There are three rules, and all three are the same sentence wearing different faces: nothing may repeat. Fill the empty cells of a 9×9 grid with the digits 1 to 9 so that
- no digit appears twice in the same row
- no digit appears twice in the same column
- no digit appears twice inside the same 3×3 box, the blocks marked off by the heavy lines
Fill the grid without breaking any of the three and you're done. The digits that were there from the start cannot be changed.
There is no arithmetic here
Digits make it look like a maths puzzle, but in sudoku 1 to 9 are just nine symbols that differ from one another. Swap them for A to I and the game works exactly the same. No sum matters, and a large digit is worth no more than a small one. What the puzzle asks for is not calculation but an eye that misses nothing.
Guessing is not part of it
A properly built puzzle has exactly one solution. If there were two, either choice would obey the rules — and that isn't solving by logic, it's just filling blanks.
Every puzzle here is checked after each clue is removed to confirm the solution is still unique, and the removal is undone the moment a second one appears. So when you're stuck, the honest sentence is never “I can't tell which one” — it's “I haven't seen it yet”.
What the words mean
| Term | Meaning |
|---|---|
| Row | A horizontal line. Nine cells |
| Column | A vertical line. Nine cells |
| Box | A 3×3 block marked off by the heavy lines. There are nine of them |
| Clue | A digit that was there from the start. It cannot be changed |
| Candidate | A digit that could still go in a cell. You jot these down as notes |
| Peers | The twenty cells sharing a row, column or box with a given cell — the reach of the rules |
Fewer clues sounds harder, but it isn't reliably so. Difficulty is decided not by how many clues there are but by which technique the puzzle demands — which is why this site prints something like “needs hidden singles” next to the difficulty label.
When you know the rules and it still won't come
People get stuck in much the same places. Four things account for nearly all of it.
- Guessing. There is only one solution, so guessing buys you nothing, and one guess sends the grid quietly wrong. The mistake usually surfaces ten cells later, by which point you can't trace it back. If you aren't sure, don't place it — write it as a note.
- Skipping notes. Counting candidates in your head means re-scanning twenty cells every time you look at one. That effort is why people end up checking only two or three cells, and puzzles that fall to two or three cells are the easiest ones.
- Watching cells and never digits. Asking “what goes in this cell?” over and over. The opposite look — picking one digit and hunting across the whole grid for where it can go — is the one that usually breaks a stuck puzzle open.
- Forgetting the boxes. Rows and columns catch the eye; the 3×3 boxes stay invisible even with lines around them. Scan the nine boxes on their own and something you missed will turn up.
If you're doing all four and it still won't move, you've reached the point where a technique is required. From here the methods have names, and the hint picks the easiest one that applies and points it out.