Sudoku
Endless unique-solution sudoku in three difficulties, with pencil marks.
How to Play
Every puzzle on this page is generated with exactly one solution — no guessing is ever required, only logic. Click or tap an empty cell to select it, then enter a digit from 1 to 9; on a keyboard you can type the digit directly, and on a phone you use the on-screen number pad. The goal is the classic one: every row, every column, and every 3×3 box must contain each digit from 1 to 9 exactly once.
Two tools support your solving. Pencil marks let you note candidate digits in a cell before committing — toggle the pencil mode, tap the candidates you want to record, and they appear small in the corner of the cell until you place a final digit. The error check, when you choose to use it, flags any entered digit that contradicts the solution, so you can catch a wrong turn before it poisons twenty minutes of work. Three difficulty levels control how advanced the required techniques are: the easiest puzzles fall to simple scanning, while the hardest demand candidate elimination and deeper pattern work. The game runs locally in your browser with no account, and your current puzzle is saved on your device, so an interrupted grid is waiting when you return.
Why Sudoku Is Worth Your Time
Sudoku is pure deduction with instant feedback — one of the few puzzle forms where you can verify your own reasoning at every step. Because each puzzle here has a unique solution, every cell is reachable by logic alone, which makes the game a clean exercise in systematic thinking: scanning, elimination, and the discipline of writing down what you know instead of what you hope. To be clear about what that does and does not do for you: solving puzzles makes you better at solving puzzles, and the broader evidence on transfer to general intelligence is covered honestly on our brain training page. A daily grid is still one of the most satisfying habits in puzzling — and our daily challenges make it easy to keep the streak alive.
Practical Tips
- Scan boxes for forced digits first. Pick a digit, find the rows and columns where it already appears, and see which box has only one cell left for it. This single technique solves most easy puzzles entirely.
- Use pencil marks before you need them. On harder grids, recording candidates early turns invisible logic into visible patterns — naked pairs and hidden singles jump out of written marks in a way they never do from memory.
- Treat the error check as a safety net, not a solving tool. Checking after every entry trains guessing. Solve in stretches, then verify, and you keep the logic muscle doing the work.
Frequently Asked Questions
Does every puzzle really have only one solution?
Yes. The generator verifies uniqueness before a puzzle is served, which guarantees that pure logic — never guessing — is always sufficient to finish.
What do the difficulty levels change?
The solving techniques required. Easy puzzles yield to basic scanning, medium puzzles need systematic candidate work, and hard puzzles require chained eliminations and more patient bookkeeping.
How do pencil marks work?
Switch to pencil mode and tap digits to record them as small candidates in the selected cell. Placing a final digit clears the cell’s marks. They are notes for you — the puzzle never requires them.
Does using the error check count against me?
No. It is an optional aid that flags digits contradicting the solution. Purists solve without it; learners often use it to catch a wrong assumption early.
Is my puzzle saved if I leave the page?
Yes. Your grid, marks, and progress are stored locally in your browser on the same device. No account is needed.
Prefer a measured challenge with a score at the end? Try the cognitive tests.