How to play Sudoku online: rules, notation, and first moves

Sudoku is a logic puzzle, not a math problem. You never do arithmetic; you only place digits. The goal is simple: fill a 9x9 grid so that every row, every column, and every 3x3 box contains the numbers 1 through 9 exactly once.

Most online Sudoku games, such as Sudoku Free, work the same way. You click a cell, pick a number, and the game checks for conflicts. If you make a mistake, you can usually erase it. The challenge is figuring out where each number goes before you place it. If you are new to the genre, you can also browse the full puzzle section to see other logic-based options.

#The basic rules you need to know

The grid starts with some numbers already filled in. These are called clues. Your job is to fill in the rest. The three rules are strict: each row must have 1-9, each column must have 1-9, and each of the nine 3x3 boxes must have 1-9. If a number appears twice in any of these three areas, the puzzle is broken.

There is always exactly one correct solution. You do not need to guess. If you feel like you have to guess, you have probably missed a logical step. This is what makes Sudoku different from many other puzzle types; it is a test of observation and deduction, not luck.

#How to mark notes

When you are not sure where a number goes, you can write small notes in the cell. Many browser games let you toggle a note mode. For example, if you think a 5 could go in three different cells in a row, you write a small 5 in each of those cells. This helps you track possibilities without committing to a wrong answer.

Notes are optional but helpful. If you are new, start with just one or two notes at a time. Do not fill every empty cell with notes; that can get messy and slow down your thinking. If you are playing on a smaller screen, such as a phone or tablet, notes can be harder to see, so use them sparingly until you are comfortable with the layout.

#Technique 1: Single candidate

Look at a cell. Count how many numbers could fit there. If only one number is possible, place it. For example, if a cell is in a row that already has 1, 2, 3, 4, 5, 6, 8, and 9, the only missing number is 7. That cell must be 7. This is the easiest way to start a puzzle.

• Scan rows: Look for rows with only one empty cell.
• Scan columns: Look for columns with only one empty cell.
• Scan boxes: Look for 3x3 boxes with only one empty cell.

#Technique 2: Hidden single

Pick a number, like 4. Look at a row. If 4 can only go in one cell in that row, then that cell must be 4, even if other numbers could also fit there. This is called a hidden single because the number is hidden among other possibilities.

You can do this for rows, columns, or boxes. For example, if a 3x3 box has five empty cells, but the number 7 can only fit in one of those five cells, place the 7 there. This technique takes a bit more work than scanning for empty cells, but it is essential for medium-difficulty puzzles.

#Technique 3: Pointing pair

This is a slightly more advanced move. Look at a 3x3 box. If all the possible spots for a number, say 8, are in the same row within that box, then 8 cannot appear anywhere else in that row outside the box. You can erase 8 from the notes in other cells in that row.

This does not place a number directly. It clears out wrong options, which often reveals a single candidate somewhere else. If you are struggling with this, do not worry. It is a technique you will learn as you play more. You can also try Catdoku Sudoku Puzzle for a different Sudoku experience.

#Common mistakes to avoid

• Guessing: If you guess and are wrong, you have to undo many moves. Stick to logic.
• Ignoring boxes: Many beginners only look at rows and columns. Remember the 3x3 boxes are just as important.
• Too many notes: If every cell is full of notes, it is hard to see what is true. Keep notes minimal.
• Not checking conflicts: After placing a number, scan the row, column, and box to make sure you did not duplicate a number.

#How to get better

Start with easy puzzles. They have more clues, so there are fewer empty cells and more single candidates. As you get faster, move to medium and hard puzzles. Hard puzzles require more techniques, like pointing pairs and hidden pairs.

You can practice on Sudoku Free or try a different style with Catdoku Sudoku Puzzle. Playing a few puzzles a day is better than one long session. Your brain gets better at spotting patterns with practice. If you enjoy this kind of logical thinking, you might also like other thinky games that challenge your memory and planning skills.

#Where to start

If you are new, pick an easy puzzle. Use the single candidate technique first. When you get stuck, look for hidden singles. Only use notes when you are unsure. If you are still stuck, take a break and come back later. Fresh eyes often spot what tired eyes miss. You can find a curated list of free options on our games page to start playing immediately.