1. Last one standing
When a row, column or box already has eight digits, the ninth square is decided. Nothing to work out — just read off which digit is missing.
This box holds 5, 3, 4, 6, 8, 9, 1 and 2. Only the 7 has nowhere else to be.
Sudoku is not a maths puzzle. You never add anything up — the digits 1 to 9 are just nine different symbols, and the whole game is working out where each one has to go.
The grid is 9 squares wide and 9 squares tall, split into nine 3×3 boxes. Fill every empty square so that:
That is the entire rulebook. The digits printed in bold when the puzzle starts are the clues — they can never be changed or removed.
Every puzzle on this site has exactly one solution, and that solution can always be reached by pure reasoning. If you ever feel the need to guess, there is a deduction you have not spotted yet — that is what the hint button is for.
Find the row, column or box that is already the most crowded. The more digits are filled in around a square, the fewer options it has left. Somewhere on a fresh Easy grid there is a square with only one possibility, and finding it is the whole game in miniature.
You only need two techniques to finish every Easy and Medium puzzle here. The rest are for when the grid stops giving things away.
When a row, column or box already has eight digits, the ninth square is decided. Nothing to work out — just read off which digit is missing.
This box holds 5, 3, 4, 6, 8, 9, 1 and 2. Only the 7 has nowhere else to be.
Pick a digit and a box, then look at every row and column running through that box. Each one that already contains your digit blocks the squares it crosses. Often only one square survives — and that square must hold the digit, even if other digits also looked possible there.
The four highlighted 7s block every square in the top-left box except one. That square is a 7.
This is the single most productive habit in sudoku. Run through digits 1 to 9 in turn, box by box, and grids open up fast.
When scanning dries up, start writing candidates: the small digits that are still legal in a square. Turn on Notes (or hold Shift while typing) and jot them in. A square that ends up with just one pencil mark is called a naked single — place it.
Pencil marks are bookkeeping, not decoration. Their real value is that they make the patterns below visible.
If two squares in the same row, column or box both come down to the same two candidates — say 4 and 7 — then those two squares will take the 4 and the 7 between them, in some order. You do not know which way round, and you do not need to: no other square in that unit can be a 4 or a 7. Rub those candidates out and see what falls over.
The same logic scales up. Three squares sharing three candidates form a naked triple.
Sometimes a digit's remaining homes inside a box all sit in the same row. You do not know which of them it is, but you know the digit is somewhere in that stretch — so it cannot appear in the rest of that row outside the box. The reverse works too: if a digit in a row can only land inside one box, it leaves the rest of that box alone.
This one clears candidates rather than filling squares, and it is usually what unsticks a Hard grid.
Selecting or placing a digit shades other squares. Nothing is ever added to the grid but the digit you typed — the colours only point at what is already there.
| Blue | Another copy of the selected digit, anywhere on the grid. Handy for asking "where are all the 7s?" before deciding where the next one goes. |
| Grey | The row, column and box of the selected square — everything it can see, and therefore everything that rules a digit out of it. |
| Red | That entry disagrees with the solution. Only shown while Flag wrong entries is on. |
| Green | The square a hint is pointing at. Amber marks the squares the hint reasons from. |
Each of the first three can be switched off under Assists if you would rather read a plain grid.
A hint names the technique, explains why it works on your grid, and highlights the squares involved. Press Apply if you would rather it made the move for you. Hints read the board as it stands, so they will tell you when something you have entered is wrong.
| 1–9 | Place a digit |
| Shift + 1–9 | Toggle a pencil mark |
| ← ↑ ↓ → | Move around the grid |
| Backspace / Delete | Clear the square |
| N | Toggle notes mode |
| H | Ask for a hint |
| U / Ctrl + Z | Undo |
| P | Pause or resume |
| ? | Open this guide |
| Esc | Close this guide |
Levels are graded by the hardest technique a puzzle actually demands, not by how many clues it starts with: