The goal
Sudoku is played on a 9×9 grid made up of nine 3×3 boxes. Some cells already contain numbers (the "givens"). Your job is to fill in every empty cell so that:
- Each row contains the digits 1–9 with no repeats.
- Each column contains the digits 1–9 with no repeats.
- Each 3×3 box contains the digits 1–9 with no repeats.
Every proper Sudoku has exactly one solution, and you never need to guess — it can always be reached with logic.
How to solve — beginner strategy
- Scan for singles. Look for a row, column or box that already has most of its digits. Often only one cell can take a missing digit.
- Check candidates. For a tricky empty cell, list which digits are still legal there. If only one is possible, it goes in.
- Use notes (pencil marks). Turn on Notes mode to jot the possible digits in a cell. As you place numbers elsewhere, candidates get eliminated.
- Look for hidden singles. Within a box, a digit may only fit in one cell even if that cell has other candidates.
- Work the intersections. If a digit in a box can only sit in one row, it can't appear elsewhere in that row — removing candidates elsewhere.
Difficulty levels
Difficulty mostly comes from how many numbers are given and which solving techniques are needed:
- Easy / Medium — solvable with simple scanning and singles.
- Hard / Expert — need candidate elimination and hidden singles.
- Master / Extreme — require advanced chains and careful pencil marks; fewest givens.
Killer Sudoku
Killer Sudoku uses the same 1–9 rules, but adds cages — dashed-outline groups of cells with a small number showing their target sum. Two extra rules apply:
- The digits inside a cage must add up to the cage's sum.
- A digit cannot repeat within a cage.
Killer puzzles often start with few or no givens, so you lean on the cage sums to break in. For example, a two-cell cage summing to 3 can only be 1 + 2, and a two-cell cage summing to 17 can only be 8 + 9.
Ready to play?
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