Sudoku Rules
The complete rulebook plus answers to the questions people always ask.
Sudoku is a logic puzzle on a 9×9 grid divided into nine 3×3 boxes. Some cells start with digits already filled in — these are the "givens." Your job is to fill every empty cell with a digit from 1 to 9 so the puzzle obeys three rules. That's it. The three rules below are the entire game; everything else — every strategy, every technique, every trick — is built on top of them.
The three rules
- 1
Every row contains 1–9, each exactly once
A sudoku grid has 9 horizontal rows of 9 cells each. After you've solved the puzzle, every row must contain each digit from 1 to 9 exactly once — no repeats, no gaps. If a row contains two 4s, or no 4 at all, the puzzle is invalid.
- 2
Every column contains 1–9, each exactly once
The same rule applies vertically. A sudoku grid has 9 columns of 9 cells. Each column must contain digits 1 through 9 exactly once when solved.
- 3
Every 3×3 box contains 1–9, each exactly once
The grid is divided into nine 3×3 sub-grids called boxes (sometimes "blocks" or "regions"). Each box must also contain digits 1 through 9 exactly once. The interaction between row, column, and box constraints is what makes sudoku a logic puzzle and not just a number-placement exercise.
What is NOT a sudoku rule
A lot of beginners think there are more rules than there really are. There aren't. Diagonals don't matter in standard sudoku (the variant where they do is called Sudoku X). Touching cells don't have to be consecutive or non-consecutive — both arrangements are fine. Numbers don't have any meaning beyond being labels — you could solve sudoku with letters A through I, or any other 9 distinct symbols. The puzzle has a unique solution by construction, so you should never have to guess; if you find yourself guessing, you're missing a logical step.
Frequently asked
Do diagonals have to contain 1–9?
No — not in standard sudoku. That's a rule for a variant called Sudoku X, where the two main diagonals each must also contain 1–9 exactly once. Standard sudoku only constrains rows, columns, and 3×3 boxes.
How many starting numbers (givens) does a sudoku puzzle have?
It varies by difficulty. Easy puzzles typically have 35–45 givens, medium 28–34, hard 22–27, expert/master 18–22, and the hardest evil-tier puzzles can have as few as 17 — which is the mathematically proven minimum for a puzzle that still has a unique solution.
Does a sudoku puzzle always have exactly one solution?
Yes — by definition. If a grid has multiple valid completions, it's not a valid sudoku puzzle. Every puzzle on Sudoku247Online is verified to have exactly one unique solution before it's served.
Can the same number appear twice in the same row or column?
Never. The whole point of the row and column rules is that each digit appears exactly once in each row and exactly once in each column. If you've placed the same digit twice in either, you've made a mistake.
Are sudoku puzzles always 9×9?
The classic version is 9×9, but smaller variants exist: 6×6 (Mini Sudoku, using digits 1–6) and 4×4 (Kids Sudoku, using 1–4). The same row-column-box rules apply, just at smaller scale. Larger variants like 16×16 and 25×25 exist too but are far less common.
Do you have to guess to solve hard sudoku?
No. Every valid sudoku has a unique solution reachable by pure logical deduction — guessing is never required. The harder the puzzle, the more advanced the techniques you need (X-Wing, Swordfish, XY-Wing, etc.), but they're all deterministic. If a puzzle truly requires guessing, it's either not a valid sudoku or has multiple solutions.
What's the difference between sudoku and Killer Sudoku?
Killer Sudoku adds cages — groups of cells (drawn with dashed outlines) that must sum to a target value, and the digits within a single cage must all be different. Standard sudoku has no cages and starts with given digits; Killer starts with no given digits and uses the cage sums as the constraint instead.
Is it okay to use pencil marks?
Yes — and most solvers use them on medium difficulty and above. Pencil marks (small candidate numbers in a cell) let you track which digits are still possible in each empty cell. They're not cheating; they're how the puzzle was meant to be solved at higher difficulties.
How long should solving a sudoku take?
Roughly: 3–7 minutes for easy, 8–15 for medium, 15–30 for hard, 25–45 for expert, and an hour or more for master/evil tiers. Times vary widely by skill level — experienced solvers can finish hard puzzles in under 5 minutes.
Is sudoku good for your brain?
Research has consistently linked regular puzzle-solving (including sudoku) to better short-term memory, faster processing speed, and improved logical reasoning in adults. It's not a cure for anything, but it's a low-effort daily habit with real cognitive benefits — particularly when paired with other mentally engaging activities.
Ready to put the rules into practice?
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