Puzzle Games 🇺🇸United States20th century

Sudoku

Number-placement logic puzzle in which a 9×9 grid partly filled with digits 1-9 must be completed so every row, column, and 3×3 box contains all digits 1-9 exactly once.

📐9×9 grid on paper or screen⏱~15 min🌤indoor

What is Sudoku?

Sudoku (数独, 'single number') is a number-placement logic puzzle. The 9×9 grid is subdivided into nine 3×3 sub-grids (called 'boxes' or 'regions'). Some cells are pre-filled with 'given' or 'clue' numbers; the solver must fill remaining cells so that every row, every column, and every 3×3 box contains all the digits 1-9 exactly once. There is exactly one unique solution to a well-formed puzzle. Sudoku's modern form was invented by American Howard Garns in 1979 under the name 'Number Place' in Dell Magazines. Japanese puzzle publisher Nikoli renamed it 'Sūji wa dokushin ni kagiru' (数字は独身に限る, 'the digits must be single') in 1984, later shortened to Sudoku. The puzzle became a global phenomenon in 2004-2005 when The Times (London) started publishing it. Today Sudoku appears in hundreds of newspapers worldwide with 30+ million dedicated solvers. Difficulty ranges from Easy (30+ clues) to Diabolical (17-24 clues + complex logic). World Sudoku Championship has been held annually since 2006; Thomas Snyder (USA) is a 3× champion.

Quick facts

How it works

Objective

Fill the 9×9 grid so every row, column, and 3×3 box contains all digits 1-9 exactly once — no digit repeated in any row/column/box.

Match structure

Solver examines given clues + applies logical techniques to deduce each cell's value. Unique solution reached without guessing (well-formed puzzles). Competition: race to solve N puzzles in fastest time.

Rules

Basic rules

Fill grid with digits 1-9
Every empty cell must contain exactly one digit from 1 to 9.
No repeat in row
Each of 9 rows must contain digits 1-9 exactly once (no repeats).
No repeat in column
Each of 9 columns must contain digits 1-9 exactly once.
No repeat in 3×3 box
Each of 9 boxes must contain digits 1-9 exactly once.
Given clues fixed
Cannot change or erase given numbers — they're part of the puzzle.
Unique solution
Well-formed puzzle has exactly one valid solution reachable by pure logic.

Advanced rules

Notes / pencil marks
Solvers write small candidate numbers in cell corners to track possibilities.
Naked/Hidden singles
Basic techniques — cell with only one possibility (naked); cell where digit can go only one place in unit (hidden).
Naked/Hidden pairs, triples
Two/three cells sharing exact same two/three candidates — eliminate from other cells in that unit.
Pointing / Claiming pairs
When candidates for a digit in a box are all in one row/column, eliminate from rest of row/column.
X-Wing, Swordfish, Jellyfish
Advanced techniques using patterns across multiple rows/columns.
XY-Wing, XYZ-Wing, W-Wing
Chain-based techniques for harder puzzles.
Coloring, Chains, Nets
Very hard puzzle techniques involving colouring cells with hypothetical values.
Uniqueness patterns
Advanced: assuming unique solution, some patterns are impossible.

Scoring

Solve time in competition; correctness required (must be valid solution).

Action	Points
Valid solution	Points for completing
Time bonus (competitions)	Faster = more points
Invalid solution	0 or negative (WSC rules)
Puzzle set	Sum of individual puzzle scores

Win condition · Fastest total time to solve set of puzzles (World Championship format).

Playing area

Called

9×9 grid on paper or screen

Dimensions

9×9 = 81 cells, subdivided into 9× (3×3) boxes.

Standard 9×9 grid with bold lines separating the 9 sub-boxes.

Equipment

Grid + given clues
Printed or digital 9×9 puzzle with initial numbers.
Pencil (paper solvers)
For notes and corrections.
Pen (for confident solvers)
Higher risk but faster.
Timer
For competitive solving.
Digital app / online puzzle sites
New York Times, Web Sudoku, Sudoku.com, etc.

Officiating

World Puzzle Federation (WPF) rules; solutions verified for correctness.

Chief Judge (WSC)
Timekeepers

Strategies

Scan for naked singles first
Look for cells with only one possible value.
Hidden singles by unit
In each row/column/box, find where each digit can go — if only one place, fill it.
Use pencil marks efficiently
Mark 2-3 candidates per cell; eliminate as you deduce.
Attack most-filled areas first
Rows/columns/boxes with most givens have fewest possibilities — solve first.
Chain techniques for hard puzzles
For evil/diabolical: use X-Wing, XY-Wing, coloring.
Verify solution
Re-check each row, column, box has all 1-9 exactly once.

Terminology

Cell: One of 81 squares in 9×9 grid.
Row: 9 horizontal cells.
Column: 9 vertical cells.
Box / Region: One of 9 3×3 sub-grids.
Unit: Row, column, or box.
Given / Clue: Pre-filled starting digit.
Candidate: Possible digit for a cell (pencil marks).
Naked single: Cell with only one possible candidate.
Hidden single: Digit that can appear in only one cell of a unit.
Naked pair: Two cells in unit sharing exactly two candidates.
Hidden pair: Two digits that appear as candidates in only two cells of a unit.
Pointing pair: In a box, candidates for a digit all in one row/column — eliminate from rest of row/column.
X-Wing: Digit forming rectangle of 4 cells across 2 rows + 2 columns; eliminates elsewhere.
Killer sudoku: Variant with 'cages' — groups of cells summing to given totals.
Samurai sudoku: 5 overlapping 9×9 grids (2 rows × 2 columns + 1 in middle).

Major competitions

World Sudoku Championship (WSC)
annual · worldwide · since 2006
US Sudoku Qualifier
annual · USA · since 2006
European Puzzle Championship (includes sudoku)
annual · Europe · since 2011

Where it is played

Governing bodies

World Puzzle FederationWPF
Founded 1999 · USA
Official site ↗
USA Puzzle Federation (USA Sudoku)
Official site ↗
Japan Puzzle Federation
Official site ↗

Famous names & records

Famous athletes

Thomas Snyder (USA, 'Dr. Sudoku') — 3× World Sudoku Champion; PhD in biophysics
Kota Morinishi (Japan) — 3× World Sudoku Champion
Jan Mrozowski (Poland) — 3× World Sudoku Champion
Wei-Hwa Huang (USA) — 4× World Puzzle Championship winner; Sudoku medallist
Peter Grabowski (Germany) — Multiple World medallist
Ulrich Voigt (Germany) — 11× World Puzzle Champion; sudoku bronze
Tantan Dai (China) — Women's WSC winner
Howard Garns (USA, 1905-1989) — Inventor of modern Sudoku ('Number Place' 1979)

Records

Fewest possible clues for unique solution
Mathematically proven — 17 clues (McGuire et al., 2012) (2012)
Total possible Sudoku puzzles
Mathematical count — 6.67×10^21 (2005)
Fastest recorded solve (competition)
Various WSC winners — sub-2 min for medium puzzles (2023)
Most WSC titles
Thomas Snyder / Kota Morinishi / Jan Mrozowski — 3 each (2018)

Learn this sport

beginner

1
Newspaper easy sudoku
Start with easy puzzles (30+ givens); learn basic scanning.
2
Master naked/hidden singles
Foundation techniques for medium puzzles.
3
Sudoku.com or Sudoku app
Digital versions with hint systems.

intermediate

1
Hard-level puzzles
Require naked/hidden pairs + triples techniques.
2
Learn advanced techniques
X-Wing, XY-Wing for evil/diabolical puzzles.
3
Local Sudoku tournaments
Puzzle events at libraries, colleges, meetups.

advanced

1
USA Sudoku Qualifier
Annual qualifier for US team to WSC.
2
World Sudoku Championship
Annual event; top solvers from 30+ countries.
3
Variant sudoku (killer, X, etc.)
Advanced puzzles used in competitions.

Frequently asked

How do you play sudoku?+

Fill a 9×9 grid so every row, every column, and every 3×3 box contains all digits 1-9 exactly once. Start with pre-filled 'given' clues (25-45 typically). Well-formed puzzles have exactly one valid solution reachable by logic — no guessing needed.

Who invented sudoku?+

American Howard Garns invented modern Sudoku in 1979 for Dell Magazines under the name 'Number Place'. Japanese publisher Nikoli renamed it Sudoku in 1984. Global explosion happened 2004-2005 when The Times (London) started daily publication.

How many sudoku puzzles are there?+

6.67 × 10^21 possible 9×9 sudoku grids exist (essentially all unique valid solutions). Countless puzzle variations can be created from these grids by removing different clue sets.

What's the fewest clues for a valid sudoku?+

17 clues — mathematically proven minimum for a unique-solution puzzle (McGuire, Tugemann, Civario, 2012, using exhaustive computer search). 16-clue puzzles always have multiple solutions.

How do you solve hard sudoku?+

Basic techniques (scanning, naked/hidden singles) solve easy-medium. Hard puzzles require: naked/hidden pairs and triples, pointing pairs, X-Wing, XY-Wing, coloring, chains. Diabolical requires long-chain deduction.

Is sudoku a sport?+

It's classified as a mind-sport by the WPF. World Sudoku Championship has been held annually since 2006; top solvers compete for cash and rankings. Not Olympic.

Sources & provenance

Last verified 2026-08-24.community