🎮 How to Play
- Fill every empty cell with a digit 1-9.
- Each row, column, and 3×3 box must contain all 9 digits exactly once.
- Click Hint for help. Click Check to verify.
- Pick difficulty: Easy / Medium / Hard.
About this tool
Sudoku: a 9×9 grid divided into 3×3 boxes. Fill each cell with 1-9 so every row, column, and box contains all 9 digits exactly once. Easy puzzles give ~36 cells, medium ~30, hard ~24.
How to play / how it works
This is a browser-based game - no install, no signup, no ads inside gameplay. State persists in your browser's localStorage so your scores and progress survive page reloads.
Tips for improving
- Practice consistently - short daily sessions beat long weekend marathons for skill acquisition.
- Reduce, don't fight, distractions - turn off notifications. Browser games are at their best when you can give them 5 minutes of full attention.
- Aim slightly above current ability - if you're succeeding 100% of the time, raise the difficulty; if below 30%, lower it. The 60-80% success zone produces the fastest improvement.
- Track your scores - this game saves your best to localStorage automatically. Looking at the trend over weeks is motivating.
Why brain training works (and what doesn't)
Decades of research show that targeted practice improves the SPECIFIC skill being trained: typing games improve typing speed, sudoku improves logical pattern matching, chess improves spatial planning. The harder question - whether these gains transfer to general cognition or daily life - has weak evidence.
Best documented benefits:
- Reaction time: trainable up to 15% with deliberate practice over 4-6 weeks.
- Working memory: trainable for the specific task (n-back), modest transfer to similar tasks.
- Typing speed: 20-40% improvement is realistic in 60 days with daily 15-min practice.
- Visual-spatial reasoning: improves with games like Tetris, chess, 2048.
Browser storage and privacy
The game saves your best scores, settings, and progress to localStorage on your device. Nothing is uploaded. Clearing your browser data (or using Incognito) resets your scores. The game does not track your input - only the final score is stored.
Frequently asked questions
Do my scores sync across devices?
No. Scores are stored locally in your browser's localStorage. To play on a new device, your high scores reset.
Will the game work offline?
Yes - once the page has loaded once, it works without an internet connection. Modern browsers cache static assets.
Is the game keyboard-only or does it support touch?
Both, where applicable. Click/tap inputs work the same as keyboard inputs.
Are there ads?
Outside the gameplay area, yes (this is how a free site stays free). No ads or popups appear during play.
Can I see how I rank against others?
Not in this version - we don't run a leaderboard server. Your scores are private to your browser.
How accurate is the Sudoku?
It applies the standard formula. Accuracy is limited only by your input precision. For decisions with material consequences (taxes, medical, legal, structural), use the result as a starting point and verify with a qualified professional in the relevant field.
Is the Sudoku free to use?
Yes. 100% free, no signup, no payment, no API key. The site is funded by display ads around the tool but not inside the calculation flow.
Are my inputs saved anywhere?
No. All inputs stay in your browser tab. Closing the tab discards them. The site uses Google Analytics for traffic measurement (anonymized) but the analytics never see what you type into the form.
Can I use the Sudoku on my phone?
Yes. The tool is responsive and tested on iOS Safari, Android Chrome, and major desktop browsers. Touch targets meet Apple's 44pt and Google's 48dp minimum.
Does the Sudoku work offline?
Yes. Once the page has loaded, it works without internet. The calculation runs in JavaScript on your device.
How do I report a bug or suggest improvement to the Sudoku?
Email hi@3tej.com with the URL of this page and a description of what you saw vs expected. We typically respond within 72 hours.
Can I share results from the Sudoku?
Take a screenshot or copy the output. The page doesn't generate shareable URLs for specific calculations - inputs stay in your browser only.
Why are the results different from another sudoku tool?
Most likely: different formula assumptions, different default values, different rounding rules, or different applicable rates. Check the methodology if both tools document it. Both can be valid for different scenarios.
The complete guide to Sudoku
Sudoku is a logic-based, combinatorial number-placement puzzle. The objective is to fill a 9×9 grid with digits so that each column, each row, and each of the nine 3×3 subgrids contain all of the digits from 1 to 9. The puzzle setter provides a partially completed grid, which typically has a unique solution.
History of Sudoku
Although Sudoku is most associated with Japan, the modern puzzle was actually designed by American architect Howard Garns in 1979 and published as "Number Place" in Dell Magazines. The puzzle gained popularity in Japan in 1986 under the name Sudoku, which is short for "Suuji wa dokushin ni kagiru" - meaning "the digits must be single." The puzzle exploded internationally in 2004 when The Times newspaper in London began publishing daily Sudoku, leading to widespread global adoption.
Sudoku solving techniques
Sudoku puzzles can be solved using a hierarchy of techniques, from beginner to expert:
- Naked Singles: A cell where only one digit is possible. Scan rows, columns, and boxes to identify cells where 8 of 9 digits are eliminated.
- Hidden Singles: A digit that can only fit in one cell of a row, column, or box, even if that cell has multiple candidates from other constraints.
- Naked Pairs / Triples: Two cells in the same unit that share exactly the same two candidates eliminate those candidates from other cells in that unit.
- Pointing Pairs: When a candidate digit appears in only one row or column within a 3×3 box, it can be eliminated from the rest of that row or column.
- X-Wing: Advanced pattern where a candidate appears exactly twice in two rows, in the same two columns. The candidate can be eliminated from those columns elsewhere.
- Swordfish, Jellyfish, XY-Wing: Expert-level patterns for the hardest puzzles.
Difficulty levels and clue counts
The minimum number of clues that produces a uniquely solvable Sudoku is 17 - proven mathematically in 2012 by Gary McGuire's exhaustive search. Most newspaper puzzles use 22-30 clues. Difficulty rating depends not just on clue count but on which solving techniques are required:
- Easy: 36-40 clues, requires only Naked/Hidden Singles
- Medium: 30-35 clues, requires Naked Pairs and Pointing Pairs
- Hard: 26-30 clues, requires X-Wing and Box-Line Reduction
- Expert/Diabolical: 22-26 clues, requires Swordfish, Forcing Chains, etc.
Sudoku variants
Killer Sudoku adds cages of cells whose sums are given. Mini Sudoku uses 6×6 grids with 2×3 boxes. Hyper Sudoku adds four extra 3×3 regions. Diagonal (X) Sudoku requires both diagonals to also contain 1-9. Samurai Sudoku overlaps five 9×9 grids.
Health benefits of Sudoku
Multiple studies have linked regular Sudoku play with improved working memory, faster information processing, and reduced risk of dementia. A 2019 study in the International Journal of Geriatric Psychiatry, surveying over 19,000 adults aged 50-93, found that those who frequently played number puzzles performed equivalent to people 8 years younger on tests of grammatical reasoning and short-term memory accuracy. While not a cure for cognitive decline, daily Sudoku is a low-cost, evidence-supported brain workout.