How to play

How to Play Sudoku

Sudoku is a logic-based number placement puzzle. The objective is to fill a 9×9 grid with digits so that each column, each row, and each of the nine 3×3 sub-boxes contains all of the digits from 1 to 9.

Rows

Each row must contain the numbers 1-9 with no repetition.

Columns

Each column must contain the numbers 1-9 with no repetition.

3x3 boxes

Each 3×3 box must contain the numbers 1-9 with no repetition.

Jump by level

Go straight to the section that matches your current solving level.

Example board

See the rule zones

5
3
7
6
1
9
5
9
8
6
8
6
3
4
8
3
1
7
2
6
6
2
8
4
1
9
5
8
7
9
Beginner tips

Start with a good rhythm

Start with rows, columns, or boxes that already have many numbers filled in.

Use pencil marks (notes) to track possible values for each cell.

Look for "naked singles" where only one number can fit.

Look for "hidden singles" where one number only fits one cell in a row, column, or box.

Do not guess. Every Sudoku puzzle can be solved with logic alone.

Solve loop

Solve loop

A repeatable loop keeps progress logical and prevents random guessing.

  1. Scan the most crowded row, column, or 3x3 box.

  2. Update candidates and collect singles.

  3. Apply one candidate-reduction pattern.

  4. Rescan from the top and repeat the loop.

Stuck protocol

Stuck protocol

When progress stops, run this sequence before considering guesses.

  • Do a full candidate cleanup in one complete unit.
  • Recheck hidden singles by candidate frequency.
  • Apply one targeted intermediate elimination.
  • Try one advanced single-digit pattern pass.
  • Use controlled bifurcation only as a tracked last resort.

Chilldoku note: in-game hints focus on core logic. Use this page to study advanced human techniques.

Technique ladder

Technique ladder

Move from simple logic to deep patterns based on puzzle difficulty.

Beginner

Foundation pass

Build consistency with placements that are forced by one unit at a time.

Easy to medium

Grid scanning

  • Logic: Sweep rows, columns, and boxes to list missing digits and remove impossible spots.
  • Use when: Start of every puzzle and after every solved cell.
  • Watch out: Do not jump randomly between areas before finishing a full scan cycle.

Naked single

  • Logic: One cell has only one candidate left, so it must be that value.
  • Use when: Any time candidate cleanup leaves a cell with one option.
  • Watch out: Recheck candidate notes immediately after placing the digit.

Hidden single

  • Logic: A candidate appears only once in a unit, even if the cell has multiple notes.
  • Use when: After scanning candidate distributions inside one row, column, or box.
  • Watch out: Players often miss this by checking cells instead of candidate frequency.
Intermediate

Candidate reduction

Remove candidates in groups so easier singles become visible again.

Medium to hard

Naked / Hidden pairs

  • Logic: Two digits locked in two cells let you remove those digits elsewhere in the unit.
  • Use when: When units are dense with notes but no singles appear.
  • Watch out: Confirm the pair is exact; extra candidates invalidate the pattern.

Pointing pairs / triples

  • Logic: If candidates in one box align to one row or column, eliminate outside the box.
  • Use when: After box-by-box candidate scanning.
  • Watch out: The elimination happens in the same row or column outside that box only.

Box-line reduction

  • Logic: If row or column candidates for a digit are confined to one box, remove inside that box elsewhere.
  • Use when: When a row or column has a digit restricted to one 3x3 region.
  • Watch out: Do not mix this with pointing logic direction; they are mirror rules.
Advanced

Pattern logic

Use candidate geometry to force eliminations across distant units.

Hard to diabolical

X-Wing

  • Logic: A digit forms a rectangle across two rows and two columns, allowing cross-line eliminations.
  • Use when: When one digit repeatedly appears as two candidates in parallel lines.
  • Watch out: All four corners must be valid candidate cells for the same digit.

Swordfish

  • Logic: Generalizes X-Wing to three rows and three columns for broader eliminations.
  • Use when: When one digit forms a 3-line interaction with aligned candidate columns or rows.
  • Watch out: Track one digit only; mixing digits breaks the fish structure.

XY-Wing

  • Logic: A pivot and two wings create a shared elimination target for one candidate.
  • Use when: When multiple bi-value cells appear and simple eliminations stall.
  • Watch out: Visibility between pivot and wings is mandatory for valid eliminations.
Expert

Control and verification

Apply disciplined checks so hard-board logic stays reliable under pressure.

Diabolical and beyond

Chain reasoning (XY-Chain / AIC mindset)

  • Logic: Follow alternating links to prove one candidate must be true or false.
  • Use when: After pattern techniques and with stable candidate notation.
  • Watch out: If one link is weak or not actually conjugate, the full chain collapses.

Uniqueness checks

  • Logic: Use unique-solution assumptions to eliminate patterns that would create multiple outcomes.
  • Use when: When rectangles or repeated candidate structures appear on hard puzzles.
  • Watch out: Apply only on valid standard Sudoku assumptions with unique solutions.

Controlled bifurcation (last resort)

  • Logic: Test one candidate branch with strict note tracking and rollback discipline.
  • Use when: Only after exhausting logic and wanting a structured fallback path.
  • Watch out: Never branch casually; capture the fork and contradictions explicitly.
Beginner-friendly strategy

How to solve Sudoku without guessing

Most beginners improve fastest when they stop looking for random numbers and instead scan the grid in a repeatable order. The goal is to reduce options one logical step at a time.

1. Start where the puzzle is already crowded

Look for rows, columns, or 3x3 boxes with many given digits. The more numbers that are already present, the fewer candidates remain for the empty cells.

  • Scan each row for the missing digits first.
  • Check whether one of those missing digits only fits one spot.
  • Move to the next most-complete row, column, or box and repeat.

2. Use notes to narrow the board

Candidate notes help you see structure. They are most useful when a puzzle reaches the point where no obvious placement is visible at first glance.

  • Add notes only where you are genuinely unsure.
  • Remove candidates as soon as a row, column, or box rules them out.
  • A short list of candidates often reveals the next logical move.

3. Look for singles before advanced patterns

Many boards open up after one or two simple discoveries. Hidden singles and naked singles are enough to solve a surprising number of positions.

  • A naked single means one cell has only one valid number left.
  • A hidden single means a number can only go in one place within a row, column, or box.
  • You rarely need to guess if you keep rescanning after each solved cell.

Common beginner mistakes

The fastest way to get stuck in Sudoku is usually not a lack of skill. It is breaking a good routine. These are the habits that most often slow new players down.

  • Jumping around the board without finishing a scan of the current row, column, or box.
  • Keeping outdated notes after the board changes.
  • Guessing too early instead of rechecking the surrounding units.
  • Ignoring easier sections because harder-looking areas feel more interesting.

Ready to practice on a real board?

Once the rules feel clear, jump into the daily puzzle for a shared challenge or start a casual board to practice at your own pace.

Play the daily challenge
Sudoku FAQ

Sudoku FAQ

These are the questions players ask most often before they settle into a comfortable solving rhythm.

What are the basic rules of Sudoku?

Each row, each column, and each 3x3 box must contain the digits 1 through 9 exactly once, with no repeats.

Do I ever need to guess in Sudoku?

A properly designed Sudoku puzzle can be solved with logic. If you feel forced to guess, there is usually an overlooked clue elsewhere on the board.

What should I do first on a new Sudoku board?

Start with the most-filled rows, columns, or boxes, then list the missing digits and test which positions remain possible.

When should I use notes?

Use notes once the easiest placements are gone and you need a lightweight way to track remaining candidates in uncertain cells.