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There are many ways how you can flex your mental muscles and have a lot of fun at the same time, but you would be hard-pressed to find one that’s as accessible, engaging, and challenging as Sudoku.
Even if you don’t like math and always rely on a calculator, there’s a pretty good chance that you’ll fall in love with Sudoku and the numerous benefits of playing it, including improved memory, increased concentration, and a better sense of time.
What Is Sudoku?
Sudoku is a puzzle game that tasks its players with filling a 9×9 grid with digits so that each of the nine 3×3 subgrids, as well as each column and each row, contains all of the digits from 1 to 9. This task is made more difficult by the fact that the typical Sudoku puzzle has certain digits pre-filled.
The present-day version of Sudoku was first published in 1979 by Dell Magazines, a company known for its many puzzle magazines and astrology magazines. It’s believed that it was designed by Howard Garns, a 74-year-old retired architect who liked to spend his free time designing puzzles, because Dell Magazines included his name on the list of contributors.
Dell Magazines published the puzzle game as Number Place. It wasn’t until Maki Kaji, the president of Japanese puzzle manufacturer Nikoli, abbreviated the Japanese version of the game (Sūji wa dokushin ni kagiru or “the digits must be single” in English) when it first appeared under its current name: Sudoku.
Sudoku quickly spread outside Japan, appearing alongside crosswords and other puzzle games in major newspapers around the world. Today, the market is saturated with books of Sudoku puzzles, Sudoku video games, and Sudoku websites, so Sudoku players of all skill levels can enjoy this addictive puzzle game anywhere and at any time.
How to Play Sudoku?
One great thing about Sudoku is that it has very simple rules:
- You must fill in all cells (there are 81 cells in total, stacked nine high and nine wide) with numbers from 1 to 9.
- You’re not allowed to use the same number more than once in any row, column, or block when filling in the empty cells.
- At least 17 numbers must be pre-filled (typically, 22 or 30 numbers are pre-filled).
The challenge comes from the fact that each cell in a Sudoku puzzle has only one correct answer because of the placement of pre-filled cells.
There are many Sudoku-solving techniques out there, but you don’t need to worry about them at all if you’re just starting and have never won a game of Sudoku before. Instead, we recommend you pick a Sudoku puzzle with a large number of pre-filled cells so that the path to the one and only correct solution is shorter.
Always start by identifying all sole candidates, which are cells that can contain only one specific number. When all other numbers but the candidate number exists in either the current block, column, or row, you’ve found a sole candidate.
From there, scan blocks, columns, and rows for cells with the fewest possible candidates and use a pencil to write them down. When you make a decision, immediately study how it affected other cells and adjust your strategy if needed.
How to Create a Sudoku Puzzle?
To create a Sudoku puzzle:
- Grab a sheet of paper, a pencil, and a ruler.
- Draw a large square and divide it into a 3×3 grid to end up with 9 smaller squares.
- Divide all 9 squares into 3×3 grids to end up with 81 cells.
- Fill in all cells according to the rules of Sudoku.
- Start randomly removing digits until you’re left with only 17 to 30 filled cells.
Alternatively, you can save yourself a lot of work by using an online Sudoku generator, such as OpenSky, which outputs a printable PDF document with up to four Sudoku puzzles per page. You can choose between four difficulty levels or let OpenSky randomly pick one for you.
Best Sites to Play Sudoku Online for Free
Creating Sudoku puzzles from scratch can get boring pretty fast, which is why it’s always a good idea to bookmark a couple of sites that let you play Sudoku online for free.
1. Sudoku Online
Sudoku Online is a simple website with free Sudoku puzzles in four levels of difficulty (easy, medium, hard, and very hard). You can either select a cell and fill it in by typing the desired number on your keyboard, or you can use the virtual numbers on the site and play with your mouse only.
Sudoku Online gives players an unlimited number of undos, the ability to restart the game with a single click, and the option to print the generated Sudoku puzzle on a sheet of paper. Unfortunately, the site isn’t optimized for mobile devices, so we can’t recommend it to mobile users who want to solve Sudoku puzzles while commuting or waiting in a line at a grocery store.
2. Web Sudoku
Web Sudoku is a straightforward online Sudoku game with a clean layout and multiple useful features. The site was created in June 2005 by Gideon Greenspan and Rachel Lee, and it quickly became one of the most popular Sudoku websites in the world.
The site is completely free, supported by the money its developers make by selling Web Sudoku Deluxe, the premium version for Windows or Mac costing $14.95, and Web Sudoku Ebooks, which offer a unique ability to create and print personalized books of puzzles.
Web Sudoku is also available as a mobile app for Android and iOS, but the website itself is well-optimized for mobile devices. Regardless of which version of Web Sudoku you choose, you can always select between four difficulty levels: easy, medium, hard, and evil. The last difficulty level is truly diabolic, and you should have a lot of practice under your belt before you even attempt it.
3. Sudoku Kingdom
If you want to compare your Sudoku skills against players from around the world, Sudoku Kingdom is the right website for you, featuring global leaderboards and rewards for players that score 100 points or more.
Sudoku Kingdom offers daily Sudoku puzzles in four levels of difficulty. Unlike most other Sudoku websites featured in this article, Sudoku Kingdom doesn’t allow its players to generate as many puzzles as they want. Instead, Sudoku Kingdom encourages players to register and come back every day for a new puzzle. Once a week, Sudoku Kingdom publishes the solutions to puzzles from the week before.
4. 247 Sudoku
247 Sudoku is an important member of 247 Games, one of the best resources for free games on the internet. Like all other online games from 247 Games, this version of Sudoku has appealing graphics, sound effects, and built-in guide for new players.
As a player, you simply select an empty square and select a tile to fill the square. You can also select a number on the board to highlight numbers and click the pencil icon to switch to note-taking mode. 247 Sudoku provides Sudoku puzzles in four difficulty levels, and the jumps between them are nicely balanced.
5. Sudoku Solver
Sudoku Solver can help you solve any Sudoku puzzle you ever come across. To use this handy tool, you simply copy the numbers from any Sudoku puzzle you can’t solve by yourself and click the large Solve Puzzle button. Instantly, Sudoku Solver will fill in all remaining squares, giving you the right solution.
In addition to helping you solve challenging Sudoku puzzles, Sudoku Solver provides a number of printable sheets with Sudoku puzzles in five levels of difficulty. The printable sheets contain minimal branding and are very clean, so you can use them for virtually any purpose.
6. Sudoku Menu
From Sudoku puzzles for complete beginners to Sudoku puzzles intended for experienced players, Sudoku Menu has it all. The site has been translated into several different languages, and it runs surprisingly well on mobile devices.
Sudoku Menu goes beyond traditional Sudoku puzzles, offering two interesting modifications of Sudoku: diagonal and odd/even Sudoku. In diagonal Sudoku, your goal is to fill the grid with numbers so that each of the rows, columns, highlighted 3×3 sections, and both diagonals contain all of the digits from 1 to 9. In odd/even Sudoku, certain squares are highlighted, and the highlighted squares may contain only odd or only even numbers.
Thanks to its memorable domain name, Sudoku.com has become one of the most popular websites with Sudoku puzzles on the internet. However, this site has a lot more to offer besides an attractive domain name. For starters, it has a clean, modern design that’s easy on the eyes and also easy on your data allowance.
Sudoku.com is available on Android and iOS as a polished app with daily Sudoku challenges, seasonal events, statistics, unlimited undos, color themes, auto-save functionality, highlighting of a row, column, and box related to the selected cell, eraser, and many other useful features. The web version isn’t nearly as feature-packed, but it gets the job done and is easy to use.