The single rule
A sudoku grid is nine rows by nine columns, divided into nine three-by-three boxes. Fill every empty cell with a digit from 1 to 9 so that no digit repeats in any row, any column, or any box. That is the entire rule set. Everything else is technique.
The digits given at the start are called clues and cannot be changed. A well-made puzzle has exactly one solution reachable by logic alone — every puzzle generated on this site is checked for that before it appears.
What sudoku is not
It is not arithmetic. The digits could be nine letters or nine colours and nothing would change; they never get added, and their order does not matter. It is also not a guessing game. If you find yourself picking one of two options at random, there is a logical step available that you have not yet found.
Playing on this board
- Tap or click a cell to select it, then tap a number on the pad or type a digit from 1 to 9.
- Tap the same digit again, or press 0 or Backspace, to clear the cell.
- The arrow keys move around the grid, so you can solve entirely from the keyboard.
- Selecting a cell highlights its row, column and box, and every other cell holding the same digit.
- Mistake warnings are on by default and turn a wrong entry red. You can switch them off for a stricter game.
Pencil marks
Pencil marks — also called candidates or notes — are small digits recorded in a cell to show which values are still possible there. They are the single biggest step up from beginner solving, because they turn an invisible memory problem into something you can look at.
Press N, or use the pencil-marks button, to switch into note mode. Any digit you then enter is stored as a candidate rather than an answer. The Fill candidates button does the bookkeeping for the whole grid at once, which is the usual starting point on harder puzzles.
Don't pencil in everything on easy puzzles
On an easy grid, full candidate lists slow you down. Scan for the obvious placements first and only start writing notes when scanning stops producing results.
Your first two techniques
Scanning for hidden singles
Pick a digit, say 7. Look at a box that does not contain a 7. Cross out every cell in that box that shares a row or column with a 7 elsewhere. If exactly one cell survives, that is where the 7 goes. This is called a hidden single, and it solves most of an easy puzzle by itself.
Work digit by digit rather than cell by cell. Doing all the 1s, then all the 2s, is far faster than staring at individual squares.
Naked singles
The mirror image: look at one empty cell and eliminate every digit that already appears in its row, column or box. If only one digit survives, it must go there. Naked singles are easy to spot once candidates are written in, and almost invisible without them.
How difficulty works here
| Level | Typical clues | Techniques needed |
|---|---|---|
| Easy | about 42 | Scanning and hidden singles |
| Medium | about 34 | Naked singles, some candidate work |
| Hard | about 29 | Pairs, locked candidates, careful bookkeeping |
| Expert | about 25 | Longer eliminations and patient elimination chains |
Clue count is a rough proxy, not a guarantee — a 30-clue puzzle can be easier than a 34-clue one depending on where the clues sit. What is guaranteed is uniqueness: every grid here has exactly one answer.
Using hints without spoiling the puzzle
The hint button does not simply fill a square. It finds the next cell that can be solved by logic, names the technique, and explains why it works — for example, that a 4 has only one remaining home in box 6. If you have already entered something that contradicts the solution, it points that out first, because no technique will work on a broken grid.
Good habits from the start
- Finish every easy placement before starting candidates. Free information first.
- When you place a digit, immediately remove it from the candidates of its row, column and box — the board does this for you.
- If you are stuck, change what you are scanning for: switch from digits to cells, or from rows to boxes.
- Never guess. If nothing is available, you have missed something, and the answer is usually a hidden single in a crowded box.