Sudoku Rules: How to Play and Solve Sudoku

Updated: Oct 18, 2026 · 🎮 Single player

Sudoku is a logic puzzle where you fill a grid of 9 rows, 9 columns and nine 3×3 boxes with the digits 1 to 9. There is just one rule: every digit appears exactly once in each row, each column and each box. Every puzzle on Zipi has a single solution and never needs guessing; logic is always enough.

▶ Play Sudoku Daily nowFree, no download, no sign-up.

The sudoku grid

A sudoku board has 81 cells divided into three kinds of region:

• Rows: nine horizontal lines of 9 cells.
• Columns: nine vertical lines of 9 cells.
• Boxes: nine 3×3 squares separated by thick lines.

Some cells are filled when the puzzle starts; these givens cannot be changed. Your job is to fill the empty cells.

Sudoku rules

1. Each cell takes one digit from 1 to 9.
2. A digit may not repeat in the same row.
3. A digit may not repeat in the same column.
4. A digit may not repeat in the same 3×3 box.

As a result every row, column and box contains each digit 1-9 exactly once. There is no arithmetic involved; the digits are just symbols.

Basic solving techniques

• Naked single: if only one digit fits a cell once you check its row, column and box, that digit goes there.
• Hidden single: if a digit can only go in one cell of a row, column or box, it goes there even if that cell has other candidates.
• Locked candidates: if all of a digit's candidates in a box lie in one row, that digit can be removed from the rest of that row outside the box.
• Naked and hidden pairs: if two cells in a region can only hold the same two digits, those digits can be removed from the region's other cells.

Example: if a row already contains 1, 2, 3, 5, 6, 7, 8 and 9, the empty cell can only be 4. That is the simplest naked single.

Difficulty levels

On Zipi difficulty is set by the most advanced technique a puzzle needs:

The Puzzle of the Day follows the week: Monday is easy, Tuesday and Wednesday medium, Thursday and Friday hard, Saturday expert and Sunday extreme.

Notes, hints and the mistake rule

• Pencil notes: jot down possible candidates in small digits, or fill them in automatically with one tap.
• Highlights: selecting a digit highlights all its copies plus the row, column and box; remaining-digit counters show how many of each are left.
• Hints: reveal the next cell that logic can find and name the technique used. If there is a wrong digit, the hint finds and clears it first. You start with 3 hints and earn one more for each puzzle solved.
• Mistake rule: with mistake highlighting on, a wrong digit flashes red and three mistakes end the puzzle. Turn highlighting off to solve entirely on your own; when the board is full, any wrong cells are shown.

Scoring

Finish under the target time and you earn 2 points for every second to spare. Solve with no mistakes and the total bonus rises by 20%. Your best time without hints is saved as a record.

Common mistakes

• Guessing: Zipi's puzzles never require it, so make a note instead of writing a digit you are unsure of.
• Stale notes: after placing a digit, remove it from the notes in the same row, column and box, or use auto-notes.
• Only scanning rows: boxes often hide the easiest deductions.
• Getting stuck on one cell: move to another region instead of staring at a hard cell.

Sudoku tips for beginners

1. Start with the fullest regions: rows, columns or boxes with few gaps give up singles easily.
2. Scan digit by digit: pick a digit and use its existing copies to see where it can go in each box.
3. Hunt hidden singles: finding the only cell for a digit in a box is often the fastest progress.
4. Make notes a habit: from Medium upwards you cannot see pairs and locked candidates without them.
5. Learn from hints: hints name the technique, so try applying it yourself in the next puzzle.
6. Climb gradually: the career mode introduces techniques in order.

Play sudoku online on Zipi

Sudoku is free in your browser on Zipi with no download. Everyone solves the same Puzzle of the Day; solved days are marked on a calendar, consecutive days grow your streak, and you can go back to a missed day. In Career mode puzzles get harder level by level; if you lose you can watch an ad to continue with one mistake left, or start again. In Free play you choose one of five difficulties for endless puzzles. Sudoku is single-player, so there is no online opponent.

FAQ