Sudoku Solver

It also tells you something most solvers do not: whether the puzzle you typed is well-formed, or whether it has no answer at all, or more than one.

What the three answers mean

• One solution. A proper Sudoku. The answer shown is the only one that fits.
• More than one solution. The clues do not pin the grid down. Usually a digit was mistyped or one was missed while copying; occasionally the source puzzle was genuinely broken.
• No solution. The clues contradict each other somewhere further along. This is different from two of the same digit in one row, which gets flagged before the solver even runs.

How it solves

It works the way a careful person does, only faster: it repeatedly picks the square with the fewest digits still legally possible, tries one, and continues — backing up the moment a square is left with no legal digit at all. Constraining the choice to the most restricted square first is what keeps the search small; a typical grid is solved in thousandths of a second.

Everything runs on your own device. Nothing you type is uploaded, and the page works offline once it has loaded.

Explain, and why it is the part worth using

Solve fills the grid in. Explain does something most solvers cannot: it works the puzzle the way a person is taught to, one named technique at a time, and tells you which one applies at each step. A last free cell, a hidden single, a naked pair, a pointing pair, an X-Wing — each step says what it is and which squares it reasons about, and each links to the page explaining that technique.

That difference matters when you are stuck rather than curious. A filled grid tells you the answer and nothing else; a named technique tells you what you could not see, which is the only thing that makes the next puzzle easier. It is the same solver the hint button uses during a game, and the same one that selects the puzzles in the nine technique lessons.

When it says there is no single answer

Two verdicts surprise people, and neither is a fault in the solver.

More than one solution means the clues you typed do not pin the grid down. Almost always a digit was missed while copying, not a flaw in the original: a published puzzle has one answer by definition, so a grid that admits two is a grid with a clue absent. Check the row and column counts against the source before assuming the puzzle is broken.

No solution means the clues contradict each other somewhere — not necessarily where you would notice. Every digit can be legal where it sits while the set as a whole leaves some square with nothing that can go in it. That is normally one mistyped digit, and re-checking the box it sits in finds it faster than re-reading the whole grid.

What it will not do

It solves the standard 9x9 grid only. Variants that add rules on top of it — Killer, Samurai, Jigsaw, hyper or diagonal Sudoku — are not supported, because their constraints are not the ones it checks. It also reads nothing from an image: a puzzle has to be typed in, which on a phone is slower than it sounds for a grid of thirty clues.

And Explain stops where the nine taught techniques stop. A puzzle needing something more intricate than an X-Wing will solve under Solve but will reach a point where Explain says so plainly rather than inventing a reason. That is deliberate: a step labelled with a technique that was not actually used would teach the wrong lesson.

A solver is a tool, not a shortcut

If you are stuck on a puzzle you want to finish yourself, a solved grid is the least useful thing to look at. Better to find the technique you are missing: the solving techniques guide covers them in the order they become necessary, and the per-level guides show which ones each difficulty actually demands.

Where the solver earns its place is checking a puzzle rather than beating it — confirming a grid you copied is valid, settling an argument about whether a puzzle has two answers, or verifying a Sudoku you have constructed yourself.

Or just play

Every puzzle generated on this site is checked for a unique solution before you ever see it, so it never needs the solver's verdict: easy, medium, hard and expert.

Last reviewed 25 October 2026