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Classic Sudoku - The Definitive 9x9 Logic Puzzle

Classic Sudoku is the gold standard of logic puzzles. Played on a 9x9 grid divided into nine 3x3 boxes, your objective is straightforward: place the numbers 1 through 9 so that every row, every column, and every box contains each digit exactly once. No guessing is required — every puzzle on Mini Puzzles can be solved through pure deduction, and each one has a single unique solution.

What makes Classic Sudoku so enduringly popular is the way it balances simplicity and depth. The rules take seconds to learn, yet the puzzles scale from gentle warm-ups to fiendishly difficult challenges that demand advanced techniques like X-Wings and Swordfish. Whether you are solving your first grid or your ten-thousandth, there is always a new pattern to discover and a faster time to chase.

On Mini Puzzles you can tackle a fresh daily puzzle that is the same for everyone, jump into unlimited randomly generated grids whenever you want more, or use the built-in solver to study solutions step by step. The interface works seamlessly on desktop and mobile, supports dark mode, and saves your progress automatically — so you can pick up right where you left off.

How to Play Classic Sudoku

The rules of Classic Sudoku are simple: fill every row, column, and 3x3 box with the numbers 1 through 9, using each number exactly once. Start from the given clues, apply logic to eliminate possibilities, and work your way to the complete solution.

No math is involved and no guessing is needed. Every puzzle has one unique answer that can be reached through careful reasoning alone.

Rules

  1. 1

    Understand the Grid

    The puzzle is a 9x9 grid broken into nine 3x3 boxes. Some cells are pre-filled with numbers — these are your starting clues and cannot be changed.

  2. 2

    One Number Per Row

    Each row of nine cells must contain every digit from 1 to 9 without repetition. If a row already has a 5, no other cell in that row can hold another 5.

  3. 3

    One Number Per Column

    The same rule applies vertically. Each column of nine cells must also contain the digits 1 through 9 exactly once.

  4. 4

    One Number Per 3x3 Box

    Each of the nine 3x3 boxes outlined by thicker borders must contain the digits 1 through 9 with no repeats. This constraint is what makes Sudoku more than just a row-and-column exercise.

  5. 5

    Select a Cell and Enter a Number

    Click or tap an empty cell to select it, then choose a number from the number pad (or use your keyboard, 1-9). The number will be placed in the selected cell. If it conflicts with an existing number in the same row, column, or box, the conflict will be highlighted.

  6. 6

    Use Notes to Track Candidates

    Toggle notes mode (pencil icon) to jot down possible candidates in a cell. Notes appear as small numbers and help you narrow down options when the answer is not immediately obvious.

Solving Strategies

Naked Singles

When only one candidate remains for a cell after eliminating all digits already present in its row, column, and box, that candidate must be the answer. This is the most fundamental technique and the one you will use most often.

Hidden Singles

Sometimes a digit can only fit in one cell within a row, column, or box, even though that cell has multiple candidates. Scan each group for digits that appear as a candidate in only one place — that is where they belong.

Scanning (Cross-Hatching)

Pick a number and scan rows and columns to see where it is already placed. Use those placements to eliminate possibilities in intersecting boxes. This visual sweep often reveals the only legal spot for that digit.

Naked Pairs

If two cells in the same row, column, or box share the exact same two candidates and no others, those two digits can be removed from all other cells in that group. This clears the way for further deductions.

Controls

Mouse / Trackpad

Click a cell to select it, then click a number on the number pad to place it. Click the pencil icon to toggle notes mode. Use Undo to revert your last action or Clear to empty the selected cell.

Keyboard Shortcuts

Use arrow keys to navigate between cells. Press 1-9 to enter a number. Press Delete or Backspace to clear a cell. Press N to toggle notes mode. Press Ctrl+Z (or Cmd+Z) to undo.

Frequently Asked Questions

What is Classic Sudoku?

Classic Sudoku is a logic-based number-placement puzzle played on a 9x9 grid. The grid is subdivided into nine 3x3 boxes. Some cells start pre-filled with digits (called "givens" or "clues"), and your goal is to fill every remaining cell so that each row, each column, and each 3x3 box contains the numbers 1 through 9 exactly once. The puzzle requires no arithmetic — only logical deduction. Every properly constructed Sudoku has one and only one valid solution.

What do the difficulty levels mean?

Difficulty is determined by the number of clues provided and the complexity of the techniques needed to reach the solution:

  • Easy — More starting clues and solvable with basic techniques like naked and hidden singles. A great entry point for newcomers.
  • Medium — Fewer clues and occasional need for scanning or simple elimination strategies. Good for building confidence.
  • Hard — Fewer clues still, requiring intermediate techniques like naked pairs, pointing pairs, or box-line reduction.
  • Expert — Minimal clues with puzzles that may demand advanced strategies such as X-Wings, Swordfish, or chains. Designed for seasoned solvers.
What strategies should I learn first?

Start with these three foundational techniques and you will be able to solve most easy and medium puzzles:

  1. Naked Singles — Check what numbers already exist in a cell's row, column, and box. If only one possibility remains, that is the answer.
  2. Hidden Singles — Within a row, column, or box, look for a digit that can only go in one cell, even if that cell has other candidates.
  3. Cross-Hatching — Choose a digit and scan its existing placements across the grid. Use row and column eliminations to find the only possible cell in a given box.

Once these become second nature, explore paired and grouped elimination techniques to tackle harder difficulties.

Any tips for solving puzzles faster?
  • - Scan the full grid first. Before placing a single digit, survey all the givens. Identify rows, columns, or boxes with the most clues — they are easiest to complete.
  • - Use notes liberally. Pencil marks reveal hidden patterns. Fill in candidates for every empty cell in a region, then eliminate methodically.
  • - Work in passes. Cycle through the grid using one technique at a time — singles first, then pairs — rather than jumping around randomly.
  • - Focus on the most constrained areas. Cells at the intersection of a nearly full row, column, and box have the fewest possibilities and are often solvable at a glance.
  • - Practice daily. Speed comes from pattern recognition, and pattern recognition comes from repetition. The daily puzzle is a great way to build consistency.
What keyboard shortcuts are available?

When playing on a device with a physical keyboard you can use the following shortcuts for a faster solving experience:

Arrow Keys Move selection between cells
1 - 9 Place a number in the selected cell
Delete / Backspace Clear the selected cell
N Toggle notes (pencil) mode
Ctrl+Z / Cmd+Z Undo the last action
Is Classic Sudoku on Mini Puzzles free?

Yes, completely free. There are no ads, no account requirements, and no paywalls. You can play the daily puzzle, generate unlimited random puzzles, or use the solver at no cost. The game runs entirely in your browser, works on any device, and your progress is saved locally so you never lose your work.

How is Classic Sudoku different from Mini Sudoku?

Mini Sudoku uses a 6x6 grid with numbers 1-6 and 2x3 boxes, making it quicker and more accessible for beginners. Classic Sudoku uses the full 9x9 grid with numbers 1-9 and 3x3 boxes, offering a deeper challenge with 81 cells to fill instead of 36. If you enjoy Mini Sudoku and want a step up in complexity, Classic Sudoku is the natural next level.

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