How to Play Sudoku

What You Need to Know Before You Start

Sudoku is a logic puzzle played on a 9×9 grid. When you open a puzzle, some cells already contain digits — these are called "givens" and serve as your starting clues. Your task is to fill every remaining empty cell with a digit from 1 to 9.

The key insight is that Sudoku has nothing to do with arithmetic. You are not adding, subtracting, or calculating anything. You are using logical reasoning to determine which digit belongs in each empty cell. Anyone can learn to play, regardless of their math background.

Understanding the Grid

Before making your first move, take a moment to understand how the grid is structured. The 81-cell grid is divided into three types of units:

• Rows: Nine horizontal lines, each containing 9 cells.
• Columns: Nine vertical lines, each containing 9 cells.
• Boxes: Nine 3×3 square regions tiling the entire grid.

Every cell belongs to exactly one row, one column, and one box. The rule that governs Sudoku applies to all three: no digit may appear more than once in any single row, column, or box.

Step 1: Survey the Puzzle

When you first open a puzzle, don't immediately start filling in numbers. Take a few seconds to scan the entire grid and notice which rows, columns, and boxes already have the most digits. Areas with many givens are the easiest to work with because they leave fewer possibilities for the remaining cells. Start there.

Step 2: Fill in Obvious Cells

Select an empty cell and check its row, column, and box. List which digits are already present across all three units. If only one digit from 1–9 is absent from all three units combined, that digit is the answer for this cell — place it in.

For example: if a cell's row contains 1, 3, 5, 8, and 9; its column contains 2 and 6; and its box contains 4 and 7; then all nine digits are accounted for except one. That missing digit is the answer.

Step 3: Use Pencil Marks for Uncertain Cells

When you cannot immediately determine the correct digit for a cell, write down all the digits that could possibly go there. These are called pencil marks or candidate notes. The Sudoku game on sotjun.com includes a memo mode for exactly this purpose.

Each time you confirm a digit in any cell, remove that digit from the candidates of every cell in the same row, column, and box. As candidates shrink, cells with only one candidate left become automatically solvable. Pencil marks turn complex puzzles into a systematic, manageable process.

Step 4: Look Within Boxes

Besides checking individual cells, also examine each 3×3 box as a whole. Ask yourself: for each missing digit in this box, which cells could it go in? If a digit can only fit into one cell within the box (because all other cells are blocked by that digit in their row or column), place it there. This technique — sometimes called a "hidden single" — is extremely powerful and works on every difficulty level.

Step 5: Repeat and Complete

Continue cycling through steps 2 to 4. Every digit you place narrows down the possibilities for other cells, which then leads to more confirmed placements. This chain reaction continues until the entire puzzle is filled in correctly.

Common Mistakes to Avoid

The most common beginner mistake is placing a digit without fully checking all three units — row, column, and box. Checking only two out of three is enough to cause an error. Make it a habit to always verify all three before writing in a digit.

Another frequent issue is guessing when stuck. Resist the urge. If no cell seems solvable, try approaching the puzzle from a different angle — focus on a different digit or a different area of the grid. A good Sudoku puzzle always has a logical next step waiting to be found.