Sudoku

How it works

A grid of nine by nine squares, split into nine three-by-three boxes. Some digits are printed already; the rest are yours to place. There is one rule: every row, every column and every box must hold the digits 1 to 9 exactly once.

Nothing has to be guessed. Every digit can be worked out from the ones already there, which is what makes sudoku a game of reasoning rather than luck.

The four difficulties

You choose at the start, and the difference is how many digits you are given.

In the first three levels every grid is solvable by reasoning, with no guessing at all: the game checks that before offering it to you. At expert that guarantee is gone, which is exactly what expert means.

Every grid is new. There is no bank of puzzles: the grid is built on the spot, and the game proves it has exactly one possible solution before giving you the first square.

Controls

• Touch or mouse: tap a square, then a digit on the pad below.
• Notes writes small candidates instead of a final digit, so you can mark what a square could hold.
• Undo steps back one move, Erase empties the chosen square.
• Hint reveals a square. You get three per grid.
• Keyboard: 1–9 to write, 0 or Delete to erase, arrows to move, Space for notes, U to undo, H for a hint, P to pause, N for a new grid.

Tapping a square lights its row, its column, its box and every square holding the same digit — the quickest way to see where a digit cannot go.

Scoring and time

Each correct square is worth 10 points. Finishing the grid is worth 500 to 3,500 points depending on the difficulty. A wrong digit turns red at once and costs 20 points; a hint costs 100.

The clock stops when you switch tabs or pause, so your best time stays honest. For each difficulty the game remembers your best score and your best time, saved in your browser.

How to actually reason it out

Look for the square with the fewest possibilities, not the first empty one. A row with seven digits already in it is missing one: fill that first.

Ask where a digit can go, not what a square can hold. They are different questions, and the first is often easier. If the 4 fits in only one place in a box, it goes there, even if that square would accept other digits too.

Use notes when you get stuck. Writing the candidates of one area makes pairs visible: two squares in a row that accept only 3 and 7 mean 3 and 7 cannot be anywhere else in that row.

Do not guess. If you cannot find the next move, change area rather than writing a hopeful digit: in the first three levels the move is always there.

Frequently asked questions

Is it free? Yes. It plays in your browser, with no signup and nothing to download.

Do I need to be online while playing? No, the grid is built by your own device.

Does it work on a phone? Yes, upright and sideways. Upright, the digit pad sits at the bottom where your thumb is.

Do the grids ever run out? No. They are generated one at a time, so no two games repeat.