In this guide
1. The basic rules of Sudoku
Sudoku is a logic puzzle played on a grid. The most common version uses a 9×9 grid, but it can also be played on 4×4, 6×6, 12×12, or 16×16 grids. Here are the three simple rules:
- Every row must contain each number exactly once (1–9 for a 9×9 grid).
- Every column must contain each number exactly once.
- Every box (the 3×3 sections in a 9×9 grid) must contain each number exactly once.
That's it. No maths, no guessing — just pure logic. Every Sudoku puzzle has exactly one correct solution.
2. Understanding the grid
A standard 9×9 Sudoku grid is made up of 81 cells arranged in 9 rows and 9 columns. The grid is further divided into nine 3×3 boxes (sometimes called "regions" or "blocks").
When you start a puzzle, some cells are already filled in — these are called given numbers or clues. Your job is to fill in all the empty cells without breaking the three rules above.
3. How to start solving
The best place to start is to look for rows, columns, or boxes that already have many numbers filled in — they leave fewer possibilities for each empty cell. Here is a simple approach:
- Scan each row, column, and box to see what numbers are already present.
- Find cells where only one number is possible — fill those in first.
- After filling a cell, re-scan the affected row, column, and box for newly restricted cells.
- Repeat until the puzzle is complete.
4. Solving techniques
Naked single Beginner
When a cell has only one possible number remaining — because all other numbers are already in its row, column, or box — that must be the answer. This is the most common technique for easy puzzles.
Hidden single Beginner
When a particular number can only go in one cell within a row, column, or box — even if that cell has multiple possibilities — place it there. Scan each unit looking for numbers that only "fit" in one place.
Naked pair Intermediate
When two cells in the same row, column, or box both contain exactly the same two possible numbers, those numbers can be eliminated as possibilities from all other cells in that unit.
Pointing pairs Intermediate
When a number can only appear in one row or column within a box, that number can be eliminated from the rest of that row or column outside the box.
X-Wing Advanced
When a candidate number appears in exactly two cells in each of two different rows, and those cells share the same two columns, the number can be eliminated from those columns in all other rows.
Swordfish Advanced
An extension of X-Wing using three rows and three columns. When a candidate appears in at most three cells across three different rows, and those cells all fall within the same three columns, the candidate can be eliminated from the rest of those columns.
5. Tips for beginners
- Use pencil marks. On our site, click "Notes" to toggle pencil-mark mode. Write small candidate numbers into cells as you narrow down the options.
- Start with 9s, 8s, or 7s. Look for the number that appears most in the given clues — it will be easiest to place the remaining copies.
- Don't guess. Every Sudoku puzzle can be solved by logic alone. If you find yourself guessing, try a different technique instead.
- Undo freely. Made a mistake? Click Undo as many times as you need.
- Take breaks. Fresh eyes often spot things you missed before.
6. Advanced strategies
For harder puzzles (Expert, Master, Extreme), you may need to combine multiple techniques:
- Naked triple / hidden triple: Like naked pairs but with three cells and three candidates.
- Coloring / chains: Follow the logical chain of a candidate through the puzzle — if placing it in one cell forces a contradiction, it must go in the other.
- Candidate elimination: Systematically write all candidates for every empty cell, then apply techniques to eliminate impossible values.
- BUG (Bivalue Universal Grave): If the puzzle has only bivalue cells remaining and one cell has three candidates, the third candidate must be the answer for that cell.
7. Frequently asked questions
Does Sudoku require maths?
No. The numbers in Sudoku are just symbols — you could use letters, colours, or shapes instead. The puzzle is entirely about logic and pattern recognition, not arithmetic.
How long does a Sudoku puzzle take?
Easy puzzles can be solved in 5–10 minutes. Medium takes 15–25 minutes. Hard puzzles can take 30–60 minutes. Expert and above may take hours — or days for extreme puzzles.
Can there be more than one solution?
A properly constructed Sudoku puzzle always has exactly one solution. Our puzzles are all algorithmically verified to have a unique solution.
What is the minimum number of clues needed?
It has been mathematically proven that a valid 9×9 Sudoku puzzle requires at least 17 given clues to have a unique solution. Our Extreme difficulty uses around 12% of cells filled (approximately 10–12 clues).
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