How to play Sudoku

The beginner loop: scan one row, column, or box; list what is missing; rule out positions; place a certain digit; then scan again.

The three Sudoku rules

1. Each row contains 1–9 once. A digit cannot repeat across a horizontal row.
2. Each column contains 1–9 once. The same restriction applies vertically.
3. Each 3×3 box contains 1–9 once. The nine bold-outlined regions must also contain every digit.

Every cell belongs to one row, one column, and one box at the same time. A number is legal only when it does not duplicate that number in any of those three groups. The starting numbers, often called givens or clues, stay fixed while you solve the empty cells.

How to start an easy Sudoku

1. Find a nearly complete group. Look for a row, column, or box with many givens. Fewer empty cells mean fewer possibilities to compare.
2. Identify the missing digits. If a row contains 1, 2, 3, 5, 6, 8, and 9, then its empty cells must receive 4 and 7.
3. Check each empty cell in all three directions. A 4 already present in the cell's column or box rules 4 out there. If only 7 remains, place 7.
4. Rescan after every placement. One solved cell changes its row, column, and box, often creating the next forced answer.
5. Move to another area when nothing is certain. Progress elsewhere may remove a candidate and unlock the area that looked stuck.

This method is more reliable than searching the entire grid at random. Work through one group carefully, make only placements you can explain, and let each certain answer create new information.

Use candidates instead of guessing

A candidate is a digit that could legally fit an empty cell based on the current grid. If a cell might be 2 or 6, write both as small notes and leave the cell unresolved. Remove a candidate when that digit is placed in the same row, column, or box.

Candidate notes are not a shortcut or a failure of memory. They make the reasoning visible and reduce the need to repeatedly reconstruct the same possibilities. In Unrot, select an empty cell, turn Notes on, and use the number pad to add or remove candidates. Turn Notes off when you are ready to commit a value.

Two beginner techniques that solve many easy puzzles

Naked single: one candidate remains in a cell

Suppose an empty cell's row blocks 1, 3, 5, and 8; its column blocks 2, 6, and 9; and its box blocks 4. The only digit left is 7, so that cell must be 7. The answer is “naked” because the cell itself has only one possible candidate.

Hidden single: one position remains for a digit

Sometimes a cell still has several candidates, but one of those digits can appear nowhere else in the same row, column, or box. If 5 can fit in only one cell of a particular 3×3 box, that cell must contain 5. The single is “hidden” among the other notes in the cell.

What to do when you get stuck

• Scan each 3×3 box for one missing digit, then repeat by rows and columns.
• Check whether a candidate appears in only one position within a group.
• Update stale notes after every placement; an old candidate can hide a new single.
• Look for cells with two candidates, but do not choose between them without a reason.
• Take a short break if you are rereading the same area without seeing new information.

A valid puzzle is designed to have one solution, but a difficult puzzle may require techniques beyond singles. For your first few grids, choose an easy difficulty and concentrate on making each placement traceable rather than finishing quickly.

Common beginner mistakes

• Checking only the row. Every placement must also be legal in its column and 3×3 box.
• Treating digits as quantities. Sudoku does not require sums; 9 is simply a different symbol from 8.
• Guessing to create movement. One unsupported entry can produce conflicts much later and make the original mistake hard to find.
• Filling every possible note immediately. Begin with nearly complete areas and add notes where they help you compare real alternatives.
• Racing the timer. Accuracy and explainable deductions matter more while you are learning the solving rhythm.

Try the method on a real puzzle

Start on Easy. First scan for rows, columns, or boxes with one or two empty cells. Use Notes when more than one value remains, and use Undo if you enter something accidentally. The current puzzle is saved on your device, so you can leave and return without turning the session into a race.

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If you want a shorter activity before a full grid, try a guided five-minute session. For a broader practice plan, read how much brain training to do each day.