How to Solve a Sudoku Puzzle

Sudoku has a single rule: fill the 9×9 grid so that every row, every column, and every one of the nine 3×3 boxes contains the digits 1 through 9 exactly once. No arithmetic is involved — it's pure logic. A well-made puzzle has exactly one solution reachable without guessing.

You start with some cells already filled, called givens. The fewer givens, the harder the puzzle, which is roughly how difficulty levels are set.

The first technique is scanning: pick a digit and look across the rows, columns, and boxes where it already appears. Those lines rule out cells, and sometimes only one cell in a box is left where that digit can go — so it must go there.

The flip side is the single candidate: look at one empty cell and eliminate every digit that already appears in its row, column, or box. If only one digit survives, that's the answer for that cell. Alternating between these two ideas cracks most easy and medium puzzles.

For tougher puzzles, note the possible digits (pencil marks) in each empty cell. Patterns then emerge: if two cells in a unit can only hold the same two digits, those digits are locked to them and can be removed from the rest of the unit — a technique called naked pairs.

Harder grids need more advanced patterns, but they never require guessing in a properly built puzzle. If you get stuck, the Sudoku solver will complete the grid and show the answer, and you can generate fresh puzzles at any difficulty to practice these techniques.