🧩 Sudoku

How to Play

Sudoku is played on a 9x9 grid cut into nine 3x3 boxes. Your job is to fill every empty cell so that each row, each column, and each of the nine boxes contains the digits 1 through 9 exactly once. There is no guessing: every digit you place is a conclusion you can defend.

Every puzzle here has exactly one solution, so reasoning alone is always enough. Difficulty is set by how many cells start empty. Easy opens 36 cells, Medium opens 45, Hard opens 54, and Expert opens 57, so the harder levels need longer chains of reasoning before anything falls into place.

Tap a keypad number or press the matching key to place it in the highlighted cell. Press P (or the pencil button) for pencil mode, where the number you pick becomes a small candidate note instead of a committed digit. Backspace erases the current cell. You get three mistakes before the game is over; the hint button reveals a correct value when you are stuck. A timer runs from load to solve, and you can check your solve time afterwards.

Example

Suppose a row already shows 3, 5, 1, 8, 2, 9, and 7, leaving two cells open. The row is missing 4 and 6, so each open cell is a 4 or a 6. Now look at the 3x3 box holding the first open cell: if the digit 6 already sits elsewhere in that box, the first open cell cannot be 6 — a box may not repeat a digit — so it must be 4. The second open cell then takes the 6.

Two cells solved with one observation — that is the whole game, repeated. Each forced digit narrows its row, column, and box, which forces another digit elsewhere, and the board clicks forward.

Strategy & Tips

• Start with cross-hatching: take one digit, say 7, scan every row and column where a 7 already sits, and see which boxes still have room for it — often just one cell.
• Fill last-empty-cell units immediately; when a row, column, or box has eight digits, the ninth is forced.
• Keep pencil notes honest — a note is a possibility, not a promise, and stale notes cause most middle-game errors.
• When a digit is restricted to one row or column inside a box, that line blocks the same digit in neighbouring boxes.
• Hunt for naked pairs: two cells in one unit sharing the same two candidates, which can then be cleared from the rest of that unit.
• Never guess. A logical path always exists on a unique-solution puzzle, and a wrong guess costs a mistake you cannot afford at Expert.

Variations & Difficulty Levels

The levels differ in more than hole count. Easy, with 36 open cells, falls to plain scanning. Medium, at 45 holes, asks you to combine a row and a box before a digit becomes clear. Hard, with 54 holes, brings in the intermediate toolkit — hidden singles, naked pairs, pointing pairs. Expert, at 57 holes, demands long reasoning chains with no visible progress for stretches.

Because every puzzle has exactly one solution, difficulty is about how deeply you reason, never trickery — a well-played game always ends with a complete grid.

Common Mistakes & Edge Cases

• Placing a digit that fits the row but breaks the box — check all three units before committing.
• Leaving a stale candidate note after the digit it pointed to has been placed elsewhere; it can mislead you for moves.
• Miscounting which digits are already present in a unit and declaring a cell forced when it is not.
• Erasing a correct digit in a panic after a mistake, then re-solving the same region twice.
• At Expert, mistaking a near-naked pair for a real one when a third cell shares one of the two candidates.

Frequently Asked Questions

Can a Sudoku puzzle have more than one solution?

In general yes, but none here — every puzzle is generated and checked to have exactly one valid solution.

What happens after my three mistakes are gone?

The game is over for that run. Start a fresh puzzle; the failed run's timer does not carry over.

Does the timer keep running in pencil mode?

Yes — pencil mode is a solving tool, not a pause button, so the clock keeps counting while you review candidates.

Why does a puzzle sometimes seem impossible?

A deduction has been missed. Revisit units with two or three candidates, or use the hint button to see the overlooked cell.

Is Expert just Medium with more empty cells?

Not really — the extra holes force intermediate techniques such as naked pairs and pointing pairs.

Can I compare solve times?

Yes, the results screen shows your time and your best for that difficulty. The related Daily Challenge uses the same format on a fresh grid each day.