How Sudoku works
Sudoku asks you to fill a nine by nine grid so that every row, every column, and every three by three box contains the digits one to nine exactly once. There is no arithmetic in it despite the numbers; the digits are just nine distinct symbols, and the puzzle is pure constraint satisfaction. You could play the identical game with nine colours.
Every grid here is generated fresh in your browser rather than pulled from a fixed library, so you will not see the same puzzle twice. The generator builds a complete solved grid, then removes clues one at a time, checking after each removal that exactly one solution still exists. That check is what separates a real Sudoku from a grid of numbers: a puzzle with two solutions cannot be solved by logic alone, only by guessing.
Difficulty levels
| Level | Starting clues | Typical solve time | Techniques needed |
|---|---|---|---|
| Easy | 45 | 5 – 10 minutes | Scanning and single candidates |
| Medium | 36 | 10 – 20 minutes | Hidden singles, pointing pairs |
| Hard | 30 | 20 – 40 minutes | Naked pairs, box-line reduction |
| Expert | 25 | 30 – 60 minutes | X-wing, chains, extended candidate work |
| Technique | What it does | First needed at |
|---|---|---|
| Scanning | Eliminates a digit from a box using rows and columns | Easy |
| Naked single | One cell has only one possible digit | Easy |
| Hidden single | A digit fits only one cell in a unit | Medium |
| Naked pair | Two cells share the same two candidates | Hard |
| Pointing pair | A digit in a box confined to one line | Hard |
| X-wing | A rectangle pattern eliminating candidates | Expert |
How to solve faster
- Scan for the most constrained digit first. Find whichever digit already appears most often on the grid and place every remaining instance you can. Constraint density does the work for you, and it is far faster than working cell by cell.
- Use pencil notes from medium upwards. Trying to hold candidate lists in your head caps you at easy puzzles. Writing candidates into the cells is not cheating; it is the standard method and it is how every technique above naked singles actually gets applied.
- Solve boxes, not just rows and columns. Beginners tend to scan horizontally and vertically and forget the third constraint. The three by three box is often where the decisive elimination is hiding.
- When you stall, look for pairs before guessing. Two cells in one unit holding exactly the same two candidates eliminates those digits from every other cell in that unit. This single technique unlocks most hard puzzles.
- Never guess on a well-formed puzzle. Every puzzle here has exactly one solution reachable by logic. If you are guessing, there is an elimination you have not spotted, and guessing wrong costs far more time than finding it.
Frequently asked questions
Is every Sudoku puzzle here solvable without guessing?
Yes. The generator verifies after every clue removal that exactly one solution remains, so each puzzle can be completed by logic alone. Harder puzzles need more advanced techniques, but never a guess.
What is the minimum number of clues a Sudoku can have?
Seventeen. This was proved by exhaustive computer search in 2012. No valid Sudoku with a unique solution exists with sixteen clues or fewer, though puzzles at seventeen are extremely difficult.
Does clue count determine difficulty?
Only loosely. A 30-clue puzzle can be easier than a 34-clue one depending on where the clues sit. Clue count sets a rough band, but which techniques are required is what actually determines difficulty.
How long should a Sudoku take?
An easy puzzle runs about 5 to 10 minutes for most people. Experienced solvers finish easy grids in under 3 minutes and expert grids in 10 to 15. Anywhere in those ranges is completely normal.
Does Sudoku involve maths?
No. The digits function as nine distinct symbols and no arithmetic is ever performed on them. The puzzle is logical rather than mathematical, which is why the same grid works identically with letters or colours.
Are my puzzles saved?
Your fastest time for each difficulty is stored in your browser. The puzzles themselves are generated fresh each time, so a specific grid cannot be resumed after you leave the page.