Solve any Sudoku instantly

How to use the solver

1. Tap a cell to select it, then tap a shape in the palette to place it. Prefer the keyboard? Select a cell and press 1 to 9 to place the matching shape, use the arrow keys to move, and Backspace to clear.
2. Place every starting clue exactly as it appears in your puzzle. You can press Load sample to see it work first.
3. Press Solve for the full answer. The shapes it worked out appear faded so you can tell them from your clues.
4. Or press Step by step to learn as you go. It reveals one cell at a time and names the technique it used, like a naked single or a hidden single. Use Next step, Auto-play, or go back to editing.
5. Press Clear to start over, or change any cell to adjust the puzzle and solve again.

Shapes, not numbers

This solver speaks Shapedoku. Instead of the numbers 1 to 9 you place the same nine glowing symbols the game uses - triangle, hexagon, circle, crystal, heart, diamond, square, crescent moon and lotus. The logic is exactly classic Sudoku underneath: every row, column and 3x3 box ends up with each symbol exactly once. If your puzzle is written with numbers, just map 1 to 9 onto the shapes in that same order.

How the solver works

Under the hood it uses backtracking, the classic approach to Sudoku. It finds the first empty cell, tries the smallest legal number, and moves on. If it ever reaches a dead end, it steps back and tries the next option. Because a well made puzzle has exactly one solution, this search always lands on the correct board. If your clues break a rule or have no possible answer, the solver tells you instead of guessing.

What Step by step actually teaches

The step-by-step mode does not just fill cells; it names the reason for each one, using the two techniques that carry most of every puzzle:

• Naked single. A cell has only one candidate left, because the other eight symbols already appear in its row, column or box. The solver tells you which cell and which symbol.
• Hidden single. A symbol has only one possible home inside a unit, even though that cell could still hold other candidates. The solver names the unit, for example row 4 or 3x3 box 7, so you can see exactly where to look.

If the board reaches a point where no single is available, the solver stops and says so rather than guessing. That moment is useful information: it means the puzzle needs an intermediate technique such as a pointing pair, a naked pair or a box-line reduction. Our solving techniques guides cover each of those, and Instant solve will still finish the grid if you just want the answer.

When to use a solver, and when not to

A solver is at its best in three situations. First, checking a finished puzzle from a book or newspaper when no answer key is printed. Second, verifying that a puzzle you set for someone else actually has a solution. Third, breaking a deadlock: enter the board as it stands, step forward one move, and see which unit you overlooked. That last use is where most of the learning happens, because the solver shows you the pattern you missed instead of the whole answer.

It is less useful as a crutch during play. If you find yourself reaching for it on every Medium board, try the opposite: keep the solver closed, place pencil marks, and only check your work at the end. Progress in Sudoku comes from the moments of being stuck.

Why it might say there is no solution

When the solver reports that a board cannot be solved, the puzzle is almost never the problem. The usual causes, in order of how often we see them:

1. A mistyped clue. One symbol entered in the wrong cell creates a contradiction several moves later. Compare your grid to the original row by row.
2. A duplicate in a row, column or box. The solver checks the rules before it starts and highlights the conflict.
3. A clue that was actually your own earlier guess. If you copied a half-finished board, remove anything you placed yourself and enter only the printed givens.
4. A puzzle with too few clues to be unique. Some casual puzzles have more than one valid solution; the solver will return one of them, which may differ from the published answer.

Want to solve it yourself?

A solver is great for checking an answer or unsticking a tough spot, but the real fun is doing it by hand. If you want to sharpen your skills, read our Sudoku tips and strategies, learn naked and hidden singles, then put them to work in a fresh game.