Sudoku Rules

Sudoku is a well-known number-based logic puzzle that has captured the hearts of millions of players worldwide. Despite having simple rules, it requires focus, strategic thinking, and patience. In this guide, we’ll explain the rules of Sudoku in detail and show you how to play it correctly, even if you’re just beginning to explore this fun and engaging game.

What is Sudoku and what is the goal?

Sudoku is a popular number puzzle that was invented in Japan. It consists of a 9×9 grid divided into smaller 3×3 blocks. Some cells are already filled with numbers from 1 to 9 (these are called “clues”), while the rest are empty. The goal of Sudoku is to fill all the empty cells with digits from 1 to 9 so that each row, each column, and each 3×3 block contains every digit only once. In other words, no number should repeat within any row, column, or block.

Sudoku doesn’t require any math — just logic, attention, and patience. It may seem tricky at first, but the rules are easy and always stay the same. The puzzle develops analytical thinking and is a great brain exercise. For many people, it becomes an enjoyable hobby.

Basic rules of Sudoku

Game board: A standard Sudoku puzzle has 81 cells (9 rows × 9 columns). Every 9 cells form a 3×3 square. At the beginning, some of the cells contain given numbers, called clues.

Filling rules: The player must fill the empty cells with numbers from 1 to 9, following three key rules (no other symbols allowed):

• No repeating numbers in a row: Each horizontal row must have digits 1–9 with no repeats.
• No repeating numbers in a column: Each vertical column must also contain all digits from 1 to 9 with no repeats.
• Unique numbers in each 3×3 block: Every 3×3 block must also contain digits 1 to 9, each used once.

In other words, the same number cannot appear more than once in any row, column, or 3×3 square. For example, if the number 7 is already in the first row, you cannot place another 7 in that same row.

What’s not allowed: There are a few things that are not allowed in Sudoku:

• You can’t place a number if it already exists in the same row, column, or block. This breaks the uniqueness rule and is a mistake.
• You can’t use numbers outside the 1–9 range. Only these digits are allowed in classic Sudoku. For example, 0 or 10 are not valid.
• Don’t guess randomly. Sudoku is a logic game, and guessing without checking will lead to mistakes. A well-designed puzzle always has a single solution that can be found logically.

How the game works: The player fills the empty cells step by step. After placing a number, make sure it doesn’t break the rules in its row, column, or block. If a rule is broken, it’s a mistake and the number shouldn’t be placed. A good Sudoku puzzle doesn’t need trial and error – the solution must be found logically at every step.

How to solve Sudoku: step-by-step

Take it step by step. Below is a general strategy for solving Sudoku, especially helpful for beginners:

1. Study the whole grid. Look at the puzzle and find where the most numbers are already filled in. Start with the row, column, or 3×3 block that has the most clues. For example, if a row has 7 out of 9 cells filled, the remaining two can often be figured out quickly. Tip: begin where there are more clues and fewer gaps.
2. Find obvious candidates. Look for any section (row, column, or block) that has only one or two missing numbers. Try to determine what’s missing. If a cell has only one possible number, write it in. Then remove that number from possible options in related rows, columns, and blocks. Repeat.
3. Use the elimination method. If you can’t find a clear number right away, eliminate candidates. Pick an empty cell that lies at the intersection of filled row and column. Cross out digits that can’t go there. If only one remains, write it in. If more than one, skip for now.
4. Work through harder sections. Move from more filled areas to less filled ones. Continue narrowing down possibilities using logic and elimination.
5. Use pencil marks. Write small numbers in empty cells showing all possible candidates. This gives a better overview and helps with complex logic. Erase candidates that are no longer possible.
6. Keep scanning the board. After placing each number, recheck the rows, columns, and blocks. Repeat the cycle: fill → update candidates → find the next move.
7. Check for mistakes. Stop and review regularly to make sure no rules are broken. If there’s a duplicate, go back and correct it. Never place a number unless you're fully sure it fits.
8. Hard situations and guessing. If all logic options are exhausted, try a number in a cell with the fewest candidates. Follow it mentally. If it leads to a contradiction, undo and try another. This is called backtracking.

Example of solving Sudoku (step-by-step)

Let’s look at a simple example to show how Sudoku is solved. Suppose we have a partially filled grid—a classic Sudoku with some clues already in place:

This is a standard Sudoku puzzle with clues. The task is to fill in the empty cells with digits 1–9 following the rules.

In the top-left 3×3 block, 7 out of 9 cells are filled. The missing numbers are 5 and 6. They must go in the remaining two spots, but it’s unclear where each belongs. It's best to leave this block and check elsewhere.

Now look at the leftmost column. It has three empty cells. The missing numbers are 1, 5, and 6. The number 1 is already in the top and middle blocks, so the only place left is the bottom-left block. Place the 1 using logical deduction.

Now return to the top-left block. Only 5 and 6 remain. With 1 placed, it becomes clearer: one cell can only take 5, and the other must be 6. Step by step, the whole puzzle gets solved.

Note: In a real Sudoku game, you often reason through several areas at once. Each move simplifies the puzzle, and the number of empty cells decreases until the board is full. When you fill the final cell, you’ve completed the puzzle!

Basic strategies for beginners

New players should learn a few basic strategies to solve Sudoku. These help you find correct digits without guessing:

• Single techniques. These are cases where a number has only one possible location. Two types:

  1. Single candidate in a cell: only one number fits in that spot.

  2. Single position for a number: in a row, column, or block, a number can go in only one cell.

  Beginners should always look for these obvious singles—they're the easiest way to make progress.

• Eliminate impossible candidates. If a cell has only one valid option left, write it in. This method is often used along with pencil marks.
• Scan the board. Regularly check rows, columns, and blocks for the last empty cell and fill it in.
• Pencil marks. Write down potential digits in cells. This helps with logic and more advanced strategies.
• Check your moves. After every step, confirm you haven’t broken the rules. Pause often and recheck with fresh eyes.

Experts say Sudoku is a game of logic, not guessing. Use step-by-step strategies and even difficult puzzles will become manageable.

Common mistakes and how to avoid them

Here are some common Sudoku mistakes and tips to avoid them:

• Guessing instead of using logic. Mistake: placing numbers randomly. Avoid it: never place a number without reason. Leave it blank if unsure.
• Overlooking duplicates in rows or blocks. Mistake: placing a number where it already exists. Avoid it: always check rows, columns, and blocks first.
• Choosing puzzles that are too hard. Mistake: starting with a puzzle that has very few clues. Avoid it: begin with easier levels and move up gradually.
• Messy notes and confusion. Mistake: disorganized pencil marks or unclear digits. Avoid it: keep your work clean and consistent.
• No structured approach. Mistake: jumping randomly around the board. Avoid it: follow a strategy—work through rows, columns, or blocks in order.

Remember, mistakes are part of learning. Review what went wrong and try again. That’s how you improve your skills.

Conclusion: try Sudoku online

Now that you know the rules, strategies, and techniques, it’s time to practice. Start with an easy level, follow the advice, and build your skills. With each puzzle you solve, you’ll improve.

Try solving a Sudoku puzzle on our website now and see how enjoyable it can be!