Sudoku Solver — Step by Step

How the step-by-step solver works

Enter any valid Sudoku puzzle — either as an 81-character string (using digits and 0 or . for empty cells) or by typing a puzzle ID from this site — and the solver will analyse it and break it down into a sequence of individual logical steps, from the first digit placed to the last.

From simple to advanced techniques

The solver works the same way a human player would: it always tries the simplest available technique first, only escalating to more complex reasoning when easier approaches are exhausted. At the simplest end, some cells can be filled immediately because only one digit is possible given what is already in their row, column, or 3×3 box. Moving up the scale, the solver looks for situations where a particular digit can only fit in one cell within a unit, then progressively applies more advanced constraint-based logic that considers interactions between groups of cells or candidates across multiple units. If no purely logical deduction is sufficient, the solver can also make a reasoned guess and backtrack if it leads to a contradiction.

What each step shows you

Every step displays the grid as it looks at that moment, with colour-coded highlighting to draw your attention to the relevant cells. A badge indicates the type of reasoning used — placement, elimination, or guess — and the explanation describes exactly why that digit can (or cannot) go in that cell. Use the navigation buttons or the arrow keys to move through the steps at your own pace.

Difficulty and rating

Each puzzle on Sudoku2PDF is assigned a rating — a numeric score that reflects the hardest logical technique the solver must use to crack it. The rating starts at 1 000 for the simplest puzzles and rises without a fixed ceiling for the most demanding ones, giving you a precise measure of challenge rather than a broad category alone. Puzzles are grouped into five difficulty bands:

• Easy — rating ≤ 1 100 — Hidden/Naked Singles only; solvable by scanning rows, columns and boxes without pencil marks.
• Medium — rating ≤ 1 300 — Locked Candidates, Naked/Hidden Pairs; light use of pencil marks recommended.
• Hard — rating ≤ 1 680 — X-Wing, Swordfish and similar pattern-based eliminations; systematic candidate tracking required.
• Fiendish — rating ≤ 2 100 — XY-Wing, XYZ-Wing and multi-step inference chains; comfortable pencil-mark management essential.
• Diabolical — rating > 2 100 — Alternating Inference Chains (AICs) and forcing nets; the toughest puzzles on the site.