The Only 3 Rules
1. Each row must contain the digits 1 through 9 exactly once.
2. Each column must contain the digits 1 through 9 exactly once.
3. Each 3×3 box must contain the digits 1 through 9 exactly once.
A standard Sudoku grid is 9 rows by 9 columns, divided into nine 3×3 boxes. Some cells are pre-filled (these are called "givens" or "clues"). Your job is to fill the rest without breaking any of the three rules above.
Rules in 30 Seconds
No repeated digits in any row. No repeated digits in any column. No repeated digits in any 3×3 box. Fill every cell. Done.
What "Box" Means
The 9×9 grid is divided into nine non-overlapping 3×3 regions, usually separated by thicker lines. These are the "boxes" (sometimes called "blocks" or "subgrids").
For example, the top-left box covers rows 1–3 and columns 1–3. The center box covers rows 4–6 and columns 4–6. Each box operates as its own independent constraint: it must contain 1–9 with no duplicates, just like rows and columns.
What Is NOT a Rule
People sometimes assume things about Sudoku that are not true. Here are the most common myths:
"You need math." Sudoku is a logic puzzle, not an arithmetic puzzle. The digits 1–9 are just symbols. No adding, subtracting, or calculating is involved.
"You have to guess sometimes." Never. Every valid Sudoku puzzle has exactly one solution reachable through deduction alone. If you are guessing, you are missing a technique.
"Speed matters." Sudoku is not a race (unless you want it to be). Going slowly and methodically is the best way to learn and to avoid mistakes.
"Diagonals matter." In standard Sudoku, diagonals have no constraint. There are diagonal Sudoku variants, but those are a separate type of puzzle.
Rules vs. Strategies
The rules tell you what a valid solution looks like. Strategies tell you how to find that solution efficiently.
For example, "naked singles" (when a cell has only one possible value) is a strategy, not a rule. The rule just says "no repeats in a row, column, or box." The strategy uses that rule to eliminate impossible values.
If you want to learn strategies, start with our tips page or the beginner guide.
Quick Examples: Valid vs. Invalid
Valid: Row 1 contains 5, 3, 4, 6, 7, 8, 9, 1, 2. All nine digits present, no repeats. This row satisfies the rule.
Invalid: Row 1 contains 5, 3, 4, 6, 7, 8, 9, 1, 5. The digit 5 appears twice, and the digit 2 is missing. This breaks the row rule.
Valid: The top-left box contains 5, 3, 4, 6, 7, 2, 1, 9, 8. All nine digits, no repeats.
Invalid: The top-left box contains 5, 3, 4, 6, 7, 2, 1, 9, 3. The digit 3 is duplicated. This breaks the box rule.
A completed puzzle must satisfy all three rules simultaneously — every row, every column, and every box must pass.
Printable Rules Summary
Here is a compact version you can print or save:
Sudoku: The Rules
Fill the 9×9 grid so that:
→ Every row has 1–9 (no repeats)
→ Every column has 1–9 (no repeats)
→ Every 3×3 box has 1–9 (no repeats)
No math. No guessing. Just logic.
Frequently Asked Questions
Can a Sudoku puzzle have more than one solution?
A properly constructed puzzle has exactly one solution. If a puzzle has multiple solutions, it is considered defective. All puzzles on VeryFreeSudoku have a unique solution.
What happens if I break a rule?
You will end up with a contradiction — a cell where no number can legally go, or a row/column/box that cannot be completed. Our online player highlights conflicts in red so you can catch mistakes early.
Are the rules the same for all difficulty levels?
Yes. Easy, Medium, Hard, and Expert puzzles all follow the same three rules. The difference is how many clues are given at the start and which techniques you need to solve them.
What about Sudoku variants?
Variants like Killer Sudoku, diagonal Sudoku, or Thermo Sudoku add extra constraints on top of the base three rules. Standard Sudoku (which is what we offer) uses only the three rules described on this page.
How do I know when I am done?
When every cell is filled and no row, column, or box has a repeated digit, you have solved the puzzle. Our online player confirms this with a completion message.