How to Play Sudoku Classic

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Sudoku Classic is a puzzle game that is easy to learn but has enough depth to keep improving at. This guide walks you through the rules, the controls and the key tips beginners need to play confidently within a couple of rounds. You can play right here on this page — the game is embedded above.

— The basics

What is Sudoku Classic?

Sudoku Classic is the full 9×9 number puzzle that has been the world's most popular logic game for two decades. The rules are simple to state: fill in the empty cells so that every row, every column and every 3×3 box contains the digits 1 through 9 exactly once. But those simple rules produce a puzzle with astonishing depth — every solve is a chain of deductions, each one unlocking the next, until the entire grid crystallises into a single valid solution.

This version offers three difficulty levels — Easy (about 35 clues), Medium (about 28 clues), and Hard (about 22 clues). Easy puzzles can typically be solved with single-cell logic (a cell where only one digit fits), while Medium and Hard puzzles require intersection techniques and pencil-mark tracking. The number of clues alone does not determine difficulty; the real difficulty comes from the logical patterns required. This is why professional Sudoku puzzles have widely varying "clue counts" but consistently rated difficulty — the setter chooses clues that force the required technique level.

— Step-by-step controls

How to play — the steps

Step 1: Pick a difficulty on the Play overlay
Step 2: Tap a cell, then a digit to fill it
Step 3: Toggle ✏️ Notes for pencil marks
Step 4: Each row, column and 3×3 box must contain 1–9

— Beginner tips

Beginner tips for Sudoku Classic

Start with the "single candidate" scan — find cells where only one digit is legally possible.
When stuck, pick a digit (say 4) and scan every row, column and box for where it must go.
Use notes / pencil marks aggressively on Medium and Hard — memory alone is not enough.
Look for "naked pairs": two cells in a row/col/box that can only hold the same two digits.
Intersections between a row and a box often force a digit into a narrow strip.
If a digit appears seven times on the board, the two remaining cells are usually easy to find.
Do not guess. Every Sudoku puzzle on this site has a unique solution reachable by pure logic.
Use digit highlighting — tap a cell with a value to see every same-value cell at once.

— Advanced strategy

Level up — advanced Sudoku Classic strategy

The highest-value habit in Sudoku is disciplined pencil-marking on any puzzle above Easy. Beginners try to solve mentally, which works for the first ten or fifteen cells but breaks down when the puzzle requires reasoning about the intersection of two or three constraints. Expert solvers pencil-mark every cell that has more than a few candidates and then use those marks to spot patterns — naked pairs, hidden pairs, X-wings, and so on — that are simply invisible without written possibilities. The tradeoff is that pencil-marking takes time; the return is that it unlocks entire classes of puzzle you cannot solve without it. Turning on Notes mode from move one on Medium and Hard puzzles is what separates the players who finish Hard from the players who give up.

The second big lever is scanning discipline. Rather than staring at one cell hoping for inspiration, expert solvers scan the board digit by digit: pick 1, look at every row, column and box, and place any 1s that are forced. Then move to 2, then 3, all the way to 9. This "single digit sweep" catches easy placements that fixed-cell staring misses, and it converts stuck moments into progress. The best solvers cycle through this scan multiple times per solve, especially after each significant placement, because a single new number can unlock a chain of previously ambiguous cells.

— Where to play

Play Sudoku Classic free right now

You can play Sudoku Classic free on GameJadoo — no download, no sign-up, works on any modern phone, tablet or computer. The game is embedded above so you can start playing while the guide is still open, or visit the full Sudoku Classic game page for related guides, achievements and share options.

— Frequently asked questions

How to play Sudoku Classic — FAQ

Is Sudoku Classic hard to learn?

No. Sudoku Classic is designed so anyone can pick it up in under a minute. The full ruleset above is short, the controls are intuitive, and most players are playing confidently by their second or third round.

Do I need to install anything to play Sudoku Classic?

No. Sudoku Classic runs directly in your web browser using HTML5. There is no installer, no download, no plugin — just open the page and play.

Can I play Sudoku Classic on my phone?

Yes. Sudoku Classic works on phones and tablets with touch controls. The controls scale to any screen size, and you can play in portrait or landscape.

What is the best strategy for Sudoku Classic?

Start with the basics — Start with the "single candidate" scan — find cells where only one digit is legally possible. As you get more comfortable, the tips section above covers the advanced techniques that separate casual play from personal-best runs.