It started as “Number Place”
Modern Sudoku first appeared in a U.S. puzzle magazine in 1979 as Number Place; a Japanese publisher rebranded it “Sūji wa dokushin ni kagiru” (“digits must be single”)—shortened to Sudoku.
Origin story
Sudoku — Calm, Mobile-First, Private
Board & Controls
Ready. Choose a difficulty and press New Game.
⏱️ 00:00
❌ Mistakes: 0
How to Play
Fill the 9×9 grid so each row, column, and 3×3 box contains the digits 1–9 exactly once.
- Tap a cell, then tap a number (1–9) to place it. Use Clear to erase.
- Toggle Notes to add pencil marks (candidates) to a cell.
- Hint reveals a valid digit if a single-candidate cell exists; otherwise it adds one safe digit.
- Use Undo to revert your last change.
Tips
- Scan for singles (only one candidate in a cell).
- Check “hidden singles” (a number that appears once in a row/col/box).
- Use notes to track candidates and avoid guesswork.
Learn more on Wikipedia: Sudoku.
How to Solve Sudoku (Step-by-Step)
The goal of Sudoku is to fill the 9×9 grid so every row, column, and 3×3 box contains the digits 1–9 exactly once. The fastest way to learn is to follow a reliable sequence of techniques—from simple to advanced—while using notes (pencil marks) to track candidates.
Setup
- Turn on Notes to add small candidates to empty cells.
- Use Auto Notes to prefill obvious candidates (optional), then keep them updated as you solve.
- Enable Highlight errors to avoid accidental duplicates.
Step 1 — Naked Singles
If a cell has only one possible digit (all others ruled out by its row, column, and box), place it.
Step 2 — Hidden Singles
Scan each row, column, and box. If a digit can appear in only one cell within that unit, it must go there.
Step 3 — Pairs & Triples
- Naked Pair/Triple: If two/three cells in a unit contain exactly the same candidates, those digits are locked to those cells; remove them elsewhere in the unit.
- Hidden Pair/Triple: If two/three digits in a unit can only go in the same cells, keep just those and remove the rest from those cells.
Step 4 — Box–Line Interactions
- Pointing: If a digit’s candidates in a box lie in a single row/column, eliminate that digit from that row/column outside the box.
- Claiming: If a digit’s candidates in a row/column all fall in one box, eliminate the digit from other cells in that box.
Step 5 — X-Wing (for tougher puzzles)
If a digit appears as candidates in exactly two cells in two different rows, sharing the same two columns, you can eliminate that digit from other cells in those columns (mirror for columns-based X-Wing).
When You’re Stuck
- Re-scan for singles after every elimination wave.
- Refresh notes—stale candidates cause dead ends.
- Use Hint to reveal a safe next step.
Keyboard & Mobile Tips
- Desktop: Arrow keys move selection; 1–9 enters digits; 0/Backspace clears.
- Mobile: Tap a cell, then use the on-screen keypad. Toggle Notes for pencil marks.
- Undo reverses your last action; use it to safely explore a line of reasoning.
5 Fun Facts about Sudoku
There are 6,670,903,752,021,072,936,960 solutions
That’s the count of valid 9×9 completed grids. Up to rotations/reflections and digit swaps, there are still over 5.4 billion essentially unique ones.
Huge search space
17 clues are the minimum
No one has ever found a valid Sudoku with 16 givens and a unique solution. In 2012, researchers proved 17 clues is the absolute floor.
Puzzle tightness
Programmable as an exact cover
Sudoku can be encoded as an exact cover problem; Knuth’s Dancing Links (DLX) algorithm can solve or generate puzzles lightning fast.
Algo nerd
Hardest puzzles need exotic logic
Puzzles like “AI Escargot” force advanced techniques (X-Wing, Swordfish, chains). Some “extreme” grids even require backtracking despite being human-crafted.
Brain-benders