Sudoku Solver

How to use the sudoku solver

1. Enter the puzzle. Tap a cell, then a number on the pad (or type it on a keyboard; the arrow keys move between cells, Backspace or 0 clears a cell). Clues show in bold. To enter a puzzle in one go, paste it as 81 characters — row by row, with 0 or . for empty cells — into the box below the grid, or press Ctrl+V while a cell is selected. Spaces, line breaks and grid lines such as | or - are ignored, so a puzzle copied from most websites or text files works as it is.
2. Watch for red. As you type, any number that appears twice in a row, column or 3×3 box turns red, so a typo shows up before you solve.
3. Press Solve. The missing numbers appear in green, and the message says whether the puzzle has exactly one solution, more than one, or none.
4. Or solve it yourself. Switch to Solve it myself: the clues are locked and the numbers you add are yours (in blue). Hint fills in the next cell that can be found by simple logic and explains the reasoning; Check compares your numbers with the solution and marks wrong ones in red without giving anything else away; Clear my digits starts over with the same clues.

The puzzle is saved on this device, so you can close the page and come back to it. Copy link puts the whole grid — clues and your own numbers — into a link you can send to someone. Nothing is uploaded: the solver runs in your browser and works offline once the page has loaded.

How the solver works

A sudoku has 81 cells in 27 groups — 9 rows, 9 columns and 9 boxes — and each group must contain 1 to 9 exactly once. The solver keeps, for every row, column and box, the set of numbers already used (as a 9-bit mask), so the candidates of any empty cell are found with a couple of bit operations. It then fills the grid by backtracking with constraint propagation:

• it always works on the empty cell with the fewest candidates. A cell with a single candidate is filled at once; a cell with none means an earlier guess was wrong, and the solver steps back;
• when every cell has two or more candidates, it tries them one by one and continues from there.

This finds the answer to any valid 9×9 puzzle in milliseconds. The solver doesn’t stop at the first solution: it keeps searching until it finds a second one or proves there isn’t one. That is how it tells you a puzzle is valid (exactly one solution), ambiguous (more than one, usually because a clue is missing or was mistyped) or impossible (no solution — for example two 5s in one row, or a cell where every number is already used in its row, column or box).

How the hints explain each step

Hints use the two techniques that solve most easy and medium puzzles, and say which one applies:

• Hidden single — a number that has only one possible place left in a box, row or column, because every other empty cell of that group already “sees” the number in its own row, column or box. The hint highlights the group and the clues that rule out the other cells.
• Naked single — a cell where eight of the nine numbers already appear in its row, column or box, so only one is left.

Hidden singles in boxes come first, because that is how most people scan a grid. When neither technique finds a cell, the puzzle needs harder methods (naked pairs, pointing pairs, X-wings and so on); the hint then says so and reveals the empty cell with the fewest candidates from the solution. If one of your numbers is wrong, the hint points at it first: logic built on a wrong number leads nowhere.

Worked examples

A classic puzzle with 30 clues

This is the example puzzle from Wikipedia’s sudoku article (it is loaded when you first open the page). In text form it reads 53..7....6..195....98....6.8...6...34..8.3..17...2...6.6....28....419..5....8..79.

The first hint is a hidden single in the top-middle box: the 8 can only go in row 1, column 6, because the 8s in row 3, column 3 and row 5, column 4 rule out every other empty cell of that box. Following the hints from there, singles alone fill all 51 empty cells — this is an easy puzzle. The solver needs 52 search steps to prove that the solution is unique:

A puzzle with more than one solution

Remove just one clue — the 8 in row 3, column 3 — and the same puzzle has more than one solution. The solver says so instead of quietly picking one. A puzzle printed with a missing or misread clue is the usual cause; compare the grid with the original.

“The world’s hardest sudoku”

The Finnish mathematician Arto Inkala published this 21-clue puzzle in 2012, and newspapers called it the hardest sudoku ever made: 8..........36......7..9.2...5...7.......457.....1...3...1....68..85...1..9....4... At the start not a single hidden or naked single exists (the hint has to reveal a cell), which is what makes it so hard by hand. The solver still finds its one solution in 24,881 search steps — a few hundredths of a second on a phone.

Sudoku facts worth knowing

• A sudoku needs at least 17 clues to have exactly one solution. Gary McGuire, Bastian Tugemann and Gilles Civario proved in 2012, with an exhaustive computer search, that no 16-clue sudoku has a unique solution. A 16-clue grid will always be reported as having more than one solution.
• Many clues don’t make a puzzle easy, and few clues don’t make it hard: difficulty depends on which techniques are needed. The Inkala puzzle above has 21 clues; many easy newspaper puzzles have fewer than 30.
• There are 6,670,903,752,021,072,936,960 completed 9×9 grids, as counted by Bertram Felgenhauer and Frazer Jarvis in 2005.

Limits

• Classic 9×9 sudoku only — not 4×4, 6×6, 16×16, Killer, jigsaw (irregular) or Samurai variants.
• Hints use singles only. On hard puzzles they will reveal cells instead of naming the advanced technique a human expert would use.
• There is no photo import: type or paste the numbers.

Frequently asked questions

Last updated: October 15, 2026