Sudoku Validator – Check Sudoku Grid Validity
Validate your Sudoku puzzle with our free online Sudoku validator. Check for duplicate numbers in rows, columns, and 3×3 boxes instantly.
How the Sudoku Validator Works
Sudoku is a logic-based number puzzle played on a 9×9 grid. The goal is to fill the grid so that each row, each column, and each of the nine 3×3 boxes contains the digits 1 through 9 exactly once. This validator checks your puzzle against these three fundamental rules.
When you click "Validate," the tool performs three separate checks:
- Row Check: Verifies no number repeats in any horizontal row
- Column Check: Verifies no number repeats in any vertical column
- Box Check: Verifies no number repeats in any 3×3 subgrid
The validator distinguishes between puzzles that are valid but incomplete (no errors but empty cells remain) and puzzles that are correct and complete (all 81 cells filled with no violations).
Example Sudoku Puzzles
Easy Puzzle (50+ filled cells)
Good for beginners, with many given numbers:
5 3 _ | _ 7 _ | _ _ _
6 _ _ | 1 9 5 | _ _ _
_ 9 8 | _ _ _ | _ 6 _
------+-------+------
8 _ _ | _ 6 _ | _ _ 3
4 _ _ | 8 _ 3 | _ _ 1
7 _ _ | _ 2 _ | _ _ 6
_ 6 _ | _ _ _ | 2 8 _
_ _ _ | 4 1 9 | _ _ 5
_ _ _ | _ 8 _ | _ 7 9
Medium Puzzle (35-45 filled cells)
Moderate challenge requiring more strategy:
_ _ _ | 6 _ _ | 4 _ _
7 _ _ | _ _ 3 | 6 _ _
_ _ _ | _ 9 1 | _ 8 _
------+-------+------
_ _ _ | _ _ _ | _ _ _
_ 5 _ | 1 8 _ _ 7 _
_ _ 3 | _ _ 6 | _ _ _
_ 8 _ | 4 _ _ _ _ _
_ 2 _ | 7 _ _ _ _ 9
_ _ 6 | _ _ 8 | _ _ _
Common Error Patterns
Watch out for these frequent mistakes:
Row duplicate: Two 7s in row 3
Column duplicate: Two 4s in column 5
Box duplicate: Two 9s in the center 3×3 box
Quick Fact: Sudoku's Surprising History
Despite its Japanese name, Sudoku wasn't invented in Japan. The puzzle was created by American architect Howard Garns in 1979 and first published as "Number Place" in Dell Magazines. It was introduced to Japan in 1984 by the puzzle company Nikoli, which gave it the name "Sūji wa dokushin ni kagiru" (numbers must be single), later shortened to "Sudoku." The puzzle became a worldwide phenomenon in 2004 when New Zealand judge Wayne Gould convinced The Times of London to publish it, sparking "Sudoku fever" across Europe and eventually the globe.
Frequently Asked Questions
What are the basic rules of Sudoku?
Fill the 9×9 grid so that each row contains the numbers 1-9 exactly once, each column contains 1-9 exactly once, and each of the nine 3×3 boxes contains 1-9 exactly once. A proper Sudoku has only one unique solution.
Can a Sudoku have multiple solutions?
A well-constructed Sudoku puzzle should have exactly one unique solution. If a puzzle has multiple valid solutions, it's considered poorly designed. Most newspaper and app puzzles are verified to have a single solution before publication.
What's the minimum number of clues needed?
Mathematicians proved in 2012 that a valid Sudoku needs at least 17 clues (pre-filled cells) to guarantee a unique solution. Puzzles with 16 or fewer clues will always have multiple solutions. Most published puzzles have 22-35 clues depending on difficulty.
How do I know if my puzzle is solvable?
A valid Sudoku should be solvable using logic alone, without guessing. If you reach a point where you must guess, either you made an error earlier or the puzzle is poorly constructed. This validator helps catch errors before they cascade through your solution.
What's the best solving strategy for beginners?
Start with "scanning"—look for rows, columns, or boxes that are nearly complete. Use "cross-hatching" to eliminate possibilities. For each empty cell, ask: "What numbers can go here?" If only one number fits, place it. Repeat until stuck, then look for "naked pairs" or "hidden singles."
Why does my puzzle show as "valid but incomplete"?
This means you haven't made any rule violations, but you still have empty cells. This is actually good news—it means your work so far is correct! Keep filling in numbers using logic. The validator will show "Valid & Complete" only when all 81 cells are filled correctly.
Can I use this validator for variant Sudoku types?
This validator checks standard 9×9 Sudoku rules. It won't validate variant puzzles like Killer Sudoku, X-Sudoku (with diagonal rules), or Jigsaw Sudoku (with irregular regions). For those variants, you'd need specialized validators that check their additional constraints.
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