Sudoku
Play free sudoku online — easy, medium and hard puzzles with hints, pencil notes, timer and mistake checking, plus an instant sudoku solver. Everything runs in your browser.
Keyboard: arrow keys move, 1-9 fills, 0 / Backspace clears. Toggle Notes to pencil in candidates.
Sudoku Rules
- The board is a 9×9 grid divided into nine 3×3 boxes. Some cells start pre-filled — these “givens” cannot be changed.
- Fill every row with the digits 1-9. Each digit appears exactly once per row.
- Fill every column with the digits 1-9. Each digit appears exactly once per column.
- Fill every 3×3 box with the digits 1-9. Each digit appears exactly once per box.
- A proper puzzle has exactly one solution and never requires guessing — only logic.
How to Solve Sudoku: Core Strategies
Scanning (cross-hatching)
Pick a digit and scan every 3×3 box for where it can go. Because the digit already appears in some rows and columns, whole swaths of each box are eliminated. When only one cell in a box remains possible, place the digit there. Scanning the boxes for 1 through 9, then the rows and columns, cracks most easy puzzles on its own.
Naked singles
For each empty cell, check its row, column and box. If eight digits are already accounted for, the last one is forced. Use pencil notes to track candidates — every time you place a digit, our notes mode automatically clears it from peer cells.
Naked pairs
When two cells in the same row, column or box hold exactly the same two candidates (say 3 and 7), those two digits must occupy those two cells. No other cell in that unit can be 3 or 7 — erase those candidates everywhere else. Naked pairs frequently unlock hard puzzles where scanning stalls.
Quick Answer
Sudoku is a logic puzzle played on a 9×9 grid split into nine 3×3 boxes. Fill every row, column and box with the digits 1-9 without repeating any digit. Our free online sudoku generates unlimited unique-solution puzzles in easy, medium and hard, with pencil notes, hints, a timer, mistake checking — plus an instant sudoku solver. Everything runs locally in your browser.
How It Works
Pick a difficulty — easy (~40 givens), medium (~32) or hard (~26) — and a fresh unique-solution puzzle is generated instantly
Click a cell (or arrow-key to it), then press 1-9 or tap the on-screen number pad to fill it
Toggle Notes mode to pencil candidate digits into cells; placed digits auto-clear from peer notes
Use Check to highlight mistakes against the true solution, or Hint to reveal one cell
Fill every row, column and 3×3 box with 1-9 to win — the timer tracks your pace
Stuck on a puzzle from elsewhere? Paste its 81-character string in the Solver tab
Key Facts
- Unlimited unique-solution puzzles: easy (~40 givens), medium (~32), hard (~26)
- Symmetric puzzles generated in your browser — verified to have exactly one solution
- Pencil mode for candidate notes that auto-clear from rows, columns and boxes
- Mistake highlighting against the true solution, plus a one-tap Check button
- Hint button fills any single cell you are stuck on
- Timer with pause, full keyboard controls, and a mobile-friendly number pad
- Solver tab: paste any 81-character sudoku string for an instant solution
- 100% client-side and free — nothing is uploaded, no sign-up
Frequently Asked Questions
How do you play sudoku?
Fill the 9×9 grid so that every row, every column and every 3×3 box contains the digits 1-9 exactly once. The puzzle starts with pre-filled "given" cells that cannot change. All the information you need is on the board — a proper sudoku is solved with logic, never guessing. Start by scanning for digits that can only fit one place in a box, then work through rows and columns with the most givens.
What is the difference between easy, medium and hard sudoku?
Difficulty mostly comes down to how many cells are given and which techniques you need. Easy puzzles start with about 40 givens and usually fall to scanning alone. Medium (~32 givens) needs pencil notes and naked singles. Hard (~26 givens) demands deeper logic such as naked pairs. Every puzzle we generate has exactly one solution regardless of difficulty.
Are these sudoku puzzles guaranteed to have exactly one solution?
Yes. The generator removes cells while a counting solver verifies the puzzle still has exactly one solution; if a removal would create a second solution, the cell is restored. Every easy, medium and hard puzzle you play has a single, unique answer.
What is a naked pair in sudoku?
A naked pair is two cells in the same row, column or box that hold exactly the same two candidates. Because those two digits must occupy those two cells, you can erase both candidates from every other cell in that unit. It is one of the most useful techniques once scanning and naked singles stop making progress on harder puzzles.
How does the sudoku solver work?
Switch to the Solver tab and paste your puzzle as an 81-character string — digits 1-9 for the given cells and 0 or . for empty ones, reading left to right, top to bottom. The solver runs a constraint-propagation backtracking search entirely in your browser and returns the completed grid plus a copyable solution string in milliseconds.
Is this sudoku free and private?
Completely free with no sign-up, and 100% private: puzzles are generated, checked and solved locally in your browser with JavaScript. Nothing you type or play is ever uploaded to a server.