Play sudoku online
Pick a level to start
Puzzle of the day. The same puzzle for everyone, per level.
A free sudoku in three levels. Every puzzle has exactly one solution, you can pencil in notes and show mistakes, and you see your time. There is also a puzzle of the day that is the same for everyone.
The rules in four sentences
The grid has 81 cells, arranged in nine rows, nine columns and nine boxes of 3 by 3. You put a digit from 1 to 9 in every cell. A digit can appear only once in each row, column and box. Some digits are given, and you fill in the rest.
How to play
Tap a cell and pick a digit, or type it on your keyboard. The arrow keys move you to another cell. With notes switched on, a digit goes into the cell as a small candidate instead of a large digit, which helps once you are down to two or three options. With ‘Show mistakes’ on, a digit turns red when it clashes with another digit in its row, column or box. Switch it off and you only find a mistake once the grid is full.
Three techniques for when you are stuck
The first is the single candidate: for an empty cell, check which digits already appear in its row, column and box. If only one is left, that is your digit. The second is the single position: if a box has only one cell where a 7 can still go, the 7 goes there. The third is pairs: if two cells in a row can only be a 3 or an 8, those two digits belong to those two cells, and you can cross the 3 and the 8 out of the other cells in that row. Notes make this visible.
How the levels are set
A level is the number of digits given at the start. Easy gives 36 to 40, medium 30 to 34 and hard 24 to 28. It is a rough measure. A puzzle with fewer digits usually takes more searching, but the technique you need can be the same as in an easier one. For a tougher game, pick hard and switch the mistake checking off.
How the puzzles are made
A generator first fills a complete, valid grid. Then it removes digits in a random order. After each one a solver counts the solutions and stops at two. If there is more than one, the digit goes back. That gives every puzzle exactly one solution, which means you never have to guess. The same input always gives the same puzzle, and that is how the puzzle of the day stays the same for everyone.
What Skillido keeps
Your best time per level stays on your own device. The game itself is not saved: close the page and the next time you start fresh. We do not count who played what, and there is no account.
More games
For short games with a score, see the brain games. For a different kind of number work, there is the mental math test. The Dutch version of this page is sudoku.
Frequently asked questions
- How does sudoku work?
- You fill a 9 by 9 grid so that each digit from 1 to 9 appears exactly once in every row, every column and every 3 by 3 box. A few digits are already there. You do not need to calculate or guess: every digit follows from the ones that are already placed.
- Does every puzzle have exactly one solution?
- Yes. When a puzzle is made, the generator removes a digit only if a solver still finds exactly one solution afterwards. So if a digit in your grid clashes with the only solution, one of your own digits is wrong.
- What is the difference between easy, medium and hard?
- The number of digits given at the start. Easy gives 36 to 40, medium 30 to 34 and hard 24 to 28. More empty cells means more work, but not always a harder technique, so a hard puzzle can go quickly.
- Is the puzzle of the day the same for everyone?
- Yes, per level. The puzzle follows from the date in Amsterdam. Anyone who picks the same level under "Puzzle of the day" gets the same puzzle today. Tomorrow there is a new one.
- Is my time saved?
- Only on your own device, as your best time per level. There is no account and nothing is sent to a server. If you clear your browser data, the times are gone.