How to play Sudoku
The board is a 9 by 9 grid split into nine 3 by 3 boxes. Some squares already contain digits; these clues are shown in bold white and cannot be changed. Your task is to fill every empty square so that each row, each column and each 3 by 3 box contains the digits 1 to 9 exactly once.
Select a square by clicking or tapping it. Its row, column and box are shaded, and every other square holding the same digit is highlighted, which makes it easy to see where a number is already used. Then enter a digit with your keyboard or the number pad. The small figure on each pad button tells you how many of that digit are still to be placed.
- If a digit clashes with another in the same row, column or box, both are marked in red.
- Turn on Notes to pencil small candidate digits into a square. Placing a real digit automatically removes that digit from the notes of related squares.
- Tapping the same digit again, or pressing Erase, clears the square.
Choose Easy, Medium or Hard from the menu. Harder puzzles give fewer clues. The timer runs while you play and pauses when the tab is hidden. Fill the grid correctly to finish, and your fastest time for each difficulty is saved.
Controls
| Desktop | Click a square, then type 1-9. Backspace, Delete or 0 erases, arrow keys move the selection and N toggles notes mode. |
|---|---|
| Mobile | Tap a square, then tap a number on the pad below the grid. Use Erase to clear a square and Notes to pencil in candidates. |
Tips and strategy
Start with the easy wins
Scan one digit at a time. Pick a number that already appears many times, then check each box that is missing it. Often the existing copies in nearby rows and columns rule out every square but one. This technique, sometimes called cross-hatching, solves most of an Easy puzzle on its own.
Look for lonely squares
Some squares can only hold a single digit because the other eight already appear in their row, column or box. Rows or boxes that are nearly full are the best places to find them.
Use notes deliberately
On Medium and Hard grids you will reach a point where simple scanning stalls. Switch on Notes and write every possible candidate into the squares of one crowded box or row. Patterns then become visible:
- Hidden singles: a candidate that appears in only one square of a row, column or box must go there, even if that square has other candidates.
- Naked pairs: if two squares in the same unit can only be the same two digits, those digits can be erased from every other square in that unit.
- Pointing pairs: if a digit's candidates within a box all sit in one row, that digit cannot appear elsewhere in that row outside the box.
Never guess
Every puzzle here has exactly one solution, so it can always be solved by logic alone. If you feel you must guess, step back and rescan instead; there is a deduction you have missed. The red conflict marks catch direct clashes, but a digit can be wrong without clashing yet, so only place a number when you are sure.
Keep a rhythm
Alternate between quick scanning for singles and slower candidate work. Each digit you place unlocks new deductions, so return to scanning after every breakthrough.
About Sudoku
Sudoku is a logic puzzle built on the idea of the Latin square, where each symbol appears once in every row and column. The modern form, with its extra rule for 3 by 3 boxes, was first published in puzzle magazines in the United States in the late 1970s under the name Number Place. It later became very popular in Japan, where the name Sudoku was adopted, and then spread around the world in the mid-2000s through newspapers.
No arithmetic is involved; the digits could just as easily be letters or colours. What matters is pure deduction, which is why the puzzle appeals to people of all ages.
Our browser version generates a new grid every time you play and checks that it has exactly one solution before showing it to you. It runs entirely on your device, with no sign-up required.
Frequently asked questions
Does every puzzle have a single solution?
Yes. Each grid is generated on the spot and tested by a solver that confirms there is exactly one valid solution before the puzzle is shown.
What is the difference between the difficulty levels?
Easy puzzles start with about 40 clues, Medium with about 32 and Hard with about 25. Fewer clues mean you need more advanced reasoning, such as working with notes.
Why is a number shown in red?
Red means that digit appears more than once in the same row, column or 3 by 3 box. Change one of the clashing squares to fix it.
Can I pause the timer?
The timer stops automatically while the browser tab is hidden. It also stops as soon as you complete the puzzle.
