Sudoku Solver — Step by Step

Updated 15 October 2026

A sudoku solver is a program that takes an unfinished 9×9 grid and returns the completed one. A step-by-step sudoku solver returns something more useful than the answer: the reasoning. It hands back an ordered list of named deductions — Hidden Single places 3 in r4c5, Naked Pair removes 7 from r2c1 — so you can see which move you missed, learn the pattern behind it, and finish the puzzle yourself instead of copying 50 digits off a screen.

This solver runs entirely in your browser. Nothing is uploaded, nothing is stored, and no account is needed. It is the same source-available technique library that powers hints inside the Suuudokuuu game, so every technique it names is one the app’s hints can show you mid-game.

Why a narrated solve beats a bare answer

Most sudoku solvers online do exactly one thing: they fill the grid. That answers “what goes here” and nothing else. If you were stuck, you are still stuck the next time the same pattern appears, because the solver never told you what the pattern was. The filled grid is also unfalsifiable to a learner — you cannot check it, you can only trust it.

A narrated solve is checkable. Every step here states the technique, the cells it looked at, and the digit it placed or eliminated, and every step links to a guide explaining that technique on a worked example. Open any step and it replays on a live board, with the pattern cells highlighted and the candidates drawn in, so you can confirm the deduction with your own eyes before you accept it. That is the difference between an answer key and a teacher.

How this solver checks your puzzle

Before anything is narrated, the grid you entered is checked for a unique solution — twice, by two independent algorithms that share no code:

• The bitmask solver is the fast path. It stores each cell’s candidates as bits in a typed array, propagates naked singles until nothing changes, then branches on the cell with the fewest remaining candidates.
• The Dancing Links solver is the cross-check. It restates sudoku as an exact-cover problem and solves it with Donald Knuth’s Algorithm X over a doubly linked matrix — a completely different search from the bitmask one.

Both are asked to count solutions, stopping at two, and the two counts have to agree. Three things can come back. If the count is zero, the givens cannot be completed into a valid grid. A digit repeated in a row, a column or a box is the obvious cause, and the validity check names that pair of cells before any counting starts, but a grid with no visible repeat can still rule out every completion; either way the solver says so instead of guessing at what you meant. If the count is two or more, the puzzle is not a proper sudoku: it has multiple valid completions, there is no single answer to narrate, and any solver that quietly returns one of them is hiding the problem from you. Only a count of exactly one gets a walkthrough. If the two solvers ever disagree, that is reported too — it would be a bug in Suuudokuuu, not in your puzzle.

How the step-by-step walkthrough is produced

Once the puzzle is verified, the solving engine is run over it one deduction at a time. It tries the techniques in a fixed order, cheapest first, and stops at the first one that fires. The detectors are the same technique library the game’s hints are built on, though the hints choose their next move differently. That is why the walkthrough reads like a human solve rather than a machine dump: a Full House is preferred over a Hidden Single, a Hidden Single over a Pointing Pair, and a Naked Quad is never invoked while an easier move is still on the board.

Every elimination is carried forward from one step to the next, exactly as you would keep pencil marks, so a chain of eliminations that only pays off several moves later is narrated in full. The order is the SE rating’s cheapest-first order, which is why the hardest step in the walkthrough is also the technique named next to the puzzle’s SE rating.

One honest limit is worth stating up front. The genuinely hardest sudoku puzzles and some 17-clue grids reach positions where no technique Suuudokuuu implements makes progress. The walkthrough does not pretend otherwise: it says so in plain words, fills the one cell with the fewest candidates from the verified solution, and carries on with logic from there. The rating is then reported as 8.5+, the top of what the engine can price.

How to use the solver

1. Enter your puzzleClick a cell and type a digit, or use the on-screen pad. Arrow keys move around the grid and Backspace clears a cell. If you already have the puzzle as text, paste it into the box below the grid — digits for the givens; dots, zeros, dashes, x, * or _ for the blanks; spaces, line breaks, | and + are ignored. If the text does not add up to exactly 81 cells, the box says how many it found instead of guessing. A share link copied from the Suuudokuuu app works in the same box.
2. Solve step by stepPress Solve step by step. The validity and uniqueness checks run first and report honestly if a digit repeats or the grid has no solution or more than one. A solved puzzle shows its SE rating and the hardest technique it needs.
3. Read the stepsEach numbered step names the technique that proves it and links to a full guide for that technique. Work down the list until you reach the move you missed.
4. Replay a step on the boardOpen any step to see it on a live board: the pattern cells are highlighted, the candidates are drawn in, and the eliminations and the placement are played back in order so you can verify the deduction yourself. Press the next-step button to move on, all the way to the finished grid.

Techniques the solver can explain

Every technique the walkthrough can name has its own guide, with a worked example board generated by this same engine. The technique index is the hub for all of them and lists them in the order the solver tries them; the five tiers below are how that order is grouped.

• Tier 1 — Singles place a digit outright and carry most easy puzzles on their own: Full House, Naked Single and Hidden Single.
• Tier 2 — Intersections work on the overlap between a box and a line and only ever remove candidates: Pointing Pair, Pointing Triple and Box Line Reduction.
• Tier 3 — Subsets reserve a group of digits for a group of cells: Naked Pair, Naked Triple, Naked Quad, Hidden Pair, Hidden Triple and Hidden Quad.
• Tier 4 — Fish, wings and chains reason across several units or along a path of cells: X-Wing, Swordfish, Jellyfish, Finned X-Wing, Finned Swordfish, Sashimi X-Wing, Sashimi Swordfish, XY-Wing, XYZ-Wing, W-Wing, X-Chain, XY-Chain, Simple Coloring and AIC.
• Tier 5 — Uniqueness and forcing chains use the one-solution guarantee or follow every case of an assumption: Unique Rectangle, Bivalue Universal Grave, Nishio Forcing Chain, Cell Forcing Chain and Region Forcing Chain.

Never solved a sudoku before? The how to play guide covers the three rules and your first moves before any of the patterns above become useful.