Sudoku for Beginners: Rules and Your First Grid

Understand the board before making a move

A standard board has 81 cells arranged in nine rows and nine columns. Bold boundaries divide it into nine smaller boxes, each three cells wide and three cells high. Some digits are supplied at the start. These clues stay fixed; your job is to fill only the empty cells while respecting every clue.

The same digit can appear many times across the whole board. What matters is its location: it cannot repeat within a single row, column or box. A completed standard grid therefore contains nine copies of each digit, distributed one per row, column and box. There is no diagonal rule unless a puzzle explicitly introduces that variation.

Choose a manageable first place to look

Do not feel obliged to begin at the top-left corner. Look for a row, column or box with seven or eight digits already filled. Count which digits from 1 through 9 are absent. With one empty cell, the missing digit is immediately determined, although checking the crossing units remains a useful habit.

With several empty cells, keep the missing digits as possibilities rather than filling them in order. A row missing 2 and 6 does not tell you which empty cell gets which number. The crossing column or box may settle that question. This distinction between a possibility and a confirmed answer is the main habit to learn first.

Follow a worked first-move example

Suppose one row reads 1, blank, 3, 4, 5, blank, 7, 8, 9. The row is missing 2 and 6. Call its two empty cells A and B. If A's column already contains a 6, A cannot also be 6. Since the row permits only 2 or 6, A must be 2.

Before entering 2, check that A's column and box do not already contain 2. In a consistent puzzle with correct earlier entries, this check succeeds. The row now needs only 6, so B becomes 6 after its own checks. If either placement clashes, stop and review the information; do not overwrite a fixed clue to make the move fit.

Learn to use notes as a thinking aid

Notes are small candidate digits you record in an unsolved cell. To create them, start with 1–9 and cross out every digit already present in that cell's row, column or box. The remaining digits are possible entries. When only one remains, you have a confirmed placement, often called a single.

Notes should reduce the amount you need to remember, not replace checking the board. A note of 4/8 means that further information is needed. After placing a 4 nearby, remove 4 from notes in the same row, column and box. Do not remove it from unrelated cells, and do not assume a digital interface updates all notes automatically.

Recognise another easy kind of certainty

You can also search for the only place a particular digit fits. Imagine a 3 × 3 box is missing a 7. Existing 7s in the rows and columns crossing that box may rule out every empty cell except one. The remaining cell must hold 7, even if several other digits initially seemed possible there.

This approach is useful when no cell has a single obvious candidate. Select one digit and check each box that lacks it. Then try another digit. Keep the reasoning explicit: the excluded cells each share a row or column with an existing copy, and the surviving cell is the box's only legal location.

Avoid the mistakes that make beginners restart

A digit can look correct in a row and still violate its box. Always make the three-part check. Another common mistake is treating the two missing digits in a unit as a known order. If neither cell is settled by a crossing constraint, leave both open and search elsewhere. Progress in another area may resolve them.

Do not interpret empty space as an instruction to guess. A Sudoku often asks you to move between different areas before one becomes solvable. If you discover a duplicate, examine your recent entries and notes. In a digital puzzle, an undo control can help when available; on paper, light pencil makes careful corrections easier.

Practise with a simple, repeatable checklist

Choose an easier puzzle while you are learning the relationship between rows, columns and boxes. Difficulty is about the deductions needed, not whether you finish quickly. Aim to understand a handful of correct moves, then build confidence by using the same checks on the next board.

1. Find a nearly complete unit and list its missing digits before testing individual cells.
2. Check each proposed entry against its row, column and box before confirming it.
3. Use notes when there are several possibilities, and refresh them after related placements.
4. After filling a cell, revisit the units it touches to look for a newly created opportunity.
5. When the grid is filled, verify that every row, column and box contains each digit once.

Worked example: follow each deduction

Read rows from top to bottom and columns from left to right, starting at 1. Read each instruction, then compare the orange changes with the previous picture. Keep undecided cells or segments open until the explanation settles them. This 4×4 number example uses digits 1–4, once in each row and column. Its boxes contain 2×2 cells.

? = undecided

This compact 4×4 Sudoku uses digits 1–4 and 2×2 boxes.

1. Follow these changes in order; the picture below includes all of them.

   • Write 1 at row 3, column 2. row 3 already contains 2, 3. Repetition is forbidden there. From this square’s remaining choices 1, 2, 3, only 1 survives.

   • Write 4 at row 3, column 3. row 3 still needs 4. Every other position in that unit has ruled it out, so this is its only remaining place.

   • Write 1 at row 2, column 3. column 3 already contains 2, 3, 4. Repetition is forbidden there. From this square’s remaining choices 1, 2, 3, only 1 survives.

2. Follow these changes in order; the picture below includes all of them.

   • Write 1 at row 1, column 1. the 2×2 box starting at row 1, column 1 already contains 4. Repetition is forbidden there. From this square’s remaining choices 1, 4, only 1 survives.

   • Write 2 at row 1, column 2. the 2×2 box starting at row 1, column 1 still needs 2. Every other position in that unit has ruled it out, so this is its only remaining place.

   • Write 3 at row 2, column 1. the 2×2 box starting at row 1, column 1 still needs 3. Every other position in that unit has ruled it out, so this is its only remaining place.

3. Follow these changes in order; the picture below includes all of them.

   • Write 2 at row 2, column 4. the 2×2 box starting at row 1, column 3 already contains 1, 3. Repetition is forbidden there. From this square’s remaining choices 1, 2, only 2 survives.

   • Write 4 at row 1, column 4. the 2×2 box starting at row 1, column 3 still needs 4. Every other position in that unit has ruled it out, so this is its only remaining place.

   • Write 3 at row 4, column 2. the 2×2 box starting at row 3, column 1 already contains 1, 2. Repetition is forbidden there. From this square’s remaining choices 1, 3, only 3 survives.

4. Follow these changes in order; the picture below includes all of them.

   • Write 4 at row 4, column 1. the 2×2 box starting at row 3, column 1 still needs 4. Every other position in that unit has ruled it out, so this is its only remaining place.

   • Write 1 at row 4, column 4. the 2×2 box starting at row 3, column 3 already contains 2, 3, 4. Repetition is forbidden there. From this square’s remaining choices 1, 4, only 1 survives.

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