Sudoku: rules and solving techniques

What is Sudoku?

Classic Sudoku is played on a 9×9 grid divided into nine 3×3 boxes. Some digits are given at the start and cannot be changed.

In this guide, a cell means one square in the grid, while a 3×3 box is one of the nine groups of nine cells.

Goal of the game

Fill every empty cell with a digit from 1 to 9 while satisfying all three Sudoku constraints. Every DIGIGMA puzzle has a unique solution and can be solved logically.

Rules

• Use only the digits 1–9.
• Each digit appears once in every row.
• Each digit appears once in every column.
• Each digit appears once in every 3×3 box.

A digit can go in a cell only if it is not already present in that cell’s row, column or 3×3 box.

How to start a grid

To get started, choose an Easy puzzle from the Sudoku library. Select an empty cell and enter a digit. If several possibilities remain, use Draft to note the candidates.

Begin with the most complete areas: a row, column or 3×3 box with only a few gaps often gives you the first deductions.

A simple step-by-step method

1. Scan rows, columns and 3×3 boxes that are nearly complete.
2. Find cells with only one possible candidate — naked singles.
3. Use Draft to record candidates in harder cells.
4. After every placement, reassess the constraints around that cell.
5. Then look for digits that have only one possible position in a row, column or box — hidden singles.
6. Repeat this cycle until the grid is complete.

A new deduction often appears near the last cell you solved. Work locally rather than scanning the whole board at random.

Core solving techniques

Beginner — Naked single

If a cell has only one candidate left, that digit is forced.

Beginner — Hidden single

A digit may appear possible in several cells at first, but only one position can satisfy all the row, column or box constraints. That digit is then forced into that position.

Beginner — Candidate elimination

Every placed digit removes that same candidate from cells in the same row, column and box. If a cell is left with one candidate, it becomes a naked single.

Intermediate — Naked pair

If two cells in the same row, column or box contain exactly the same two candidates, those digits are locked into those two cells. Remove them from the other cells in that unit.

Intermediate — Hidden pair

If two digits can appear only in the same two cells of a unit, those cells are reserved for that pair. Any other candidates in those two cells can be removed.

Intermediate — Box-line interaction

If every possible position for a digit inside a 3×3 box lies on the same row, that digit can be removed from the rest of that row outside the box. The same logic works with columns.

More advanced patterns exist, but Easy and Medium puzzles usually do not require them. If you get stuck, clean up your candidates and look again for singles before reaching for rarer techniques.

When you’re stuck

• Recheck rows, columns and 3×3 boxes that are nearly complete.
• Review your Draft candidates and remove anything that is no longer possible.
• Look for hidden singles one digit at a time.
• Check box-line interactions where a digit has only a few possible positions.
• Avoid guessing: DIGIGMA puzzles are designed to progress by deduction.

Hint can point you towards a useful next step if you want help. Reveal completes the grid. Neither is needed for normal solving.

Common mistakes

• Guessing too early instead of looking for a logical deduction.
• Checking a row but forgetting the corresponding 3×3 box.
• Keeping candidates that have already become impossible.
• Filling every cell with candidates and never updating them.

Practical tips

• After every placement, immediately inspect the surrounding row, column and box.
• When you get stuck, choose one digit and trace its possible positions through the 3×3 boxes.
• Draft is a working tool, not a score: a few clean candidates are more useful than a cluttered grid.
• Once these habits feel natural, try them on a puzzle from the Sudoku library.

Interactive tutorial

The tutorial below uses the same interactions as a normal game. You will finish a row, find a missing digit in a 3×3 box, and solve a cell constrained by both its row and column. Then you can open the Sudoku library and play a full puzzle.

FAQ

Is guessing necessary in Sudoku?

No. On a DIGIGMA puzzle, useful placements can be deduced logically.

What should I do when I get stuck?

Clean up your candidates, look for new singles, then trace one digit at a time through the 3×3 boxes.

What are pencil marks?

On DIGIGMA, this is called Draft. These are the digits that are still possible in a cell until you have enough information to decide.

Can a Sudoku have more than one solution?

DIGIGMA puzzles do not: they are built with a unique solution.

How long should a Sudoku take?

It depends on the difficulty and your pace. The goal is to solve cleanly, not to race the clock.