Sudoku
Sudoku is a number puzzle on a 9×9 grid divided into nine 3×3 boxes. Fill every empty cell with a digit from 1 to 9 so that each row, each column and each box contains every digit exactly once.
How to play
Every Sudoku here has exactly one solution, checked by a separate solver before it ships, and none of them needs a guess. The level is set by the hardest technique the puzzle truly needs, not by how many digits are given.
- Each row contains the digits 1 to 9 once.
- Each column contains the digits 1 to 9 once.
- Each 3×3 box contains the digits 1 to 9 once.
- The given digits cannot be changed.
Playing on this site
- Select a cell, then type a digit or tap it on the number pad. Arrow keys move the selection.
- Turn on Notes (or press N) to pencil in candidates instead of answers.
- Hint first points out any wrong digits, then names the next technique and shows where it applies before giving anything away.
- Your puzzle is saved in this browser, so you can come back to it later.
Techniques by level
Every technique the solver knows, from easiest to hardest. A puzzle's level is the hardest one it needs.
Easy
- Hidden single
- In the highlighted area, one digit fits in only one cell.
- Naked single
- The highlighted cell has only one possible digit left.
Medium
- Pointing
- Inside a box this digit fits only in one line, so it can't go elsewhere on that line.
- Claiming
- On this line the digit fits only inside one box, so it can't go elsewhere in that box.
- Naked pair
- Two cells share the same two candidates, so those digits leave the rest of their area.
- Hidden pair
- Two digits fit only in the same two cells, so those cells hold nothing else.
- Naked triple
- Three cells hold only three digits between them, so those digits leave the rest of their area.
- Hidden triple
- Three digits fit only in the same three cells, so those cells hold nothing else.
Hard
- X-Wing
- A digit is limited to the same two columns in two rows (or the reverse), so it leaves the rest of those lines.
- XY-Wing
- Whatever the middle cell holds, one of the two wings gets the shared digit, so cells seeing both wings can't.
- Naked quad
- Four cells hold only four digits between them, so those digits leave the rest of their area.
- Hidden quad
- Four digits fit only in the same four cells, so those cells hold nothing else.
- Swordfish
- A digit is limited to the same three columns in three rows (or the reverse), so it leaves the rest of those lines.
- XYZ-Wing
- One of these three cells must hold the shared digit, so cells seeing all three can't.
Expert
- Simple coloring
- Following pairs of a digit splits its cells into two colors; one color is true, which rules out the highlighted spots.
- Jellyfish
- A digit is limited to the same four columns in four rows (or the reverse), so it leaves the rest of those lines.
- XY-Chain
- A chain of two-candidate cells ends on the same digit at both ends; one end has it, so cells seeing both ends can't.
Questions
How are the Sudoku levels decided?
A solver that works like a person tries techniques from easiest to hardest. Easy puzzles need only singles; Medium adds pairs, triples and box–line interactions; Hard needs patterns such as X-Wing and Swordfish; Expert needs chains and coloring. A puzzle's level is the hardest technique it cannot be solved without.
Will I ever have to guess?
No. Every puzzle can be finished by logic alone, at every level.
Is this Sudoku free?
Yes. Every level is free to play here, with no account and no sign-in.