Sudoku the rules

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Fill the grid so that every row, every column and every 3 × 3 box has the digits 1 to 9, each once. Some digits are given. Every puzzle here has one solution and can be solved by logic alone — no guessing.

The rule

The grid has 9 rows, 9 columns and 9 boxes of 3 × 3 squares (the thick lines). Each of them holds 1, 2, 3, 4, 5, 6, 7, 8 and 9 exactly once. So a square "sees" its row, its column and its box: no digit may appear twice in what it sees.

The middle square sees its row, its column and its box (lit up).

Trick 1: one digit left

Look at what a square sees. If eight different digits are already there, the ninth is the only one left: write it.

Row 3 has 8, 1, 3, 7, 5 and 4; column 6 adds 6; the box adds 2. Only 9 is left.

Trick 2: one place left

Turn it round: pick a digit and a box (or a row or column). Cross out every square the digit cannot go — because that row or column already has it. If only one square is left, the digit goes there.

Where does 9 go in the middle left box? The 9s in column 1, column 3 and row 6 rule out every square but one.

Notes

When it gets hard, write notes: the small digits that could still go in a square (✎ Notes, or hold Shift). A square with only one note left is that digit. When you write a digit, the game takes it out of the notes in its row, column and box.

Notes in the first two rows: two squares (ringed) have only one note left, 3.

Medium and Hard

Easy puzzles need only the two tricks above. Medium puzzles also need these:

Locked digits: if a digit can only go in one row of a box, it cannot go anywhere else in that row (and the same with columns, and the other way round).
Pairs: two squares of a row that can both only be 4 or 7 take the 4 and the 7; no other square of the row can have them. And two digits that fit in only the same two squares of a row own those squares.

Hard puzzles also need triples (the same with three squares and three digits), the X-wing (a digit with only two places in each of two rows, in the same two columns: it cannot go anywhere else in those columns) and the Y-wing (three squares with two notes each, like 1 2, 2 3 and 1 3, that rule out a digit where two of them meet). None of them needs a guess.

Red digits

Two equal digits in the same row, column or box turn red. The game does not say whether a digit is right — only when two of them clash. When the grid is full without a clash, the puzzle is solved.

A 1 written in the middle left box clashes with the 1 in its box and the 1 in its row.

The puzzle of the day

Three puzzles a day — Easy, Medium and Hard — the same for every logged-in member (guests get puzzles of their own, without times); they change at midnight, Swedish time. Logged in with Discord, the time from opening a puzzle to solving it counts for today's list, and solving at least one each day keeps your streak. You can also practise with a new puzzle every time.

Playing here

Pick a square, then a digit on the buttons beside (or under) the grid. The small number on a button is how many of that digit are still to place.
✎ Notes writes small digits; ⌫ Erase clears a square. Undo with ↶ or Ctrl+Z.
Keyboard: arrow keys move, 1–9 writes (Shift + digit: a note), Delete or 0 erases, N switches notes on and off.
The puzzle you are solving is kept in the page address, so a reload keeps it. Nothing is stored in your browser.
Logged in with Discord, your first solved puzzle on a level unlocks rewards: Easy — digit colours, Medium — 3D squares, Hard — a crown on your HAKOO avatar.

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