How to play
Fill the empty cells with the digits 1 to 9. Every row, every column and each of the nine 3×3 boxes must contain each digit exactly once. The digits printed at the start — the givens — cannot be changed.
Each puzzle has exactly one solution, and it can be reached by logic alone. You never need to guess and backtrack.
Tap a cell, then a digit on the pad. Switch the pencil on (N on a keyboard, or hold Shift while typing a digit) to write small candidate notes instead. When you place a digit, matching notes in the same row, column and box disappear for you; you can turn this off in the settings.
Strategy and tips
- Scan by digit, not by cell. Pick a digit that already appears often and check each box that still lacks it. When only one cell in a box can take it, you have a hidden single — the move that finishes every easy grid on this site.
- When a cell can take only one digit because the other eight already appear in its row, column or box, you have a single candidate. Medium grids are built so that you need this view at least once.
- Start writing notes when the obvious moves dry up. Notes make patterns visible: two cells in a unit that share the same two candidates form a naked pair, and those two digits can be removed from every other cell of that unit.
- Look for pointing pairs. If a digit’s only possible cells inside a box lie in one row, it can be removed from the rest of that row outside the box. The reverse — a row that confines a digit to one box — is called box–line reduction.
- Expert grids need at least one pattern such as X-Wing, Swordfish, XY-Wing or XYZ-Wing. The technique guide walks through each of them.
- Stuck? The hint does not just reveal a random digit. It names the technique behind the next placement, and if your grid already contains a mistake, it points at the mistake first.
Questions
- How are the difficulty levels decided?
- By the hardest technique the solver needs. Easy grids fall to hidden singles alone and start with 36–40 givens. Medium needs single candidates and starts with 29–32. Hard needs locked candidates, pairs or triples. Expert needs X-Wing, Swordfish, XY-Wing or XYZ-Wing. Which ideas you must use matters more than the number of givens.
- Is there always only one solution?
- Yes. A grid is only published if a logical solver can complete it with every step forced, which also proves that the solution is unique.
- What is the difference between a numbered sheet and the free game?
- A numbered sheet, such as Sudoku #7, Hard, is the same grid for every visitor, so you can compare times or return to it later. The free game builds a new random grid each time you press New game.
- Will I lose my progress if I close the tab?
- No. Your digits, notes, timer and undo history are saved in this browser as you play, and the puzzle continues where you stopped. Nothing is sent to a server.
- Can I play with a keyboard?
- Yes. Arrow keys move, 1–9 enter digits, Shift plus a digit writes a note, N toggles notes, Backspace or 0 clears a cell, H asks for a hint, and Ctrl+Z and Ctrl+Y undo and redo.