Sudoku guide: make every elimination count

The rules

The grid is 9×9. Each row, column and 3×3 box must contain 1 through 9 exactly once. Given digits are fixed. Pencil marks record possibilities; a mark is not a promise that a digit belongs there.

Easy, normal and hard aim for 42 / 34 / 28 givens respectively. A removal is kept only if the solution remains unique, so a puzzle may retain more than its target. Clue count does not guarantee a particular solving technique. The diagrams distinguish a single-row example from full boards taken from an actual generated puzzle.

Getting started

Start with a nearly full row or box. List its missing digits, then check each empty cell’s column as well. If a row lacks only 2 and 6 and one empty cell’s column already contains 2, that cell must be 6 and the other must be 2. Connect a house-wide fact to the constraints on each cell.

After placing a digit, revisit its row, column and box. Removing that digit from their candidates may expose the next placement. The four full-board diagrams below share one actual generated puzzle and show every candidate permitted by the current givens. The single-row example is drawn on its own.

Naked single

Fill a cell with only one candidate. This row already contains 1 through 8, leaving 9 for its final cell. Subtract the used digits from the row’s required set. On a full board, check the column and box too. If a cell has no candidates, review earlier entries instead of forcing a value into it.

Hidden single

Place a digit when a house has only one location for it. In row 2 of the full board, only column 9 includes candidate 4. That cell is 4 despite its notes {4,6,8}. Check which given excludes 4 from each other empty cell in the row. This counts locations for a digit, rather than candidates in a cell.

Naked pair

When two cells in a row contain exactly the same two candidates, remove those digits elsewhere in that row. Row 6, columns 4 and 5, both have {2,9}. Column 2 loses 2 and 9 from {2,3,6,9}, leaving {3,6}; column 9 loses them from {2,6,9}, leaving 6. You do not need to decide the pair’s order. An extra candidate in either cell would break the premise.

Pointing candidates

If a box confines a digit to one row, remove it from that row outside the box. The upper-left box allows 1 only in row 3, columns 1 and 2. Either placement uses row 3, so r3c5 cannot be 1. All candidates are shown: check that none of the box’s other seven cells includes 1 before using the deduction.

X-Wing

Look for an X-Wing when two rows each allow a digit in exactly the same two columns. Rows 1 and 4 allow 7 only in columns 4 and 5. Either diagonal assignment uses both columns, eliminating 7 from r3c5. Check for extra 7 candidates in the base rows before treating the four corners as a pattern.

Common mistakes

Check your notes before trusting a single. An unjustified deletion can create a false lone candidate. Gather the used digits from the suspect cell’s row, column and box, then compare the remainder with its notes. After deleting a value or undoing, check whether earlier eliminations still have valid premises.

Return to singles after an elimination. Searching only for pairs or X-Wings can hide a new simple placement. A missing duplicate underline is not proof of correctness. If these techniques do not advance a difficult board, note which cells need more information and inspect another region.

Records and challenges on this site

You have 3 hints per puzzle. Each fills the selected cell and adds 60 seconds to the ranked time. Undo does not refund the hint. Comparing a hint with your own notes can reveal which constraint you missed.

The server rebuilds the starting puzzle and replays your entries before accepting completion. Rankings are separate by difficulty and use the shorter time including penalties. The submission round is valid for 120 minutes; leaving the tab does not pause elapsed time. When daily challenges are enabled and Sudoku is selected, everyone receives the same daily puzzle. The date changes at UTC midnight.

Frequently asked questions