The complete rule
Fill every empty cell with a digit from 1 to 9. Each horizontal row, vertical column and outlined 3×3 box must contain every digit exactly once. A placement is valid only when it satisfies all three units at the same time; matching the row alone is not enough.
Three useful Sudoku habits
Scan before guessing. Look for rows, columns or boxes with only one or two empty cells.
Work across the whole grid. A number placed in one box often unlocks possibilities elsewhere.
Use elimination. When a digit cannot appear in eight cells of a unit, it belongs in the ninth.
A worked mini-example
Suppose a row is missing 4 and 7. The first empty cell's column already contains 7, so that cell must be 4. The second empty cell then becomes 7. This is a forced placement: the row gives the two possibilities and the intersecting column removes one.
A repeatable first pass
Choose one digit and inspect every 3×3 box. Existing copies of that digit rule out matching rows and columns in the other boxes. Place it only where one cell remains possible, then rescan the affected row, column and box before moving on.
When that pass no longer produces a forced move, inspect nearly complete units and add candidate notes selectively. Keep notes small and update them after every confirmed placement. The linked naked-pairs guide explains one way candidate notes can create a later elimination.
Accessibility and input
Select a cell, then use the on-screen number controls or the supported keyboard input. The active cell and fixed clues use more than colour alone to communicate state. Progress checking reports whether the current entries are consistent without requiring a timer. If the interactive grid is uncomfortable to use, choose the spacious printable version and include an answer sheet only when needed.
Continue learning
Read our guide to scanning a Sudoku grid without guessing, learn to use candidate notes cleanly, or study naked pairs. Recent puzzles remain available in the seven-day archive.
Sudoku FAQs
How do you play Sudoku?
Fill every empty cell so each row, column and 3×3 box contains the digits 1 through 9 exactly once.
Do I need to guess?
A well-formed Sudoku can be solved logically. Start by scanning for forced placements and use candidates only when the next move is not immediately visible.
Is the Fun IQ Sudoku free?
Yes. The daily puzzle and progress checking are free and require no account or download.