Sudoku solver and online sudoku

How it is calculated

How this sudoku works

A sudoku has 9 × 9 cells in nine 3 × 3 boxes. The goal: every row, every column and every box contains each number from 1 to 9 exactly once. The bold givens are fixed; you fill in the rest.

How to play and solve sudoku online

1. Pick a level (Easy, Medium, Hard or Expert) and tap “New sudoku” – or choose “Enter my own”, type in a puzzle from a newspaper or book and press “Use this puzzle”.
2. Tap a cell and choose a digit from the number pad, or type 1–9; ✎ or N switches to notes.
3. Stuck? “Hint” shows the next logical step, “Check” marks wrong entries and “Solution steps” explains every step to the end.

Example: A puzzle with 26 givens leaves 81 − 26 = 55 empty cells. If row 3 is only missing 4 and 7, and column 5 already has a 7, the empty cell R3C5 must be 4 – the solver calls this a “naked single”.

Controls

• Phone and tablet: tap a cell, then a number on the big pad below. ⌫ clears the cell, ✎ toggles notes mode, ↶ undoes your last move.
• Keyboard: arrow keys move the selection, 1–9 enters a number, Delete or Backspace clears the cell. N toggles notes, Shift + number adds a note directly, Ctrl + Z undoes.
• Notes: small numbers in a cell track possible candidates. When you place a number, it is removed automatically from the notes in its row, column and box.
• Conflicts: if a number appears twice in a row, column or box, both cells turn red right away. “Check” also compares your entries with the solution and marks wrong ones.

The sudoku solver: steps with explanations

“Hint” shows the next logical step, “Solution steps” shows every step to the end. The solver works like a person and always uses the simplest technique that makes progress:

• Naked single: only one number is still possible in a cell.
• Hidden single: a number fits in only one place in a row, column or box.
• Pointing and claiming: if a number inside a box is limited to one row, it cannot appear in the rest of that row – and vice versa.
• Pairs and triples: if two (three) cells share exactly two (three) candidates, those numbers are removed from the rest of the unit.
• X-Wing: if a number sits in two rows only in the same two columns, it is removed from those columns everywhere else.
• Backtracking: when no technique helps, the computer tries values systematically. This only happens with very hard puzzles and is shown in the steps.

Difficulty levels

The level depends on the hardest technique needed: Easy uses singles only, Medium adds pointing, claiming and pairs, Hard adds triples and X-Wing, Expert needs more than these. Every generated puzzle is checked before you see it: it has exactly one solution. The givens are placed symmetrically, as in newspaper puzzles. If the generator cannot hit the chosen level in time, it shows the real level.

Solve your own sudoku

“Enter my own” clears the grid. Type in the givens from a newspaper or book and press “Use this puzzle”. The tool checks that the puzzle is solvable and unique, rates its difficulty and explains every step if you want.

Printing, saving and privacy

“Print” outputs only the title and the grid – no ads, no buttons. Your game is kept in your browser’s localStorage and is back after a reload. Nothing is sent to a server. New puzzles are generated on your device with the browser’s random generator (crypto.getRandomValues).

Fancy another puzzle? The Wordle solver helps with the word game.

More puzzles: make a crossword, build a word search or play hangman online.

Frequently asked questions

Sources and legal basis

• Wikipedia: Sudoku
• Wikipedia: Sudoku solving algorithms
• MDN: Crypto.getRandomValues()

As of: 27 September 2026