Sudoku Online — Fill the Grid, No Two Ways About It

It's a free online Sudoku game that generates classic 9×9 puzzles across a 3×3 grid of boxes right in your browser. A backtracking algorithm builds each board and checks it has exactly one solution before showing it to you. You can pick from four difficulties — Intro, Easy, Medium and Hard — each starting with a different number of pre-filled clues.

Yes. All four difficulties, pencil notes, hints, undo/redo and the timer are completely free, with no account, install or payment required. Your board autosaves locally, so you can close the tab mid-puzzle and pick it back up later at no cost. Challenge Mode (three mistakes and you're out) is also free to toggle on or off.

Yes. A backtracking generator fills the full 9×9 grid, removes numbers down to the target difficulty, and then verifies the remaining puzzle has one and only one valid solution before it's ever shown to you. If removing a number would create a second possible solution, that cell stays filled instead. This means you'll never hit a dead end that requires a blind guess.

No. Your current board, notes, elapsed timer, mistake count, hint count and difficulty are all saved to local storage after every move. Refreshing the page or closing the tab and coming back later drops you right back into the same puzzle, still running. Starting a new difficulty is the only thing that clears the saved board.

Yes. The 9×9 board scales to fit any screen with large touch targets for both the grid and the number pad, and the whole game — puzzle generator included — runs offline in your browser with no server round-trip. Arrow keys and the on-screen number pad both work for input, whichever you have handy.

Turn on note mode, then tap a number to add or remove it as a small candidate mark inside a 3×3 mini-grid drawn within that cell — exactly like scribbling in the margins of a paper puzzle. Notes are separate from your final answer, so entering a real number later automatically clears that cell's marks. You can flip note mode on and off as often as you like mid-solve.

When you finish a puzzle you earn 1 to 3 stars: 3 stars for zero mistakes and zero hints used, 2 stars if mistakes plus hints add up to 2 or fewer, and 1 star otherwise. Switch on Challenge Mode and you're playing with three hearts — your third mistake ends the run immediately instead of just being logged. Optional real-time conflict checking can flag a duplicate the moment you place it, which helps you avoid burning a life in Challenge Mode.