cycle back to the beginning after each breakthrough, and you’ll solve puzzles more efficiently and with fewer errors than ever before.

The Master Checklist

Here is the complete checklist. Each step includes what to do, what to look for, and when to move to the next step. The golden rule is: always return to Step 1 after any successful technique application.

Step 1: Scan for Naked Singles

What: Look for cells that have only one possible value based on the digits already present in their row, column, and box.

How: Examine cells in nearly complete rows, columns, and boxes. A row with 8 digits filled has a naked single in the empty cell. Boxes with 7-8 filled cells are prime candidates.

Technique page: Naked Single

When to move on: When you cannot find any more cells with a single possible value through quick visual scanning.

Step 2: Scan for Hidden Singles

What: For each digit 1-9, check each row, column, and box to find units where that digit can only go in one cell.

How: Pick a digit (start with the one that appears most frequently on the board). Scan each row: in how many cells can this digit be placed? If only one, it’s a hidden single. Repeat for columns and boxes. Then move to the next digit.

Technique page: Hidden Single

When to move on: When you’ve checked all 9 digits across all rows, columns, and boxes without finding any hidden singles.

Step 3: Fill in Pencil Marks

What: For all remaining empty cells, note every candidate (possible value) that doesn’t conflict with existing digits in the cell’s row, column, and box.

How: Go cell by cell, or unit by unit. For each empty cell, check which of 1-9 are already present in its row, column, and box. The remaining digits are the candidates.

Why now: Pencil marks make Steps 4-9 possible. Without them, spotting pairs, triples, and advanced patterns requires holding too much information in working memory.

Pro tip: If you’re solving on paper, write small and neatly. On an app, use the notes or candidates feature. Some solvers only pencil-mark areas where they’re stuck rather than the entire grid — this is fine for medium puzzles but full marks are recommended for hard and above.

When to move on: When all empty cells have their candidates noted. Proceed immediately to Step 4.

Step 4: Look for Naked Pairs and Triples

What: Search each row, column, and box for sets of 2 cells sharing exactly 2 candidates (naked pair) or 3 cells sharing exactly 3 candidates (naked triple). Eliminate those candidates from other cells in the same unit.

How: In each unit, identify cells with small candidate lists (2-3 candidates). Check if any pair of cells shares the same two candidates — that’s a naked pair. For triples, look for three cells whose combined candidate set has exactly three digits.

Technique pages: Naked Pair | Naked Triple | Naked Quad

After finding one: Make the eliminations, then return to Step 1. The eliminations may create new naked or hidden singles.

When to move on: When no naked subsets are visible in any unit.

Step 5: Look for Hidden Pairs and Triples

What: Search each unit for 2 digits that can only go in the same 2 cells (hidden pair) or 3 digits restricted to the same 3 cells (hidden triple). Eliminate other candidates from those cells.

How: For each unit, examine where each digit can go. If digits 3 and 8 can only go in cells A and B within a box, that’s a hidden pair — remove all other candidates from A and B.

Technique pages: Hidden Pair | Hidden Triple | Hidden Quad

After finding one: Make the eliminations, then return to Step 1.

When to move on: When no hidden subsets are found in any unit.

Step 6: Check Pointing Pairs/Triples and Box-Line Reduction

What: Look for candidates confined to a single line within a box (pointing) or confined to a single box within a line (box-line reduction).

How:

• Pointing: For each candidate in each box, check if all positions for that candidate fall in one row or column. If so, eliminate the candidate from that row/column outside the box.
• Box-line reduction: For each candidate in each row/column, check if all positions fall within one box. If so, eliminate the candidate from that box outside the row/column.

Technique pages: Pointing Pair | Pointing Triple | Box-Line Reduction

After finding one: Make the eliminations, then return to Step 1.

When to move on: When no pointing or box-line patterns exist.

Step 7: Try Fish Patterns (X-Wing, Swordfish, Jellyfish)

What: For each candidate, look for fish patterns — rows (or columns) where the candidate appears in limited positions that align across columns (or rows).

How:

• X-Wing: Find a candidate that appears in exactly 2 cells in two different rows, with those cells in the same two columns. Eliminate the candidate from other cells in those columns.
• Swordfish: Extend to 3 rows and 3 columns.
• Jellyfish: Extend to 4 rows and 4 columns.
• Also check for finned variants.

Technique pages: X-Wing | Swordfish | Jellyfish | Finned X-Wing

After finding one: Make the eliminations, then return to Step 1.

When to move on: When no fish patterns are found for any candidate.

Step 8: Try Wing Patterns

What: Look for XY-Wing, XYZ-Wing, and W-Wing patterns among bivalue cells.

How:

• XY-Wing: Find a pivot cell with 2 candidates that sees two wing cells, each sharing one candidate with the pivot and sharing one common candidate with each other. Eliminate the common candidate from cells that see both wings.
• XYZ-Wing: Similar to XY-Wing but the pivot has 3 candidates.
• W-Wing: Find two cells with the same two candidates connected by a strong link.

Technique pages: XY-Wing | XYZ-Wing | W-Wing

After finding one: Make the eliminations, then return to Step 1.

When to move on: When no wing patterns are found.

Step 9: Try Single-Digit Patterns, Coloring, and Chains

What: Apply the most advanced techniques in your toolkit.

How:

• Skyscraper/Two-String Kite/Empty Rectangle: Focus on one candidate at a time. Map its positions and look for chain-based eliminations. (Skyscraper | Two-String Kite | Empty Rectangle)
• Unique Rectangle: Look for four cells forming a rectangle in two rows and two columns with specific candidate patterns. (Unique Rectangle | BUG)
• Coloring: For a candidate, trace strong links and assign alternating colors. Look for contradictions or traps. (Coloring)
• Chains: Trace inference chains through strong and weak links across multiple candidates and cells. (Chains | ALS)

After finding one: Make the eliminations, then return to Step 1.

When to move on: If you’ve exhausted all known techniques and the puzzle remains unsolved, review for errors (see the Error Detection section below).

The Checklist Flowchart

Here’s the same checklist represented as a decision flow:

The critical principle is the return to Step 1 after every success. Even a single elimination from an advanced technique can create a cascade of simple singles. Always harvest the easy fruit before looking for harder patterns.

Step-to-Technique Reference Table

This table maps each checklist step to the specific technique pages you should study to master that step.

When to Restart the Cycle

The checklist is a cycle, not a linear path. You will restart from Step 1 many times during a single puzzle. Here’s when:

After every digit placement: Any time you fill in a cell (whether from a naked single, hidden single, or technique-driven deduction), immediately scan the affected row, column, and box for newly created naked singles.

After every candidate elimination: Removing a candidate from a cell might create a naked single in that cell, a hidden single in the unit, or unlock a pair/triple that was previously invisible.

After completing a full cycle without progress: If you go through Steps 1-9 without finding anything, it’s time to suspect an error rather than give up. See the Error Detection section.

The overhead of restarting is minimal — checking for naked and hidden singles is fast, especially when you know which areas were just affected. The payoff is enormous, as advanced eliminations frequently unlock cascades of simple placements.

Handling Dead Ends

When you’ve cycled through the entire checklist and found nothing, don’t panic. You have several options.

Re-Verify Pencil Marks

The most common cause of a dead end is an error in your pencil marks. A missing candidate or a candidate that should have been eliminated can block entire chains of deduction. Go through the grid systematically:

1. For each filled cell, verify it doesn’t conflict with any other cell in its row, column, or box.
2. For each empty cell, re-derive the candidate list from scratch. Compare it with your existing marks.
3. Pay special attention to cells you filled recently — errors tend to be fresh.

Expand Your Technique Search

If your marks are correct, you may need a technique outside your current comfort zone. Common situations:

• Stuck after Steps 1-6? You probably need a fish pattern (Step 7). Study X-Wing specifically.
• Stuck after Step 7? Wing patterns (Step 8) are likely needed. Study XY-Wing.
• Stuck after Step 8? Advanced techniques like Coloring or Unique Rectangle are the next frontier.

For a comprehensive guide to which techniques to learn and in what order, see our technique progression guide.

Accept the Puzzle’s Level

If you’ve exhausted every technique you know and verified your marks are clean, the puzzle may simply require a technique you haven’t learned yet. This is not failure — it’s a signpost showing you what to study next. Check the puzzle’s difficulty rating and compare it to your technique range.

Signs You’ve Made an Error

Errors are inevitable, especially on hard puzzles. Recognizing them early prevents cascading mistakes.

A Cell With Zero Candidates

If any empty cell has no valid candidates after pencil-marking, you’ve made an error somewhere. A valid Sudoku always has at least one candidate for every empty cell.

A Duplicate in a Unit

If you notice (or your app highlights) two instances of the same digit in a row, column, or box, an error occurred. Trace backward to find the incorrect placement.

A Digit With No Valid Cell in a Unit

If a digit (say 7) has no possible cell in a particular row, column, or box, something is wrong. Either you’ve misplaced a 7 elsewhere or you’ve incorrectly eliminated 7 from valid cells.

The Puzzle “Breaks”

If you reach a state where two cells in the same unit must both contain the same digit, the puzzle has “broken” due to an earlier error. This is different from a cell with zero candidates — it means your placements have created an impossible configuration.

No Progress on a Known-Solvable Puzzle

If the puzzle comes from a reputable source and is labeled as solvable with techniques at your level, but you’re completely stuck after a thorough checklist cycle, an error is the most likely explanation. Recheck recent placements before exploring more advanced techniques.

When Advanced Techniques Are Needed

Not every puzzle requires advanced techniques, and knowing when to look for them (versus when to look harder for simple patterns) saves significant time.

Easy puzzles (Easy): Steps 1-2 only. If you feel stuck on an easy puzzle, you’re missing a single — re-scan more carefully.

Medium puzzles (Medium): Steps 1-6. Pairs, pointing, and box-line cover the vast majority of medium puzzles. If stuck, re-verify your pencil marks before looking for triples.

Hard puzzles (Hard): Steps 1-8. Fish and wing patterns become necessary. The first time through Steps 7-8 may take a while as you build pattern recognition.

Expert puzzles (Expert): Steps 1-9. Expect to use single-digit patterns, unique rectangles, and occasionally coloring.

Evil puzzles (Evil): The full checklist including chains and ALS. These puzzles test the complete range of techniques and require patience and precision.

Adapting the Checklist to Your Level

You don’t need to know all 9 steps to use this checklist. Simply use the steps you’ve learned and stop where your technique knowledge ends.

If you only know Steps 1-3 (singles and pencil marks), use those steps and accept that some puzzles will be beyond your current ability. As you learn new techniques from our technique progression guide, add them to your personal checklist one at a time.

The checklist framework remains the same at every level — the only difference is how many steps you can execute. A beginner using Steps 1-2 and an expert using Steps 1-9 are following the same systematic process. The beginner just has a shorter toolkit.

Speed Optimization

Once you’re comfortable with the checklist, here are ways to execute it faster.

Skip Steps You Don’t Need

On an easy puzzle, don’t bother with pencil marks. On a medium puzzle, you may only need selective marking in problem areas. Let the puzzle’s difficulty guide how much machinery you deploy.

Batch Your Scans

Instead of scanning for naked singles one cell at a time, scan entire rows in sequence (row 1 left to right, row 2 left to right, etc.). This systematic approach ensures you don’t miss cells and builds a rhythm.

Focus on Recent Changes

After placing a digit, you don’t need to re-scan the entire grid. Focus on the row, column, and box that were affected. This targeted re-scan is much faster than a global scan.

Keep Track of Where You Are

Mentally note which rows, columns, and boxes you’ve checked for each technique. This prevents wasted re-scanning and helps you work through the grid systematically.

Putting It All Together

The checklist is a tool, not a straitjacket. As you gain experience, the steps will become automatic. You won’t consciously think “I’m on Step 4, looking for naked pairs” — you’ll naturally scan for the right patterns at the right time.

The key principles to internalize are:

1. Simple first: Always exhaust simple techniques before reaching for complex ones.
2. Return to basics: After every success, cycle back to singles.
3. Update as you go: Keep pencil marks accurate by eliminating candidates immediately after each placement.
4. Trust the process: If the puzzle is well-constructed, the checklist will find the path. Dead ends mean errors, not impossible puzzles.

This systematic approach works for every Sudoku puzzle, from a casual easy solve during your morning coffee to the most grueling evil challenge. The only variable is which steps you need — and that’s determined by the puzzle, not by luck.

Frequently Asked Questions

What is the correct order to apply Sudoku techniques?

The proven order is: (1) naked singles, (2) hidden singles, (3) pencil marks, (4) naked pairs/triples/quads, (5) hidden pairs/triples/quads, (6) pointing pairs and box-line reduction, (7) fish patterns like X-Wing and Swordfish, (8) wing patterns like XY-Wing, and (9) advanced techniques like Coloring and Chains. Always return to Step 1 after each successful elimination.

When should I add pencil marks?

Add pencil marks when you can no longer find any naked or hidden singles through visual scanning alone. For easy puzzles, you may never need them. For medium puzzles, selective pencil-marking in stuck areas is often sufficient. For hard and above, full pencil marks from the beginning save time in the long run and make advanced patterns visible.

How do I know if I’ve made an error?

Key signs include: a cell with no remaining candidates, a duplicate digit in a row/column/box, a digit with no valid position in a unit, or complete inability to make progress on a puzzle that should be solvable at your technique level. When you detect an error, check your most recent placements first — errors are usually fresh.

Should I guess if I’m stuck?

No. Well-constructed Sudoku puzzles from reputable sources always have a complete logical solving path. If you’re stuck, either you’re missing a technique application, you’ve made an error in your pencil marks, or the puzzle requires techniques beyond your current skill level. Re-verify your marks, restart the checklist carefully, and consult our technique progression guide if you think you need to learn a new method.

How often should I restart the checklist cycle?

After every successful technique application — whether it places a digit or eliminates candidates. Even a single elimination from an advanced technique like an X-Wing might create new naked singles or hidden singles. Always harvest the simple eliminations before continuing with harder techniques. On a hard puzzle, you might cycle through Steps 1-2 dozens of times during a solve, and that’s exactly how it should work.