Free Online Sudoku
Three difficulty levels, three lives, pencil notes and a timer. Every puzzle is generated fresh with exactly one solution. Pick a level, click a cell, and start filling in digits.
Keyboard: 1-9 to enter, Backspace to erase, arrows to move, N to toggle notes.
How to get better at Sudoku
Sudoku is a logic puzzle, not an arithmetic one, and the two habits that separate beginners from confident solvers are scanning and candidate tracking. Scanning means picking a digit and sweeping the grid for rows, columns and boxes where it already appears, then looking for the one box where only a single cell can still hold it. On Beginner puzzles, scanning each digit from 1 to 9 in turn will usually carry you most of the way to the finish.
When scanning stalls, switch to candidates. Turn on Notes and, for each empty cell in a tight region, write down every digit that could legally go there. Two patterns then appear again and again. A naked single is a cell with only one candidate left, so it must be that digit. A hidden single is a digit that appears as a candidate in only one cell of a row, column or box, so it must go there even if that cell has other candidates. Medium puzzles are built around these two ideas.
Hard puzzles add pairs and pointing. If two cells in a box share exactly the same two candidates, those digits are locked to those cells and can be removed from the rest of the box. If all the candidates for a digit within a box sit in one row, that digit can be removed from the rest of that row outside the box. Work slowly, update your notes after every placement, and remember that with three lives a guess is expensive; a correctly built puzzle never needs one.
Frequently asked questions
What are the rules of Sudoku?+
Fill the 9 by 9 grid so that every row, every column, and every 3 by 3 box contains the digits 1 to 9 exactly once. Some cells are given at the start; the rest you deduce. A correctly constructed puzzle has exactly one solution and never requires guessing.
How do the three lives work?+
Each time you enter a digit that does not match the solution, the cell turns red and you lose one life. After three wrong entries the round ends. You can then retry the same puzzle from the start or generate a new one. Notes (pencil marks) never cost a life.
What is the difference between Beginner, Medium and Hard?+
The levels differ in how many clues you start with: about 40 for Beginner, 32 for Medium, and 26 for Hard. Fewer clues means more cells to deduce and longer chains of reasoning. All three levels are generated with a single unique solution.
Are the puzzles random?+
Yes. Every puzzle is generated in your browser when you start a round: a complete valid grid is built first, then clues are removed one at a time while checking that the puzzle still has exactly one solution. You will not see the same puzzle twice.
How do I use notes?+
Turn on Notes with the pencil button or by pressing N, then enter digits to mark the candidates you are considering for a cell. When you place a correct digit, that digit is automatically removed from the notes in the same row, column and box.
Where is my best time saved?+
In your browser's local storage, separately for each difficulty. Nothing is sent to our servers, so clearing site data or switching devices resets it.
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