Sudoku — Free Online Puzzle Game

How This Sudoku Works

Tap any cell, then enter a digit from the on-screen pad or your keyboard; arrow keys walk the selection around the grid and ⌫ (or Backspace) clears a cell. The bold digits are givens and cannot be edited. Your goal is the classic constraint: every row, every column, and every 3×3 box must end up holding 1 through 9 exactly once.

Each board is generated fresh while you wait — a full valid solution is built first, then clues are removed one by one, with a solver re-checking after every removal that exactly one solution remains. Easy leaves you about 41 clues, Medium about 31, and Hard cuts down to roughly 23. The interface quietly helps: selecting a cell shades its row, column, and box; every copy of the selected digit lights up across the grid; any digit that clashes with a visible duplicate flashes red immediately; and a pad key fades out once all nine of that digit are placed. The timer counts up from your first look at the board, pauses whenever the How to Play card is open, and your fastest solve is tracked separately per difficulty.

Techniques That Crack Harder Grids

• Scan by digit, not by cell. Tap a placed digit to light up all its copies, then examine each 3×3 box that lacks it. Two copies in neighbouring rows often leave exactly one legal cell in the third box — the fastest placements in the game come from this cross-hatch.
• Hunt naked singles after every placement. A cell whose row, column, and box already contain eight different digits has only one candidate left. Each digit you commit creates new singles nearby, so re-check the neighbourhood you just touched.
• Let the pad counters set your order. A digit key that has gone dim is done; a digit with seven or eight copies placed is one or two scans from finished. Clearing digits to completion shrinks the search space for everything else.
• Absence of red is not proof of correctness. The conflict highlight only catches duplicates visible right now. A digit can sit conflict-free and still be wrong for the final solution — if a chain of entries suddenly jams a whole box, suspect an earlier "safe-looking" guess and erase back.
• On Hard, park your eyes on the busiest band. With only ~23 clues, whole boxes start empty. Begin in the three-row band with the most givens, fill what logic forces, and let the sparse regions wait — guessing early on Hard is how a 10-minute solve becomes 25.