Free Sudoku Solver with Step-by-Step Solutions

What Is a Sudoku Solver?

A sudoku solver takes the givens of a 9×9 sudoku and completes the grid so that every row, column, and 3×3 box contains the digits 1 through 9 exactly once. Most online solvers only hand you the finished answer. This one also shows the step-by-step reasoning— the same named techniques human solvers use — on a live candidate grid you can edit yourself, so you can find the exact move you missed and actually get better at sudoku instead of just copying the answer.

How to Use the Sudoku Solver

1. Enter your puzzle — click a cell and type. Arrow keys move around the grid, digits fill cells, Backspace clears. You can also paste an 81-character puzzle string onto the grid, load one of the five examples, or type a string into the import field.
2. Read the candidates — every empty cell fills with the small digits that are still legal there. Turn them off any time with the Show candidates switch.
3. Check the puzzle — tells you whether your grid is a proper sudoku with one solution, has several solutions, contradicts itself, or cannot be finished at all, and shows a difficulty rating.
4. Solve instantly — fills the complete answer and confirms whether the solution is unique.
5. Solve step by step — produces a numbered list of moves. Click any step to see the grid and its candidates exactly as they looked at that point, with the eliminated pencil marks struck through in amber.
6. Undo anything — the Undo button walks back through your last fifty grid changes, including applied steps.

Candidates and Pencil Marks, Explained

A candidateis a digit that could still legally go in an empty cell — one that does not already appear in that cell’s row, column, or 3×3 box. On paper, solvers write these in tiny numerals in the corner of the cell, which is where the name pencil marks comes from. Every technique past the beginner level is really a statement about candidates, so seeing them is the difference between following an explanation and just trusting it.

This solver draws the candidates for you as a 3×3 mini-grid inside each empty cell, always in the same position: 1 top-left, 5 in the middle, 9 bottom-right. That fixed layout means you can scan a whole row for a digit without reading every cell. A cell with only one candidate left is outlined — that is a naked single, ready to be written in.

You can edit the marks by hand. Click a small numeral to strike it out, click it again to bring it back, or press Shift plus that digit while the cell is selected. Striking a candidate does not change the puzzle — it only changes your working notes, exactly like erasing a pencil mark on paper. Use it to test an idea (“if 4 cannot go here…”) or to keep track of eliminations you have already made. Reset pencil marks throws away every strike and returns to the auto-computed candidates.

Two visual cues are worth knowing. A candidate struck through in gray is one youremoved. A candidate struck through in amber is one the currently selected solving step removes, so you can see the exact effect of a naked pair or an X-wing before it happens. A red dot in the corner of a cell means it has no candidates at all — the grid is broken there and nothing can be written in that cell.

How the Difficulty Rating Works

The rating is not a guess based on how many givens the puzzle has — clue count is a poor predictor. Instead the solver actually solves the puzzle with human techniques, always preferring the simplest move available, then grades it by the hardest technique it was forced to use. Easy means naked and hidden singles were enough. Medium means it needed naked pairs or pointing pairs. Hard means hidden pairs or an X-wing showed up. Expert means even those stalled and the solver had to fall back on trial and error, which is a signal that the puzzle needs chain-based logic like a swordfish, XY-wing, or coloring. The rating also reports how many logical steps the whole solve takes.

The Solving Techniques, Explained

The step-by-step mode applies these techniques in order, always preferring the simplest move available — the same priority a good human solver uses:

A Worked Example

Load the X-wing demo example and click Solve step by step. The rating reads Hard, hardest technique X-wing, so you know before you start that singles alone will not finish it. Click through the early steps and watch the candidate grid thin out as singles get written in. Somewhere in the middle you will hit the X-wing step: the description names the four corner cells, and on the grid the candidates it kills are struck through in amber. Click Apply this step to my gridon any placement step to write it in for real, then keep solving by hand from there — Undo puts it back if you change your mind.

The Easy example (the classic Wikipedia puzzle) is a good contrast: it falls to singles alone in about fifty moves, and its candidate grid never gets crowded. The Expert example is Arto Inkala’s famous 2012 puzzle, where even hidden pairs and X-wings run out of road and the solver has to say so honestly.

Frequently Asked Questions