The grid

Sudoku is played on a 9×9 grid divided into nine 3×3 boxes. Some cells are pre-filled with digits — these are the givens, and they are fixed. Everything you do is fill the empty cells with the digits 1 through 9.

The three constraints

There is exactly one rule, stated three ways. When you fill in a digit, all three must hold:

  1. Row: each of the nine rows contains the digits 1 to 9 exactly once — no repeats.
  2. Column: each of the nine columns contains 1 to 9 exactly once.
  3. Box: each of the nine 3×3 boxes contains 1 to 9 exactly once.

The three constraints are equal partners. Beginners mostly watch rows and forget boxes — yet most of the information that cracks a puzzle comes from the box constraint.

What a puzzle promises you

A properly constructed sudoku has exactly one solution, and that solution can be reached by logic alone — you should never have to guess. If a puzzle seems to require a coin flip, either it is a badly made puzzle or you have not yet found the deduction that unlocks it. For how to find those deductions, see sudoku strategy.

How a game plays out

  1. Scan the givens. Start with the digit that appears most often — it has the fewest places left to go.
  2. Place the digits you can prove. Each placement removes possibilities and often enables the next.
  3. Repeat until the grid is full. A finished grid has every row, column and box containing 1 to 9.

The one mistake almost every beginner makes

New players fill in cells because a number "fits". Fits is not the same as must. A digit can fit in three cells of a box and still be wrong in two of them — the game is about proving which one is forced, not about finding any legal-looking spot. Slow down, and only place a digit when no other cell in that row, column or box can take it.

Difficulty levels and what they mean

Difficulty in sudoku is not about how many cells are empty — it is about which techniques the solution requires. Easy puzzles fall to simple scanning; medium ones need hidden singles; hard ones need pairs, triples and beyond. That is why an easy grid with thirty givens can be harder than a medium one with twenty-six.

Ready to try one? Sudoku has easy through expert grids in the browser, no account needed. And if you enjoy the logic, Nonogram is the natural next puzzle — the puzzle games guide compares all the types.