Sudoku Hard - Play Free Online Puzzles

Play hard Sudoku on LiveSudoku when you are ready for deeper logic. Hard boards are still solved without guessing, but singles alone will not carry the whole puzzle. You will use pencil marks, locked candidates, naked pairs, and hidden pairs to remove possibilities until the next answer appears. If you miss a step, the smart Hint system becomes an in-puzzle Sudoku helper, marking the next move and explaining the logic behind it. For offline solving, see our printable hard sudoku puzzles.

Start with complete pencil marks

On hard Sudoku, pencil marks are not just a convenience. They are the map of the puzzle. Fill candidates carefully, then update them whenever a new digit is placed. If you are new to this, start with the pencil-mark tutorial.

Use Locked Candidates

Locked Candidates Type 1 and Locked Candidates Type 2 help you remove a digit from a row, column, or box when its possible positions are restricted. This is one of the most important hard-level techniques.

Find Naked Pairs and Locked Pairs

A Naked Pair appears when two cells in the same row, column, or box share the same two candidates. Those two digits must belong to those two cells, so you can remove them from the rest of that unit. A Locked Pair is the same idea, but the two cells also fall inside the same 3x3 box - so the elimination applies to both the line and the box.

Watch for Hidden Pairs

A Hidden Pair appears when two digits can only fit in the same two cells inside a row, column, or box. Even if those cells have extra candidates, the hidden pair lets you remove everything except those two digits.

Hard Sudoku is where the puzzle starts asking you to reason about groups of cells, not just one cell at a time. Medium puzzles often reveal their next move through singles. Hard puzzles make you eliminate candidates first, then use those eliminations to uncover the next placement.

If this level feels too heavy, warm up with Medium Sudoku. If you are already comfortable with pairs and locked candidates, try Evil Sudoku for advanced patterns such as X-Wing, Swordfish, and XY-Wing.

Do not guess

A good hard Sudoku still has a logical path. If you feel forced to guess, pause and check candidates in the busiest rows, columns, and boxes.

Clean candidates after every placement

Most hard-level breakthroughs come from a small elimination. After placing a digit, remove that digit from the same row, column, and box before looking for the next technique.

Save advanced patterns for Evil Sudoku

Save X-Wing, Swordfish, and XY-Wing for Evil Sudoku. They are rarely needed at hard level. If you find yourself reaching for them, you may have missed an easier elimination.

LiveSudoku was created in 2006 by Mr. Izenberg and has offered free online Sudoku puzzles for nearly two decades. The goal is simple: clean Sudoku boards, helpful tools, no downloads, and puzzles for every level from beginner to expert.