What are the rules of Sudoku?
Sudoku is played on a 9 × 9 grid split into nine 3 × 3 boxes. Some cells are given; fill the rest so that every row, every column and every box holds each number from 1 to 9 exactly once. Nothing is left to chance: the given numbers allow only one way to finish the grid, and you can always reason your way to it.
Tap a cell, then a number. Turn on Notes to pencil small candidates into a cell instead; placing a number clears it from the notes it rules out. A number that repeats in its row, column or box is outlined as soon as you place it.
How do the Sudoku levels work?
Each level is set by the hardest technique a puzzle needs, not by how many numbers it gives. Our solver works each puzzle the way people do, always trying the simplest technique first and a harder one only when every simpler one is stuck. The level is the hardest one it had to use.
| Level | Needs at least once | Numbers given |
|---|---|---|
| Easy | Only singles | 34–35 |
| Medium | pointing pair, box-line reduction, naked pair or hidden pair | 24–30 |
| Hard | naked triple, hidden triple or X-Wing | 24–32 |
| Expert | naked quad, hidden quad, Swordfish, XY-Wing or XYZ-Wing | 24–30 |
Easy puzzles also keep at least 34 numbers. The harder levels give a similar count, which is why a puzzle with few numbers isn't automatically a hard one.
How do you solve Sudoku?
- Singles. A cell where only one number fits its row, column and box; or a number that fits in only one cell of a row, column or box. Easy puzzles need nothing else.
- Pointing. If a box's only cells for a 4 lie in one row, the 4 of that row is in this box, so no other cell of the row can be a 4.
- Pairs. Two cells of a row that can only be 3 or 8 hold the 3 and the 8 between them, so every other cell in the row can cross both out.
- X-Wing. If a number can go in only two cells of each of two rows, and those cells share the same two columns, the number is in those columns at those rows, and nowhere else in the two columns.
- Wings and Swordfish. The Expert techniques combine three cells or three rows in the same way. Notes make them visible; without notes, Expert puzzles are very hard to see through.
How is your Sudoku time calculated?
Your result is the time from Start until the last number is in and nothing repeats. Next to it are the times you pressed Check, which marks any wrong number, and Hint. A hint fills in the square you have picked and, when a single shows its number, says which one. If that square is already filled, the hint fixes a wrong number elsewhere or places a number you could have found with a single. There is no rating, because no published typical time exists for puzzles graded this way.
Does every Sudoku here have exactly one answer?
Yes. The 160 puzzles were made ahead of time and graded by the solver, and the site's tests grade each one again and count its answers with a separate search: exactly one every time. Each game shows a stored puzzle in a new disguise, with the numbers relabelled and the rows and columns shuffled within their bands, which changes neither its answer count nor its level.
There are 6,670,903,752,021,072,936,960 ways to fill a Sudoku grid, counted by Bertram Felgenhauer and Frazer Jarvis in 2005, and a puzzle needs at least 17 given numbers to have only one answer, which Gary McGuire, Bastian Tugemann and Gilles Civario proved by an exhaustive computer search in 2012.
Where does Sudoku come from?
The puzzle first appeared as “Number Place” in Dell Magazines in 1979, usually credited to Howard Garns. The Japanese publisher Nikoli took it up in 1984 and gave it the name Sudoku, and it spread worldwide after The Times in London began printing it in 2004.
For a Sudoku with sums, try Kakuro; for one with arithmetic cages, Calcudoku.