sudoku · 6 min read

How to Play Sudoku

Master the grid through the absolute placement of non-repeating digits.

Sudoku is the world's most-played logic puzzle: a 9×9 grid where you fill in digits so each row, each column, and each 3×3 box contains every digit from 1 to 9 exactly once. No arithmetic, no guessing — just careful reasoning.

This guide walks through a real easy puzzle from start to finish, explaining the rules and the two foundational solving techniques along the way.

The Puzzle

Here is the puzzle we'll solve. The bold black numbers are the “givens” — they are fixed. Your job is to fill the 41 blank cells. Notice the thicker grid lines marking the nine 3×3 boxes.

The starting puzzle: 40 givens, 41 blanks to fill.

The Three Rules

Every Sudoku obeys the same three rules, no matter how easy or expert.

Rule 1: Each ROW must contain digits 1 through 9, each exactly once

Allowed: row contains digits 1-9, each exactly once.

Not allowed: digit 8 appears twice. Every row must contain each digit only once.

Rule 2: Each COLUMN must contain digits 1 through 9, each exactly once

Allowed: column contains all nine digits, no repeats.

Not allowed: digit 7 appears twice in the same column.

Rule 3: Each 3×3 BOX must contain digits 1 through 9, each exactly once

The grid has nine 3×3 boxes — three across and three down. The thicker grid lines mark their boundaries.

Allowed: top-left 3×3 box contains 1-9 with no repeats.

Not allowed: digit 5 appears twice in the same 3×3 box.

How to Start: Two Foundational Techniques

On easy puzzles you almost never need anything more than these two techniques.

Naked single — pick a blank cell, list every digit already in its row, column, and 3×3 box. If only one digit is left over, that's the answer for the cell.
Hidden single — pick a digit and a single row, column, or box. If only one cell in that group can hold the digit (every other cell is blocked), that cell must hold it.

The walkthrough below shows both techniques on real cells from our puzzle.

Walkthrough

Step 1 — Naked single at row 1, column 4

Look at the empty cell at row 1, column 4 (the fourth cell of the top row). We'll scan its row, column, and 3×3 box and see what digits are still possible.

Row 1 already contains 1, 2, 3, 5, 9. So this cell can only be 4, 6, 7, or 8.

Column 4 already contains 2, 3, 6, 8. That further narrows it to 4 or 7.

The top-middle 3×3 box already contains 1, 2, 7, 8. Only 4 survives all three scans.

Combined, the row, column, and box block every digit except 4. Place it.

Step 1 complete: row 1, column 4 = 4.

Step 2 — Hidden single in the top-left box: where does 8 go?

Now look at the top-left 3×3 box. It already has the digits 3, 5, and 1. Six digits are still missing. Let's ask: where can the digit 8 go in this box?

The top-left box: which of its blanks can hold an 8?

Check each blank cell in this box:

(row 1, col 1): column 1 already has an 8 (at row 8) → blocked.
(row 1, col 3): column 3 already has an 8 (at row 9) → blocked.
(row 2, col 1): column 1 has an 8 → blocked.
(row 2, col 2): row 2 already has an 8 (at column 6) → blocked.
(row 2, col 3): row 2 has an 8 → blocked.
(row 3, col 1): column 1 has an 8 → blocked.

Only one cell — row 1, column 3 — can hold the 8. Place it.

Step 2 complete: row 1, column 3 = 8.

Step 3 — Hidden single in row 5: where does 7 go?

Row 5 reads 2 _ 4 3 9 _ _ _ 1. It needs the digits 5, 6, 7, and 8. Let's ask: where can 7 go?

Row 5: which blank can hold a 7?

The four blanks in row 5 are columns 2, 6, 7, and 8. Check each:

(row 5, col 6): column 6 already has a 7 (at row 3) → blocked.
(row 5, col 7): column 7 has a 7 (at row 6) → blocked.
(row 5, col 8): the middle-right 3×3 box already has a 7 (at row 6, col 7) → blocked.

Only column 2 can hold the 7 in this row. Place it.

Step 3 complete: row 5, column 2 = 7.

The Solved Grid

Continuing with the same scanning technique, every remaining blank gets filled in. Here is the completed puzzle:

Solved! Every row, column, and 3×3 box contains digits 1-9 exactly once.

Tips for Beginners

Scan one digit at a time. Pick a digit (say 1). Find every row, column, and box that already contains it. The other groups are where 1 still needs to land — and often only one cell qualifies.
Look for crowded houses. A row, column, or box that already has 7 or 8 of its 9 digits is the easiest place to spot a forced move.
Pencil in candidates. When a cell could be 2 or 3 different digits, lightly note them in the corner. As nearby cells fill in, candidates get eliminated and the cell becomes a naked single.
Never guess. Every well-made Sudoku has exactly one solution, reachable by pure logic. If you find yourself guessing, look harder for a forced move you missed.
Build pattern recognition. Start with easy puzzles. The same scanning technique works at every difficulty — harder puzzles just hide their forced moves better.

Ready to give it a real shot? Hit the button below to play your first Sudoku.

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