Sudoku rules

What is sudoku?

Sudoku is a logic puzzle played on a 9×9 grid. The grid is divided into nine 3×3 boxes. Some cells come pre-filled with digits — these are called givens. Your job is to fill every empty cell so that each row, each column, and each 3×3 box contains the digits 1 through 9 exactly once.

That is the entire rule. One rule. No math, no adding, no calculating. The digits could be letters or symbols — the logic would be identical. Sudoku was originally called "Number Place" when Howard Garns created it in 1979 for Dell Magazines. It took off globally after Japanese publisher Nikoli picked it up in 1986, and the name sudoku (数独, "single number") stuck.

The one rule

Every row, every column, and every 3×3 box must contain the digits 1 through 9 exactly once. That means:

1. No digit repeats in any row (all nine cells across must be different).
2. No digit repeats in any column (all nine cells down must be different).
3. No digit repeats in any 3×3 box (the thick-bordered sections).

If a cell sits at the intersection of row 3, column 5, and the center box, the digit you place there must not already appear anywhere in row 3, column 5, or that center box. Every placement satisfies all three constraints simultaneously.

Solving a simple example

Imagine a 4×4 sudoku (the logic is the same, just smaller). The grid has four rows, four columns, and four 2×2 boxes. Each unit must contain 1, 2, 3, 4 exactly once.

┌───┬───┐   ┌───┬───┐
│ 1 │ _ │   │ _ │ 4 │
│ _ │ 3 │   │ 1 │ _ │
├───┼───┤   ├───┼───┤
│ _ │ 1 │   │ 4 │ _ │
│ 4 │ _ │   │ _ │ 1 │
└───┴───┘   └───┴───┘

Row 1 has 1 and 4. The missing digits are 2 and 3. Column 2 already has 3 (from row 2), so R1C2 must be 2 and R1C3 must be 3.

Column 1 has 1 and 4. The missing digits are 2 and 3. Row 2 already has 1 and 3, so R2C1 must be 2. Then R3C1 must be 3.

Keep going — each placement removes a possibility somewhere else, and the grid fills itself through elimination. The same logic applies to the full 9×9 grid, just with more cells to track.

How to play on this site

Click or tap an empty cell to select it. Type a digit (1-9) or use the on-screen number pad. The digit fills the cell. If it conflicts with an existing digit in the same row, column, or box, the conflicting cells highlight in red.

Use pencil marks to note candidates: hold Shift while typing a digit, or toggle pencil mode with the pencil icon. Pencil marks are small numbers in the corner of a cell — they remind you which digits are still possible there.

Arrow keys move between cells. Backspace or Delete clears the selected cell. Press U or Ctrl+Z to undo. Press H to use a hint.

Difficulty levels

All puzzles on this site are solvable through logic alone — no guessing required at any difficulty level. Harder puzzles just require more advanced elimination techniques.

Beginner tips

• Start with the most-filled rows, columns, and boxes. If a row already has 7 of 9 digits, finding the remaining two is straightforward.
• Focus on one digit at a time. Pick a number (say, 5) and scan the entire grid for where it can and cannot go. This cross-hatching technique is the backbone of all sudoku solving.
• Use pencil marks early. On medium and harder puzzles, trying to hold candidates in your head leads to mistakes. Write them down.
• Check your work after every few placements. One wrong digit early on can cascade into an unsolvable mess. Using Check on this site catches errors before they spread.
• Skip and come back. If a region is stuck, work on a different part of the grid. New placements elsewhere often unlock the stuck area.

Play modes

• Classic — Untimed. Three hints. Undo and check available. Good for learning.
• Timed Trial — Countdown clock scaled by difficulty. Three hints. Tests your speed.
• Challenge — No hints, no check, no undo. Pure solving.

Frequently asked questions

Related puzzle rules

• Killer Sudoku rules — Sudoku with cage sums, no given digits
• KenKen rules — Arithmetic cages on a Latin square grid
• Futoshiki rules — Inequality constraints between adjacent cells