Sudoku

About Sudoku

Sudoku is the world's most popular logic puzzle, and arguably the single cleanest puzzle ever designed. The rules fit on a postcard, the only required mental tool is logic (no arithmetic beyond counting), and the depth genuinely scales from "ten-minute coffee break" to "international competition." The game was popularised in Japan in the 1980s by puzzle publisher Nikoli, who gave it its current name (a contraction of "suuji wa dokushin ni kagiru", meaning "the numbers must remain single"), but the underlying concept goes back to the Latin square work of Swiss mathematician Leonhard Euler in the 18th century. The modern boom started in late 2004, when British puzzle developer Wayne Gould placed his computer-generated puzzles in The Times of London. Within a year every major newspaper in the English-speaking world had a daily Sudoku, and the app stores have not stopped since.

Our HTML5 version is a full-feature single-player implementation: three difficulty levels with different clue counts, a hint system (limited per game, so you have to think first), pencil-mark notes for tracking candidates, a three-mistake limit that forces careful play, undo support, and a best-time tracker that saves per-difficulty records to your browser. The puzzle generator builds a fresh, valid board every game, so you will never see the same puzzle twice. Everything runs locally in your browser, no account required, and the game works equally well on a desktop keyboard and a phone touchscreen.

How to Play Sudoku

The goal is to fill the 9x9 grid so that every row, every column, and every one of the nine 3x3 boxes contains all the digits from 1 to 9 with no repeats. A starting puzzle gives you some of the digits already filled in (the "clues"); your job is to deduce the rest from logic alone. Every valid Sudoku puzzle has exactly one solution that can be reached without guessing.

To play, tap or click any empty cell on the grid to select it. Then tap a number from the number pad below (or press 1 to 9 on your keyboard) to place that digit in the selected cell. If you change your mind, tap the same number again to clear it, use the Erase button, or press Backspace. Use the Notes button to switch into pencil-mark mode, which lets you mark candidate digits in a cell without committing to a single answer; this is essential for the Medium and Hard difficulties. The Hint button reveals the correct value in the currently selected cell, but each game limits how many hints you can use, so save them for when you are genuinely stuck.

The Three Difficulty Levels

• Easy (40 clues): Most cells can be solved by direct scanning. A patient beginner can finish in five to fifteen minutes. This is the right tier to learn the controls and the basic technique of "if every other row in this box already has a 3, the 3 has to go here."
• Medium (32 clues): Requires you to track candidates using pencil marks. Many cells cannot be solved by scanning alone; you have to combine information from two or three rows and columns. Typical solve time is twelve to thirty minutes.
• Hard (26 clues): Demands the intermediate techniques. Naked pairs, pointing pairs, and X-wings start to appear. Plan on twenty minutes to an hour. There are no impossible puzzles at this tier (the generator only produces valid one-solution boards), but you will need every tool in your kit.

Strategy Guide: 7 Techniques That Cover Most Puzzles

1. Scan rows, columns, and boxes for forced placements. For each digit 1 to 9, look at every row, column, and 3x3 box. If a digit already appears in eight of the nine cells of a unit, the ninth cell must be that digit. This is the most basic technique and solves most of an Easy puzzle by itself.
2. Use pencil marks early. Once scanning stops producing placements, fill in candidate notes for every empty cell. Yes, it is tedious. It is also the foundation of every advanced technique. Most people who get stuck on Medium puzzles got stuck because they skipped this step.
3. Look for naked singles. After pencil marks are in, scan for any cell that has only one candidate digit. That digit is the answer for that cell. Place it, then update the surrounding pencil marks and look for the next naked single.
4. Hunt for hidden singles. Even when a cell has multiple candidates, a digit might appear as a candidate in only one cell of a row, column, or box. That digit must go in that cell, regardless of what other candidates the cell shows.
5. Spot naked pairs. If two cells in the same row, column, or box both have exactly the same two candidates (and only those two), then those two digits are locked into those two cells. You can remove them from the candidates of every other cell in that unit.
6. Use pointing pairs. If a digit's candidates within a 3x3 box are all in the same row or column, that digit must go somewhere in that row or column inside the box. You can therefore remove that digit from the candidates of every cell in that row or column outside the box.
7. Always work the easiest cell first. When you have multiple options, place the digit that has the most certainty. Building from your strongest deductions outward is much faster than guessing on a half-pencilled cell and unwinding the mess later.

Common Mistakes I See Players Make

• Guessing instead of deducing. Sudoku is a pure logic puzzle. Every valid Sudoku has exactly one solution that can be reached without guessing. If you find yourself wanting to "try a 5 here and see what happens," you have missed a deduction; back up and look again.
• Skipping pencil marks on Medium and Hard. Trying to hold the candidate sets in your head past about three pencilled cells is asking for mistakes. Use the Notes button.
• Forgetting to update notes after every placement. Each digit you place eliminates that digit from the candidates of nine other cells (its row, column, and box neighbours). Update the pencil marks immediately; stale notes are worse than no notes.
• Burning hints on cells you could solve. Hints are finite. Use them when you are genuinely stuck on the whole board, not as a shortcut on the first hard cell you encounter.
• Playing tired. Sudoku is the kind of puzzle that feels easy when you are alert and impossible when you are not. If you are making three careless mistakes in a row, take a five-minute break and come back.

Why Sudoku Is Worth Your Time

Sudoku has become a default daily ritual for millions of people for a real reason: the puzzle hits a sweet spot between mental engagement and relaxation that is hard to find anywhere else. Unlike crosswords, Sudoku requires no prior knowledge, which makes it the rare puzzle that translates across languages and cultures with no adjustment. Unlike chess or Go, the game has a defined ending point, so a single session has a natural endpoint and doesn't expand to fill all available time. And unlike most casual phone games, Sudoku rewards careful thinking rather than fast reflexes, which makes it a good fit for both kids learning logical reasoning and older adults who want to keep their attention and working memory active.

Research on cognitive activity in older adults consistently finds that regular engagement with novel logic puzzles correlates with better maintenance of attention and processing speed over time. The honest caveat is that the effects are smaller than the brain-training industry claims, and the benefits are highly task-specific (getting good at Sudoku does not automatically make you better at, say, chess). But as one component of a varied daily mental routine, Sudoku is one of the most accessible and enjoyable options. See our longer piece on whether casual browser games actually train your brain for the honest version of the research.

Further Reading

• Do Casual Browser Games Actually Train Your Brain? The honest research on logic-puzzle cognitive benefits.
• How to Improve Your Memory with Browser Games Sudoku as part of a daily mental routine.
• 7 Best Casual Games to Play in 5 Minutes Why Sudoku is better for longer sessions than short breaks.
• Free Memory Games for Seniors How to introduce Sudoku to older players.