Sudoku solver
Type a puzzle in and it will tell you three things: whether it is well-formed, what technique comes next, and — only if you ask — the answer.
The useful answer is usually not the answer
Most solvers fill in the grid and finish the puzzle for you, which resolves the frustration and removes the point at the same time. Being told that a hidden single is available in the middle box is a different kind of help entirely.
So this reports the next technique first. If it says "hidden single", the deduction is there and you have missed it; go and find it. The full solution is behind a separate button because most of the time you should not press it.
It also names the hardest technique the whole puzzle will need, which tells you whether the one you are stuck on is within what you have learned.
Checking a puzzle from somewhere else
The most valuable thing here is the verdict on whether a puzzle is well-formed at all. Paste in one from a newspaper or another site and it will say whether it has exactly one solution, more than one, or none.
More than one means the puzzle is broken and no amount of skill will finish it without guessing. None means a clue has been mistyped or misprinted, which happens more often than you would think.
That check is the same one this site runs on its own puzzles before printing them, so it is the same standard applied to somebody else's.
How to enter a grid
Type digits into the squares, leaving the blanks empty, and move around with the arrow keys or the tab key. Anything that is not a digit from one to nine is ignored rather than rejected, so pasting a grid with dots or dashes for blanks works.
The check runs as you type and the verdict updates with it. A grid that is still incomplete simply has many solutions, which is not an error — it becomes meaningful once the clues are all in.
Nothing you type is uploaded. The solver runs on your own machine, which also means the page keeps working with the network off.
Common questions
How do I know if a Sudoku is valid?
It must have exactly one solution. Paste it in here and the verdict says whether it has one, more than one, or none. More than one means it cannot be solved by reasoning.
Can the solver tell me the next move without spoiling it?
That is the default. It names the technique that is available — a hidden single, a naked pair — without saying where. The full solution is a separate button.
Does the solver work offline?
Yes. It runs entirely in your browser and makes no request to any server, so once the page has loaded it keeps working with the network off.