The rules
Put a digit between 1 and 9 into every blank square. In the finished grid, every row, every column and every 3×3 box holds each of those nine digits once, so a repeat anywhere is a mistake.
Digits that come printed with the puzzle, the givens, stay fixed. Every grid on this site has exactly one correct completion: the generator discards any layout that allows a second one.
What the levels mean
Our levels are set by the hardest step a solver needs, not just by how many digits are given. Easy grids can be finished with hidden singles alone: a digit that has only one possible home in a row, column or box. Medium grids also need the single-candidate move, where a cell has only one digit left that fits.
Hard grids require at least one elimination move: a pointing pair, a box–line reduction, or a naked or hidden pair or triple. Expert grids need one of the long-range patterns somewhere along the way, such as an X-Wing, a Swordfish, an XY-Wing or an XYZ-Wing.
A way of working that helps
Start by scanning one digit at a time across the whole grid. Pick the digit that already appears most often and ask, box by box, where it can still go. This finds hidden singles quickly and costs no notes.
When scanning dries up, switch on notes and pencil in the candidates for a single box or line rather than the whole grid. Once two squares of one row, column or box are down to the same pair of candidates, that pair belongs to them, and you may strike both digits everywhere else in that row, column or box.
If you are stuck, press the hint button. It shows the next cell that can be decided and names the technique, so you learn the move rather than just receive the digit. If one of your entries is wrong, the hint points at that cell first.
Controls
Choose a square with a tap and pick its digit from the number pad under the board; the pencil button flips into note mode. With a keyboard, the arrows move around, 1–9 writes a digit, Shift with a digit pencils it in as a note, N switches note mode, Backspace wipes the square, H calls a hint, Ctrl+Z steps back and Ctrl+Y forward again.
Questions
Will I ever have to guess?
No. Every grid here is checked by a solver that only uses logical deductions, so a reasoned next step always exists.
What happens if I close the page?
The game is saved in your browser after every move. Open the same page again and you continue where you stopped, with your undo history.
Why do two easy grids feel different?
The level is fixed by the techniques required, but the number of givens and how they are spread still varies. Some easy grids open up quickly, others make you scan a little longer.
Can I turn off the red conflict marks?
Yes. Open the settings in the toolbar and switch off conflict highlighting or the same-digit highlight.