Sudoku Solver with Step-by-Step Explanations
Enter your sudoku and this solver shows the next logical move and why it works, using the same techniques a person would: Hidden Single, Naked Pair, X-Wing and more. It explains one step at a time, so you can get unstuck without seeing the whole answer, or show every step at once. It is free and runs in your browser.
Type the given numbers into the grid, or paste the whole puzzle as 81 characters (use 0 or . for empty cells). The puzzle below is an example.
Turn on JavaScript to use the solver. The worked example below can be read without it.
Worked example
Here is how the solver explains the example puzzle above, a Hard puzzle. The first moves are singles; the first move that needs a candidate pattern comes later.
- R3C2 = 4 · Hidden Single (only one place for it)
In box 1, the only cell that can hold a 4 is R3C2. Every other empty cell shares a row, column, or box with the 4 at R1C5, R2C9, and R4C1, so it can't be a 4. → R3C2 is 4.
- R1C2 = 5 · Hidden Single (only one place for it)
In box 1, the only cell that can hold a 5 is R1C2. Every other empty cell shares a row, column, or box with the 5 at R8C1 and R2C5, so it can't be a 5. → R1C2 is 5.
- R1C4 = 1 · Hidden Single (only one place for it)
In box 2, the only cell that can hold a 1 is R1C4. Every other empty cell shares a row, column, or box with the 1 at R4C5, so it can't be a 1. → R1C4 is 1.
- R5C7 = 2 · Pointing Pair (lined up inside a box)
In box 3, 2 can only go in R1C8 and R2C8. They all sit on column 8. So box 3's 2 is somewhere on column 8, and the rest of column 8 outside the box (R5C8, R6C8, and R9C8) can't be 2.
In row 5, the only cell that can hold a 2 is R5C7. Every other empty cell shares a row, column, or box with the 2 at R4C2 and R3C6, so it can't be a 2. For R5C8, 2 was removed as a candidate in step 1. → R5C7 is 2.
Press "Next step" above to continue from here, or "Show all steps" to see the whole solution.
How the solver works
The solver searches for the easiest technique that makes progress, in the order a person learns them:
- Singles: Hidden Single and Naked Single
- Box and line interactions: Pointing Pair and Box/Line Reduction
- Pairs and triples: Naked Pair, Hidden Pair, Naked Triple
- Fish and wings: X-Wing, Swordfish, XY-Wing
- Chains and uniqueness: Skyscraper, W-Wing, Unique Rectangle
When a placement needs candidates to be removed first, it shows each elimination as a numbered step and leaves out any step the answer does not depend on.
Why it explains instead of just filling in
Most sudoku solvers fill in the whole grid at once. That gives you the answer but not the reason, so the next hard puzzle is just as hard.
This solver is built for learning. Every explanation comes from the same solver that grades and hints the puzzles in the Sudoku AI Coach app, and each technique name links to a page that teaches it with an example.
Tips for using it
- Stuck in a puzzle from a newspaper or another app? Enter only the given numbers, then press "Next step" once and try to continue on your own.
- The highlighted cells show where to look: the light yellow area is the row, column or box in focus, the orange cells are the reason, and the blue cell is the answer.
- If the solver says the puzzle has more than one solution, a clue is probably missing or mistyped.
Frequently asked questions
Is this sudoku solver free?
Yes. It is free, needs no sign-up, and runs entirely in your browser. Your puzzle is not sent to a server.
Can it solve any sudoku?
It can solve any valid sudoku with one solution. Almost all puzzles are solved with human techniques; for the rare position beyond them, it tells you the answer for one cell and says that an advanced technique is needed.
Does the solver guess?
No. Every step is a logical deduction, such as a Hidden Single or an X-Wing, and the explanation says exactly why the number goes there or why a candidate is removed.
How do I enter a puzzle quickly?
Paste the puzzle as 81 characters, reading row by row, with 0 or a dot for each empty cell. You can also type into the grid and move between cells with the arrow keys.
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