Sudoku

• Fill every row, column, and 3×3 box with the numbers 1 through 9, without repeats. The original numbers cannot be changed.
• Select a square, then tap a number or type 1–9. Notes toggles pencil marks. Use Erase, Delete, or Backspace to clear a square; Undo takes back your last entry.
• Repeated numbers are highlighted automatically. Check compares your entries with the solution. Hint reveals one square. Hints and checks are included in your shared result.
• Every generated puzzle has exactly one solution. Easy gives more starting numbers; Hard gives fewer. The daily puzzle uses Medium and is the same for everyone on its UTC date.
• Keyboard: arrow keys select a square, N toggles notes, and Ctrl/Cmd+Z undoes. Your current puzzle and up to 100 undo steps save in this browser.

Find the only possible number

Choose an empty square and rule out every number already present in its row, column, and 3×3 box. If only one number remains, you can place it. For example, if those three groups together already contain 1 through 8, the square must be 9. You can also look for a number that has only one possible position within a whole row, column, or box.

Use notes as possibilities

Pencil marks record candidates; they are not solved entries. When you place a number, revisit related notes and remove possibilities that no longer fit. A board without repeated numbers can still contain a wrong choice, because duplicate highlighting only checks immediate conflicts. Check compares entries against the solution, while Hint supplies one square. Both count as assists in a shared result, so use them when you want help learning the next step.

Questions players ask

Does no red highlighting mean every entry is correct?

No. Highlighting finds immediate duplicates. Check compares your entries with the solution.

Do notes fill a square?

No. Notes record possible numbers. Switch Notes off before entering the square’s answer.

Why is difficulty fixed in the daily puzzle?

The daily uses Medium so everyone can compare the same puzzle for that UTC date. Free play offers all three difficulties.

Keep playing and learning

Try Nonograms to use row and column constraints on a picture, with a worked overlap lesson.