How to Play Classic Sudoku — Rules from 6×6 to 25×25

Cubedoku Guides · Published 2026-06-10

Sudoku is one of the world's most popular logic puzzles for a simple reason: the rules take thirty seconds to learn, yet the game can challenge you for a lifetime. No maths is required — the digits are just symbols, and you could play the same puzzle with letters or colours. That makes Sudoku a genuinely all-ages game: children, commuters and grandparents all solve the same kind of grid, just at different sizes and speeds.

The three rules

A classic 9×9 Sudoku grid is divided into nine rows, nine columns, and nine 3×3 boxes. The goal is to fill every empty cell with a digit from 1 to 9 so that:

each row contains every digit exactly once,
each column contains every digit exactly once,
each 3×3 box contains every digit exactly once.

That's the entire rulebook. Every puzzle starts with some cells already filled in — these are called givens or clues— and a properly made puzzle (every puzzle in Cubedoku is verified this way) has exactly one solution reachable by logic alone. You never need to guess.

Grid sizes in Cubedoku Classic

Cubedoku's Classic mode offers four grid sizes, and the size you pick changes the feel of the game more than the difficulty setting does:

6×6 — digits 1–6 with 2×3 boxes. The friendliest entry point: small enough for younger players or a five-minute coffee break, but still real Sudoku with all three constraints.
9×9 — the standard grid everyone knows from newspapers. The reference size for learning solving techniques.
16×16 — digits 1–16 with 4×4 boxes. The same logic, but scanning takes longer and your working memory gets a proper workout.
25×25 — a giant 5×5-box grid for marathon sessions. Endurance solving: the techniques are identical, the discipline required is not.

Your first solve, step by step

Open an Easy 9×9 puzzle and try this routine:

Pick the most crowded region. Find the row, column or box with the most givens. The fewer empty cells a region has, the easier it is to finish.
Ask “where can this digit go?” Take a digit that appears often in the grid and check each box that is still missing it. Often the row and column constraints leave only one possible cell — that placement is forced.
Ask “what can this cell be?” For a nearly surrounded empty cell, list which digits 1–9 are not already in its row, column and box. If only one digit survives, fill it in.
Repeat. Every digit you place creates new forced placements elsewhere. Easy puzzles can be solved entirely with these two questions.

When the two basic questions stop producing answers, it's time for pencil marks and pattern techniques — see our beginner tips and intermediate techniques guides.

Difficulty levels

Each size comes in four difficulties — Easy, Medium, Hard and Expert. Difficulty in Cubedoku is driven by how many cells are removed from the solved grid, calibrated per size so that “Hard” means the same relative challenge on a 6×6 as on a 25×25. Easy puzzles need only the two basic questions above; Expert puzzles expect pattern-based eliminations and long deduction chains.

Helpful features while you learn

Cubedoku checks your entries in real time — a wrong digit is highlighted immediately, so you learn from mistakes instead of discovering them twenty minutes later. Notes mode lets you record candidate digits in a cell, and the hint button can nudge you past a wall (hints are disabled in Daily Challenges to keep the leaderboard fair). If you want the purist experience, Perfect mode tracks whether you complete a puzzle with zero wrong entries.

Where to go next

Once 9×9 feels comfortable, you have two directions: scale up to 16×16 and 25×25, or change the geometry entirely. The Cube wraps Sudoku around a 3D cube, Skyscraper swaps boxes for line-of-sight clues, and Blitz adds a ticking clock. All of them build on exactly the rules you just learned.

Keep reading

Or put it into practice — play Cubedoku free in your browser, no download or account needed.