Sudoku
Play unlimited Sudoku puzzles in your browser. Easy, medium and hard difficulties, conflict highlighting, check and solve — every puzzle has a unique solution.
Click a cell, then type 1–9 (or use the pad). Arrow keys move, Backspace clears. Every puzzle has exactly one solution.
The rules
Fill the 9×9 grid so that every row, every column, and each of the nine 3×3 boxes contains the digits 1–9 exactly once. The pre-filled “given” numbers are fixed; everything else you deduce by logic alone — no guessing is ever required.
Cells that break a rule are highlighted in red as you type, and the same number elsewhere is gently shaded so you can scan for placements.
Solving techniques
- Naked single: a cell where only one digit is possible given its row, column and box.
- Hidden single: a digit that can only go in one cell of a row, column or box even though that cell could in theory hold others.
- Pointing pairs: when a box restricts a digit to one row/column, eliminate it from the rest of that line.
- Scan, don't guess: well-formed Sudoku always yields to logic — if you're guessing, there's a deduction you missed.
Difficulty & unique solutions
Difficulty here is set by how many cells are revealed (more givens = easier). Crucially, every generated puzzle is verified to have exactly one solution — a backtracking solver counts solutions while digging holes and refuses any removal that would create ambiguity. A proper Sudoku with fewer than 17 givens is mathematically impossible to make unique, which is why even “hard” keeps a sensible floor.
A bit of history
The modern puzzle appeared as “Number Place” in a 1979 US magazine, was popularised in Japan in 1984 as Sūji wa dokushin ni kagiru (“the digits must be single”) — shortened to Sudoku — and went global in 2004. There are 6,670,903,752,021,072,936,960 distinct completed grids, so you won't run out.