Sudoku is logic, not luck: givens are clues and you eliminate until the grid is complete. Only three repeating constraints apply, but they combine into long chains of reasoning. Below is the standard ruleset and how this site's controls map to it. To practice, open the Play sudoku board; for habits, see Sudoku strategy.
Grid and boxes
The board has nine rows and nine columns—81 cells. Thick lines mark nine 3×3 boxes. A valid solve uses 1–9 once in every row, once in every column, and once in every box.
Two cells that share a row, column, or box cannot hold the same digit. That overlap rule drives both conflict checks and candidate notes.
Givens and entries
Some cells start filled (givens). You cannot change them. You place 1–9 only in empty cells until the grid is full and legal.
Use pencil mode for small candidates when you are unsure; turn Pencil off to commit an answer. Undo steps back one move; Check highlights rule breaks without solving for you.
When you are done
The puzzle is complete when all 81 cells are filled and no row, column, or box repeats a digit. Conflicts mid-solve can be fixed—you do not have to restart the whole grid.
Tools on this site
Play from the home page. The side panel sets difficulty; the toolbar offers pencil notes, hints, check, undo, new game, and daily puzzle. Hints explain one logical step; daily uses a date-fixed classic board separate from random new games.
Common questions
Is sudoku the same as a magic square?
No. Sudoku only forbids repeats in rows, columns, and boxes. Magic squares require sums along diagonals—different puzzle family.
Can I change the given clues?
No. Givens are part of the puzzle definition. Erasing a given will be treated as your own entry and may show as a conflict.
Can a digit appear twice in one row?
Never. Each digit appears at most once per row, column, and box. Check uses that rule to flag clashes.
How are pencil notes different from real digits?
Pencil shows small candidates inside a cell for reasoning. With Pencil off, a tap writes the final answer and replaces notes in that cell.
Does Hint fill the whole grid?
No. Hint describes one logical move on the current board and highlights relevant cells. It coaches; it does not auto-complete the puzzle.
Does passing Check mean I solved it?
Check only means nothing entered so far breaks the rules. Empty cells can remain—you still need to finish the grid logically.
How does Daily relate to difficulty?
Daily is a fixed classic puzzle for the calendar date. Random new games use the difficulty preset you pick in the side panel.
Can I fix a wrong digit?
Yes. Clear the active cell with Delete or Backspace, or use Undo for the last step. Only cells you filled are editable—not givens.
Do I need math besides 1–9?
No arithmetic. You only track which digits are still allowed in each row, column, and box.
Do I need an account to save progress?
No. In-progress grids live in browser storage on this device; they do not sync when you switch browsers or clear data.
