Card Game Nook

How to Play Sudoku

Fill a 9 × 9 grid so that every row, every column and every 3 × 3 box contains the digits 1 to 9 exactly once. The digits printed at the start are fixed clues. A properly built sudoku has exactly one solution and can always be reached by deduction, never by guessing.

The rules, step by step

  1. 1Fill every cell with 1 to 9The grid has 81 cells. When it is full and legal, the puzzle is solved.
  2. 2No repeats in a rowEach of the nine rows must contain every digit from 1 to 9 exactly once.
  3. 3No repeats in a columnThe same applies down each of the nine columns.
  4. 4No repeats in a boxEach of the nine 3 × 3 boxes must also contain 1 to 9 exactly once.
  5. 5Clues are fixedThe digits present at the start are part of the puzzle and cannot be changed.

Try it — the rules above, running

Level
Time
0:00
Clues
41
Empty
40
Puzzle
#20260726

Your statistics · easy

Played
0
Won
0
Win rate
0%
Streak
0
Best streak
0
Best time
—

This level

40 clues. Solvable throughout with naked and hidden singles — no guessing, no chains.

Keyboard

Arrows
Move the selected cell
1 – 9
Enter a digit
Backspace
Clear the cell
P
Pencil marks on or off
F
Full screen

Where do you start a sudoku?

  1. 1Find the fullest boxLook for the 3 × 3 box with the most clues. It has the fewest possibilities and usually gives up a digit immediately.
  2. 2Scan one digit at a timeTake the digit that appears most often in the clues. For each box that does not have it, cross out the rows and columns where it already sits. If one cell survives, it is a hidden single.
  3. 3Fill in the naked singlesAny cell whose row, column and box already contain eight different digits has only one possible value.
  4. 4Pencil in the restWhen scanning stops producing digits, write candidates into the empty cells and look for pairs.

What techniques come after singles?

  • Naked pair — two cells in a unit share the same two candidates, so no other cell in that unit can hold either digit.
  • Hidden pair — two digits that can only go in the same two cells of a unit; everything else can be erased from those cells.
  • Pointing pair — a digit confined to one row within a box can be eliminated from the rest of that row.
  • Box-line reduction — the same argument in reverse, from a row or column into a box.

How do you win more often?

  • Scan for hidden singles by digit, not by cellPick a digit, look at each box, and cross off the rows and columns where it already appears. Very often exactly one cell survives — that is faster than examining cells one at a time.
  • Pencil in a whole box at a timeCandidates are only useful when they are complete. Half-filled pencil marks lead to false deductions.
  • Look for pairs before triplesTwo cells in a unit with the same two candidates lock those digits out of every other cell in that unit. It is the cheapest technique beyond singles and it unlocks most hard puzzles.
  • Work the most-constrained unitThe row, column or box with the fewest empty cells is where the next deduction almost always is.

Common questions about sudoku

Can a sudoku have more than one solution?
A valid sudoku cannot. If a grid admits two solutions it is not a sudoku, and no amount of logic will settle the ambiguous cells.
Is sudoku a maths puzzle?
Not really. The digits are just nine distinct symbols — you could play with nine colours and nothing would change. It is a constraint-satisfaction puzzle, not arithmetic.

Ready to stop reading? Play sudoku on the full board, with statistics, deal numbers and unlimited undo.