How to use the sudoku solver
Click a square and type a number from 1 to 9. The cursor moves on to the next square, and you can use the arrow keys to jump around. On a phone, tap a square and use the number buttons beside the grid. Leave squares you don’t know empty; Delete, Backspace or 0 clears a square.
If two numbers clash, such as two 7s in the same row, column or 3×3 box, both turn red, so typing mistakes are easy to spot before you solve.
Press Solve to fill in the whole grid. The numbers the solver adds are shown in blue, so you can tell them apart from the ones you typed. Press Clear to start a new puzzle, or load an example to see how it works.
Hints: learn the next step instead of the whole answer
Hint fills in one square and explains why it must be that number. Most hints are one of the two basic sudoku moves. Either a square has only one number left that fits, because every other number already appears in its row, column or box, or a number has only one place left in a row, column or box.
Very hard puzzles sometimes reach a point where neither simple move works. Then the hint takes the square with the fewest options and tells you its number from the solution, and says so.
Checking a puzzle has one solution
A well-made sudoku has exactly one solution. Check the puzzle tells you whether yours has one solution, more than one or none at all. More than one usually means a number was missed when copying it; none means a number was typed in the wrong place.
The solver uses constraint propagation, the same logic as the hints, and then a fast backtracking search that always tries the square with the fewest options first. Even the hardest newspaper puzzles take a few milliseconds, and it all runs in your browser.
Questions and answers
Can it solve any sudoku?
It solves any standard 9×9 sudoku that has a solution, from easy to the hardest published puzzles, usually in a few milliseconds.
What if my puzzle has more than one solution?
The solver fills in one of them and tells you there are others. Check the puzzle shows whether a grid has exactly one solution, more than one or none.
How do I type the numbers on a phone?
Tap a square, then tap a number button under the grid. The Erase button clears the square you tapped.
Why are some squares red?
A red square has the same number as another square in its row, column or 3×3 box. Fix the clash, then press Solve or Hint.
Does the solver need an internet connection?
Only to load the page. Solving, hints and checks all run on your device, and nothing you type is sent anywhere.
Like strategy puzzles? Try Four in a Row against the computer.