🧩 Sudoku
Fill in numbers 1-9 so each row, column, and 3x3 box contains 1-9 without repetition!
🎉 Congratulations!
You solved the Sudoku puzzle!
Invented in Indiana, Perfected in Japan, Named in Latin-ish
Sudoku's bones are Latin squares, studied by Euler in the 18th century. The modern puzzle — Latin square plus the 3×3 box constraint — first appeared as "Number Place" in New York's Dell Pencil Puzzles in 1979, invented by a retired architect named Howard Garns. Japan's puzzle publisher Nikoli imported it in 1984, named it sūdoku ("the digits must be single"), and crucially made puzzles by hand with symmetric clue patterns. A New Zealand judge, Wayne Gould, spotted it in Tokyo in 1997, spent six years writing a generator, and talked The Times of London into printing one in 2004 — igniting the global craze that hasn't really ended.
The math beneath is delightful: there are about 6.67×10²¹ valid completed grids, and a 2012 computer proof established that no puzzle with only 16 clues can have a unique solution — 17 is the floor, and thousands of valid 17-clue puzzles are catalogued. Every puzzle this page generates has exactly one solution, so pure logic always suffices.
The Technique Ladder
Solvers grade techniques in tiers; climb only when the tier below stops producing moves:
- Naked single — a cell whose row, column and box eliminate everything but one candidate. Fill it.
- Hidden single — a digit with only one home left in a row, column or box. The workhorse: most medium puzzles fall to hidden singles alone. Scan digit by digit (all the 1s, then all the 2s…) rather than cell by cell.
- Pointing pairs / box-line reduction — when a digit's candidates within a box all sit on one line, erase that digit from the rest of the line. Pure candidate surgery; often triggers a cascade.
- Naked pairs — two cells in a unit sharing the same two candidates lock those digits out of the unit's other cells.
- X-Wing, Swordfish, XY-Wing — cross-unit elimination patterns for hard grids. All are generalizations of the same idea: candidates constrained to a rectangle or chain exclude those digits elsewhere.
The hint button here is a tutor, not an answer key: it picks a provable cell and shows the reasoning (capped at five per game). If you keep getting the same hint type, that's the technique your scanning routine is missing.
Interface Features Built for Deduction
- Pencil notes (press P): mark candidates in-cell; notes are auto-erased when a related cell gets filled — the bookkeeping that makes paper Sudoku tedious, done for you.
- Smart highlighting: selecting a cell lights its row, column, box, and every matching digit — tap a digit first to see its whole-board distribution when hunting hidden singles.
- Conflict shake: duplicates flash red immediately, so errors surface at the move, not at the broken endgame.
- Full keyboard play: arrows to move, 1–9 to enter, 0/Backspace to clear, P for notes.
- Auto-save and dark mode: close the tab and resume later, timer intact; the moon icon switches themes for late-night solving.
Frequently Asked Questions
Do I need to be good at math?
Not at all — the digits are just nine distinct symbols; the puzzle would work with fruit. Sudoku tests constraint elimination, which is also why it crosses languages and ages so effortlessly.
Is guessing ever required?
Never, on these puzzles — unique solutions guarantee a logical path. If you're stuck, you've most likely missed a hidden single or under-used pencil marks; one hint with its explanation usually reveals which scanning habit to fix.
How do I know I'm improving?
Track hints, not time: three consecutive hint-free solves at your current difficulty is the signal to move up. Speed follows technique automatically.
Is daily Sudoku actually good for the brain?
Regular practice supports working memory (tracking candidates), pattern recognition, and sustained focus, with reasonable evidence for cognitive maintenance in older adults. Gains stay near the trained skill — treat it as an enjoyable daily rep, not a miracle.