Sudoku
Sudoku became a global phenomenon in 2004 when The Times of London started publishing it daily, though the puzzle format dates to 1970s France and earlier Latin square concepts. It's a pure logic puzzle — no math required despite the numbers. Every puzzle has exactly one solution.
Sudoku Rules
The board is a 9×9 grid divided into nine 3×3 boxes. Some cells are pre-filled with digits 1 through 9. Your job is to fill in the empty cells.
Each row must contain every digit from 1 to 9 exactly once. Each column must also contain every digit from 1 to 9 exactly once. Each 3×3 box must also contain every digit from 1 to 9 exactly once.
There is always exactly one valid solution. If you can place a number, you can prove it logically — guessing is never required on a well-constructed puzzle.
Sudoku Strategy & Tips
Scan for naked singles
Look for cells where only one digit is possible. Check what numbers already appear in that cell's row, column, and box. If eight of the nine digits are already placed, the remaining one goes in that cell. This is the most basic technique and solves a large portion of easy and medium puzzles.
Use hidden singles
Sometimes a digit can only go in one place within a row, column, or box — even if that cell could theoretically hold other numbers. Scan each row, column, and box for digits that have only one possible location. This is the second-most-common technique.
Work the boxes with the most givens
A 3×3 box that already has 6 or 7 numbers filled in only has 2-3 cells left. Start there — it's the easiest place to make progress. Then use those new numbers to unlock cells in neighboring boxes.
Pencil mark when you're stuck
Write small candidate numbers in empty cells. When a cell has only 2-3 possibilities, write them in. Pairs of cells in the same row/column/box with the same two candidates (naked pairs) let you eliminate those numbers from other cells in that group.
Frequently Asked Questions
Is Sudoku a math puzzle?
No. Sudoku uses digits but involves zero arithmetic. You could replace the numbers 1-9 with nine colors, letters, or symbols and the puzzle would work identically. It's purely a logic puzzle about placement and elimination.
Can a Sudoku puzzle have more than one solution?
A properly constructed Sudoku has exactly one solution. If a puzzle has multiple valid solutions, it's considered defective. Every puzzle on this site has a unique solution.
What makes a Sudoku puzzle hard?
Difficulty depends on which solving techniques are required, not how many cells are pre-filled. Easy puzzles can be solved with naked and hidden singles. Hard puzzles require techniques like X-Wings, Swordfish, or XY-Chains. The number of givens (pre-filled cells) matters less than their placement.
What is the minimum number of clues for a valid Sudoku?
17. It was proven in 2012 by Gary McGuire that no valid Sudoku puzzle can have fewer than 17 given numbers. There are tens of thousands of known 17-clue puzzles.