AI SUDOKU
AI Sudoku
Easy to Expert · Daily Challenge · AI Hints
Difficulty
Difficulty
Medium
Mistakes
0
Time
0:00
Every puzzle is logic-solvable — you never need to guess.
New here? Read the full how to play Sudoku guide for the rules, first moves and beginner tips.
Each puzzle is built by a randomised backtracking generator, then verified by a solver that confirms exactly one solution exists. Difficulty is set by how many clues remain and which techniques are required to finish.
The AI Hint engine reads your board live: it computes the candidate digits for every empty cell, applies logical eliminations (naked pairs, pointing pairs), then finds the next forced move — a naked single or hidden single — and names the technique so you learn the pattern.
Easy needs only single-candidate scanning. Hard demands the advanced techniques covered in the strategy guide.
Does every Sudoku puzzle have one solution?
Yes. In a valid Sudoku, the 9×9 grid must be filled so each row, each column and each 3×3 box holds the digits 1 to 9 exactly once, and a correct puzzle has precisely one arrangement that satisfies all of those constraints. Every puzzle in this game is built by a randomised backtracking generator and then checked by a solver that counts the solutions; if more than one is found, clues are kept until exactly one remains.
That uniqueness guarantee matters because it means you can always reach the answer by pure logic and never have to guess. When you apply a technique correctly, you can be confident the deduction is forced rather than a lucky branch, which is what makes the puzzle fair and solvable from start to finish.
What do the Easy, Medium, Hard and Expert difficulties mean?
Difficulty in Sudoku is governed by how many starting clues are revealed and which solving techniques the puzzle demands. In this game Easy shows about 45 clues, Medium about 34, Hard about 28 and Expert about 24, so harder levels start emptier and force longer chains of reasoning. Easy puzzles usually fall to simple scanning and naked singles, where only one digit can fit a cell. Medium and Hard introduce hidden singles and candidate elimination such as naked pairs and pointing pairs, where a digit is confined to one line of a box.
Expert puzzles dig the grid down close to the minimum number of clues that still leaves a unique solution, so they reward patient, careful pencil-mark bookkeeping more than quick scanning. The difficulty you pick also sets how many AI hints you are given for that puzzle, with fewer hints on the harder levels. Whatever the level, every puzzle stays fully logic-solvable without ever needing to guess.
How does the AI Hint engine work?
The AI Hint engine reads your current board live rather than just revealing the answer. First it computes the candidate digits for every empty cell, the numbers not already used in that cell's row, column or 3×3 box. It then applies logical eliminations: naked pairs, where two cells in a unit share the same two candidates and so remove those digits from their neighbours, and pointing pairs, where a digit confined to one box-line is cleared from the rest of that row or column.
After those eliminations it searches for the next forced move, a naked single (only one candidate left in a cell) or a hidden single (a digit that fits only one cell in a unit). It names the technique, explains why the move is forced and fills it in, so you learn the pattern. Hints are limited per puzzle.
What are pencil marks in Sudoku?
Pencil marks are the small candidate numbers you jot inside an empty cell to track which digits could still legally go there. They are the core bookkeeping tool of harder Sudoku: by recording candidates you can spot patterns like naked pairs, hidden singles and pointing pairs that are impossible to see at a glance.
In this game you add pencil marks by switching to Candidate mode with the pencil button, then tapping numbers on the pad to toggle them on and off for the selected cell. If you prefer not to track them by hand, tick Auto Candidate Mode and the board fills and updates every cell's candidates automatically as you play, recomputing them whenever you place or erase a digit. Pencil marks never count as final answers; they are just your working notes toward the solution.
Is this Sudoku free to play online?
Yes. This is a completely free online Sudoku game with no sign-up, no account, no app download and no paywall. It runs entirely in your web browser, so you can play instantly on a phone, tablet or desktop with nothing to install, and the 9×9 board and number pad scale to fit any screen size.
Everything is included at no cost: unlimited puzzles across Easy, Medium, Hard and Expert, a daily challenge that gives everyone the same seeded puzzle, pencil marks, mistake highlighting, undo, a timer and the AI hint engine. You can also switch between a light and dark theme to suit your surroundings, and your theme choice is remembered between visits. There are no ads gating play and no premium tier, so the full game is available to everyone the moment the page loads.
Is there a daily Sudoku puzzle?
Yes — tap Daily Challenge for a fresh daily Sudoku that gives everyone the same seeded puzzle each day, playable at Easy, Medium, Hard or Expert. Or hit New Puzzle any time for unlimited random boards.
Are there more free AI games on LK Forge?
Yes. AI Sudoku is part of a small collection of free, browser-based games on LK Forge that share the same no-sign-up, no-download approach. If you enjoy this puzzle, you can try AI Tic Tac Toe, which pits you against a computer opponent that plays the classic game optimally, along with AI 2048, Color Lines, Brain Puzzles and Memory Match, or browse the full games hub to see everything currently available.
Like Sudoku, the other games run entirely in your browser on phone, tablet or desktop, are free to play with no account and no paywall, and work the moment the page loads. New games and tools are added over time, so the hub is the best place to check what is on offer. You can reach it from the Games link in the navigation bar at the top of this page or in the footer below the puzzle.