🔢

Live · Ready to playpuzzle

Sudoku Classic

🔄Rotate your phone for the best experience

About 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.

Beyond the base rules, this Sudoku Classic includes the quality-of-life features every serious solver expects: a notes mode for pencil marks, digit highlighting (so tapping a 7 shows every 7 already on the board and every cell in the same row, column and box), and a timer that saves your best solve time per difficulty separately. Those features shorten the mechanical work of tracking possibilities and let you focus on the interesting part — the reasoning.

Sudoku suits both quick five-minute breaks and longer meditation-style sessions. A Hard-difficulty puzzle can easily run 15-25 minutes even for experienced solvers, while an Easy puzzle can be cleared in three or four minutes once you know the standard techniques. The game runs entirely in your browser, needs no download and no account, and works equally well on phone, tablet and desktop. Your best times are stored locally so every session gives you something to improve on.

— How to play

How to play Sudoku Classic

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

— Controls

Sudoku Classic controls

Desktop (mouse & keyboard)

Use arrow keys, WASD, the mouse or spacebar where the game requires it. Specific controls match the "How to play" steps above — each step describes the exact input the game expects.

Mobile (touchscreen)

Tap, hold, swipe or drag — whichever your finger naturally does for the action described in the steps. Sudoku Classic is mobile-first and works in portrait or landscape on any modern phone or tablet.

— Strategy & tips

How to win

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.

— Game features

Why you'll love it

Full 9×9 board with authentic Sudoku rules
Three difficulty levels: Easy, Medium, Hard
Notes / pencil-mark mode with a toggle
Digit + row / column / box highlighting on selection
Timer and best-time tracking per difficulty

— Origin & history

The story behind Sudoku Classic

The 9×9 Latin-square puzzle with 3×3 subgrids was originally published as "Number Place" in the American puzzle magazine Dell Pencil Puzzles & Word Games in May 1979, likely designed by Howard Garns. The format was picked up by the Japanese publisher Nikoli in 1984, where it was given the name "Suuji wa dokushin ni kagiru" (roughly "the numbers must be single"), abbreviated to Sudoku. The puzzle remained popular in Japan through the 1990s and then exploded internationally in late 2004 when The Times of London began publishing daily Sudoku puzzles, quickly followed by newspapers worldwide. By 2005, Sudoku was a global craze; by 2010, it was a fixture of every newspaper puzzle page in the English-speaking world. Its enduring appeal is that it requires no vocabulary, no cultural knowledge and no maths — just pure pattern recognition and logical deduction, which travels perfectly across languages and cultures.

— Advanced strategy

Master-level Sudoku Classic

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.

— Frequently asked questions

Sudoku Classic FAQ

What is the minimum number of clues for a valid Sudoku?

Mathematicians proved in 2012 that the minimum number of clues for a valid Sudoku with a unique solution is 17. Puzzles with 16 or fewer clues always have multiple solutions, which violates the "unique solution" rule that makes Sudoku a genuine puzzle. Most published Hard-level Sudokus have 22-26 clues; anything below 20 tends to require very advanced techniques and is more common in specialist puzzle collections than newspapers.

Do I need to guess on Hard difficulty?

No — well-designed Sudoku puzzles at every difficulty are solvable by pure logic without guessing. If you feel forced to guess, it usually means you have missed a technique like a naked pair or a pointing intersection. The puzzles in this game are drawn from valid full solutions with unique-solution masking, so a purely logical path exists on every one. Learning to spot the harder techniques is exactly the reward for playing Hard difficulty.

Why does the timer save separately per difficulty?

Because a 4-minute Easy solve and a 20-minute Hard solve are completely different achievements. Storing a single "best time" would either make Easy times look artificially fast or make Hard times feel undervalued. By separating the three timers, you have three different long-term goals to work on and each level rewards focused practice.

— Strategy guides

Read our Sudoku Classic guides

All guides →

Best Brain-Training Puzzle Games for Adults (2026)
The best free brain-training puzzle games for adults in 2026 — memory, logic, math and reflex picks that actually build measurable cognitive skills.
Best Games to Play in Boring Zoom Meetings (Discreet Picks)
The best discreet browser games to play during boring Zoom meetings — quiet, one-handed, tab-hideable and safe to close instantly if your name gets called.
Best Free Games for Chromebook — Works at School (2026)
The best free browser games for Chromebook in 2026 — fast on 4GB RAM, no download, no Play Store needed, and unblocked on most school Chromebooks.

— Similar games