Solve any Sudoku puzzle instantly. Enter your puzzle, see step-by-step solutions with named techniques, and learn solving strategies for all difficulty levels.
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This Sudoku solver instantly solves any valid 9×9 Sudoku puzzle using constraint propagation and backtracking algorithms. Enter your puzzle by clicking cells and typing numbers, then hit Solve to see the complete solution. Use Step-by-Step mode to watch the solver work through the puzzle one logical deduction at a time, showing you exactly which technique it uses at each step — from simple Naked Singles to advanced X-Wings.
Sudoku is a logic-based number-placement puzzle. The objective is to fill a 9×9 grid with digits 1 through 9 so that each row, each column, and each of the nine 3×3 boxes contains all digits from 1 to 9 exactly once. The puzzle begins with some cells pre-filled (clues), and the solver must deduce the remaining values using logical elimination. Efficient solvers use named techniques like Naked Singles, Hidden Singles, Naked Pairs, and X-Wings — the same strategies human experts use.
Sudoku Constraint Rule
Candidates(r,c) = {1..9} \ (Row(r) ∪ Col(c) ∪ Box(r,c))Whether you're stuck on a newspaper Sudoku or an expert-level puzzle, enter the numbers and get the complete solution in milliseconds.
Our step-by-step mode teaches you the actual named techniques (Naked Single, Hidden Pair, X-Wing) that expert solvers use. Watching the solver work is the best way to improve your own skills.
Stuck partway through a puzzle? Enter what you have so far and the solver will check for errors or fill in the rest. Real-time conflict highlighting catches mistakes before they cascade.
Practice with 4×4 grids for beginners, 6×6 for intermediate players, or tackle the standard 9×9. No other online solver offers this flexibility.
Finished a Sudoku but not sure if it's correct? Enter your completed grid and click Validate to instantly verify every row, column, and box.
Stuck on a newspaper or app Sudoku? Enter what you have so far and use Step-by-Step mode to see the next logical move.
Watch the solver demonstrate techniques like Naked Pairs, Pointing Pairs, and X-Wings on real puzzles to level up your skills.
Creating Sudoku puzzles? Use the solver to confirm your puzzle has exactly one valid solution.
Use the 4×4 grid and step-by-step mode to introduce younger players to Sudoku logic with a smaller board.
The most fundamental trick is elimination: for each empty cell, list which numbers are NOT already in its row, column, or 3×3 box. Often this leaves only one possibility (a Naked Single). For harder puzzles, look for Hidden Singles — a number that can only go in one cell within a unit.
The Rule of 3 (Naked Triple) occurs when three cells in the same unit collectively contain only three candidate numbers. You can eliminate those three numbers from all other cells in that unit. It's a powerful intermediate technique.
There's no single formula, but Sudoku is a Constraint Satisfaction Problem. The constraint is: each digit 1–9 must appear exactly once in every row, column, and 3×3 box. Solvers use constraint propagation combined with backtracking when logic alone fails.
A minimum of 17 clues is required to guarantee a unique solution. This was proven mathematically in 2012 by McGuire's team at University College Dublin. With fewer than 17 clues, puzzles always have multiple solutions.
Almost all published puzzles can be solved using pure logic without guessing. However, some mathematically valid puzzles require advanced techniques that feel like guessing. Computer solvers use backtracking as a systematic fallback.
The 159 Rule is a variant Sudoku constraint: the digit in column 1 tells you the column position of the 1 in that row, column 5 maps to the 5, and column 9 maps to the 9. This is NOT a standard rule — it's used in variant puzzles.
You can play unlimited free Sudoku puzzles at Sudoku91, which offers five difficulty levels from Easy to Evil, plus variants like Killer Sudoku and Mini 6×6. All puzzles run directly in your browser with no downloads or sign-up required.