Calculators

Games

How to Play Sudoku

To begin a sudoku puzzle look for rows, columns, or 3 x 3 boxes that have only one or two numbers missing. Check which numbers might fit in a cell given fixed numbers in that row, column, and 3 x 3 box. Start with the easiest placements and work toward the harder ones. Use logic to determine where the remaining numbers belong. Avoid guessing by using sudoku strategies you know or find online.

Game Features

The CalculatorSoup^® online sudoku game includes these features:

• One unique solution for each sudoku puzzle
• Three levels of difficulty - easy, medium, and hard - to match your skill level
• Pencil-in mode to track possible number candidates
• Pause button if you want to take a break
• Undo button that reverses your most recent move
• Clear button that erases the contents of a cell
• Printable puzzle option if you prefer solving with paper and pencil
• Play any number of puzzles - we don't limit how many you play each day!

Sudoku Strategies

Sudoku strategies are logical techniques used to eliminate possibilities in order to get the correct number in each cell. They range from basic elimination methods to more advanced pattern recognition. The CalculatorSoup^® online sudoku game has three levels of difficulty (easy, medium, and hard) where each increase in the level of difficulty introduces new solving strategies.

Easy puzzles can be solved using only the naked singles strategy. Look for empty cells where, based on the numbers already placed in that row, column, and 3 x 3 box, only one number from 1 to 9 is possible. Fill in that number and repeat the process until the puzzle is complete.

Medium puzzles include the hidden singles strategy as well as naked singles. Look within each row, column, or 3 x 3 box for a number that can only go in one specific cell, even if that cell has multiple candidates. For example, if 7 can only fit in one cell within a particular row (even though that cell might also have other possible numbers), then 7 must go there.

Hard puzzles include naked pairs along with naked singles and hidden singles. Identify two cells in the same row, column, or box that contain exactly the same two candidate numbers (like cells that can only be 4 or 7). Since these two numbers must occupy these two cells, you can eliminate those numbers as candidates from all other cells in that same row, column, or 3 x 3 box. This elimination often creates new naked or hidden singles.

History of Sudoku

The concept of sudoku can be traced all the way back to Latin squares, mathematical grid structures studied by Leonhard Euler in the 18th century. Latin squares require each symbol to appear exactly once in every row and column, forming the foundation for modern sudoku rules.

Modern sudoku was first created by Howard Garns for Dell Magazines during the 1970s. It gained worldwide popularity after Japanese puzzle company Nikoli adopted the game and renamed it "sudoku" meaning "single number." The name stuck when the puzzle spread globally and newspapers began featuring daily sudoku puzzles.

Who Plays Sudoku?

Sudoku attracts a wide audience of puzzlers who enjoy mental challenges and logical thinking.

Students can use sudoku to develop problem-solving skills and logical reasoning. Adults often play during long commutes, lunch breaks, or quiet evenings as a form of mental exercise and stress relief. Seniors in particular enjoy sudoku because it provides cognitive stimulation that may help them feel mentally sharp.

Sudoku games appeal to people who prefer logic over luck, and puzzlers often feel satisfied when they finish a puzzle. Many players find solving sudoku puzzles meditative and relaxing.

Sudoku Variations

There are several variations of sudoku that build on the traditional rules:

• Killer Sudoku: Groups of cells must also add up to specific sums while still following standard sudoku placement rules
• Diagonal Sudoku: Both main diagonals across the sudoku grid must contain each number 1 through 9 exactly once
• Irregular Sudoku: The standard 3 x 3 boxes are replaced with irregular shapes of nine cells each
• Mini Sudoku: Uses a 4 x 4 or 6 x 6 grid
• Giant Sudoku: Uses 12 x 12, 16 x 16, or even 25 x 25 grids

Sudoku Solvers

Sudoku solvers automatically complete sudoku puzzles using programmed logic techniques. These tools apply the same strategies human players use, such as naked singles, hidden pairs, and pointing pairs, but work much faster. Solvers help players check their work, learn new techniques by following step-by-step solutions, or solve puzzles that seem impossible to crack.

Sudoku Puzzle Generation

Sudoku puzzle generators often start with a complete and valid 9 x 9 sudoku grid. Generators then strategically remove numbers to create puzzles with specific difficulty levels. During this process, generators actually solve each potential puzzle using logical techniques (naked singles, hidden singles, naked pairs, etc.) to verify the difficulty matches a specific target level. Sudoku puzzle generators also ensure that each puzzle has exactly one valid solution.

Existing sudoku puzzles can be transformed into entirely new challenges for players using several mathematical operations that preserve their solvability:

• You can swap any two rows within the same horizontal band of 3 x 3 boxes, or any two columns within the same vertical band of 3 x 3 boxes. You can also swap entire vertical or horizontal bands of 3 x 3 boxes with each other.
• Any systematic remapping of numbers also creates a new valid puzzle. For example, you can replace all 1s with 7s, all 7s with 3s, and all 3s with 1s throughout the entire grid while still ensuring the integrity of the puzzle.
• The entire sudoku grid can be rotated 90, 180, or 270 degrees, or reflected horizontally, vertically, or diagonally to create new puzzle layouts that preserve the logical structure and difficulty.

Through these transformations, a single base puzzle can generate thousands of variants that appear completely different but have similar solving patterns.