Sudoku Rules: How to Play
The complete rules of Sudoku in five steps — no strategy, no jargon, just how the game works.
After reading this you can sit down with any Sudoku puzzle and legally fill in cells without guessing.
Sudoku is a logic puzzle with exactly three rules — once you understand them the entire game falls into place. No arithmetic, no sums, no diagonals. Just "each digit once per row, once per column, once per 3x3 box."
Quick answer
After reading this you can sit down with any Sudoku puzzle and legally fill in cells without guessing.
Key points
- ▸ The board is a 9x9 grid (81 cells) split into nine 3x3 boxes by thicker lines. Some cells start with digits printed in them — these are clues and cannot be changed.
- ▸ Rule 1 — rows: each row of 9 cells must contain every digit from 1 to 9 exactly once, no repeats.
- ▸ Rule 2 — columns: each column of 9 cells must contain every digit from 1 to 9 exactly once.
- ▸ Rule 3 — boxes: each of the nine 3x3 boxes must contain every digit from 1 to 9 exactly once.
- ▸ A digit is legal in a blank cell only if it does not already appear in that cell's row, column, or box. If all three checks pass, it's a valid candidate.
- ▸ A proper Sudoku has exactly one solution — you never need to guess. If you feel like guessing, you missed a deduction.
- ▸ Simplest opening technique: pick a digit (say 1), scan all nine boxes, and fill in any box where only one cell can hold that digit. Repeat for 2 through 9.
Examples
- Rule check in actionYou want to place a 5 in an empty cell. Look at its row — no 5 present, OK. Look at its column — no 5 present, OK. Look at its 3x3 box — already has a 5. Illegal. Try another digit or another cell.
- Forced placementBox 7 (bottom-left) has cells for 4 missing digits. Three of those cells already see a 2 in their row or column. The fourth cell is the only place a 2 can go — place it.
- Scanning 1-9After placing your clues, scan each digit from 1 to 9 across the whole board. For every digit, check each box — if only one cell in the box can legally hold it, place it. One pass usually yields 5-15 cells on an easy puzzle.
When to use which tool
- CYAN · STABLE — You can recite the three rules and check if a digit is legal in a cell — ready to play.
- GOLD · GUARDED — You keep forgetting to check the 3x3 box — slow down and do all three checks before each placement.
- MAGENTA · CRITICAL — You placed a digit and a cell lit red — you broke a rule; undo and rescan.
Related
Frequently asked questions
› Is there math in Sudoku? Trust & accuracy
No. The digits 1-9 are symbols — you could replace them with any nine unique characters and the game would be identical. There is no addition, no multiplication, no relationship between digits.
› Do diagonals need to contain 1-9 too?
Not in standard Sudoku. The two main diagonals have no constraint. Variants like "Sudoku X" or "Diagonal Sudoku" add that rule, but they are separate puzzle types.
› What happens if I place a wrong digit? Troubleshooting
On Kefiw Sudoku, the cell will highlight red if it breaks a row, column, or box rule. If no conflict highlights but the digit is still wrong for the final solution, you will hit a contradiction later and need to backtrack. That is why guessing is discouraged.
› How many clues do Sudoku puzzles start with? How-to
It varies by difficulty. On Kefiw: Easy 40, Medium 32, Hard 28, Expert 24. The mathematical minimum for a uniquely-solvable puzzle is 17 (proven in 2012) — most human-solvable experts stay above 22.
› Can I take notes of possible digits? Trust & accuracy
Experienced players pencil-mark candidates — small numbers in a cell showing which digits could legally go there. Kefiw Sudoku does not have built-in pencil marks yet; use mental candidates or scratch paper until the feature ships.