Sudoku Tips

The Golden Rule: Never Guess

The most important principle in Sudoku is to never place a digit unless you are completely certain it belongs there. Every well-made Sudoku puzzle is designed to be solved through logic alone. If you feel stuck, it means there is a clue somewhere that you have not yet noticed — not that you need to guess.

Guessing creates a cascade of errors that can be very difficult to trace back. The right approach when stuck is to switch your focus to a different row, column, or box rather than guessing in the current one.

Tip 1: Start Where the Most Digits Are

Look for rows, columns, or 3×3 boxes that already contain many filled-in digits. The more digits present, the fewer candidates remain for each empty cell, making it easier to pinpoint the correct value. Completing these "crowded" areas first often unlocks other cells through a chain reaction.

For example, if a box already contains seven of the nine digits, the two empty cells can only hold the two missing digits. Check the corresponding rows and columns for those two digits to determine which cell gets which.

Tip 2: Use Pencil Marks (Candidate Notes)

For cells where you cannot immediately determine the answer, write down all the digits that could possibly fit. These are called pencil marks or candidate notes. The Sudoku game on sotjun.com has a built-in memo mode that lets you mark multiple candidates in a single cell.

Every time you confirm a digit somewhere on the board, go back and erase that digit from the candidates of every cell in the same row, column, and box. Keeping your pencil marks accurate and up to date is the key to solving Medium and Hard puzzles without errors.

Tip 3: Find Naked Singles

A naked single is a cell where only one candidate remains after all other digits have been eliminated. There is no decision to make — just fill it in immediately. With well-maintained pencil marks, naked singles become easy to spot at a glance.

After filling in a naked single, immediately update the candidates of related cells, which often creates new naked singles nearby. This chain effect can resolve large portions of the puzzle quickly.

Tip 4: Find Hidden Singles

A hidden single occurs when a particular digit can only go in one cell within a row, column, or box — even though that cell may still have several candidates written in it.

For example, if the digit 7 can only fit in one cell within a given box (because the other cells are blocked by a 7 in the same row or column), then that cell must contain 7, regardless of its other candidates. Scanning each unit for this pattern — "where else could this digit go?" — is one of the most effective techniques in Sudoku.

Tip 5: Analyze Boxes Before Rows and Columns

Beginners often focus exclusively on rows and columns, but 3×3 boxes deserve equal attention. Within each box, check which digits are still missing and use the rows and columns passing through the box to eliminate impossible positions. Starting your analysis with the boxes that already have the most digits filled in gives you the quickest wins.

Tip 6: Build a Consistent Routine

Every time you place a confirmed digit, follow the same routine: remove it from candidates in the same row, same column, and same box. Before committing to a digit, double-check all three units one last time. This habit takes only a few extra seconds per move and dramatically reduces the chance of making an error that forces you to restart.

Consistency is especially valuable in longer puzzles. Skipping a step once is how most errors happen — and in Sudoku, a single wrong digit can hide for many moves before revealing itself.