Hard Sudoku

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Finished hard puzzles are recorded in your browser (your last 25 games), so you can watch yourself improve. Nothing leaves your device.

See every level, your streaks and your recent games on the Sudoku stats page.

How difficult is Hard Sudoku?

Hard Sudoku starts with about 29 of the 81 cells filled in, leaving around 52 empty cells. With roughly a third of the grid given, there are long stretches where no cell is obviously forced, and simple scanning will run out of moves early.

The difficulty is not memorizing exotic patterns. It is staying organized: keeping full pencil notes accurate and spotting a few elimination patterns, mainly naked pairs, hidden pairs and pointing pairs.

Times vary a lot from player to player; treat them as a rough guide, not a target. For a deeper comparison, see Sudoku strategies for every level.

How Hard Sudoku works

The rules are the same as ever (see Sudoku rules), but the workflow changes:

1. Clear the easy moves. Scan for naked and hidden singles until nothing new appears.
2. Write full pencil notes. Fill in the candidates for every empty cell. This is the foundation of everything that follows.
3. Eliminate candidates. Look for pairs and pointing pairs that let you remove digits from the notes of other cells.
4. Collect the new singles. Each elimination can leave a cell with one candidate, or a digit with one possible home.
5. Repeat. Keep the notes updated after every placement.

Each puzzle has exactly one solution and can be solved by logic. If it feels like you must guess, there is an elimination you have not found yet.

Tips for solving Hard Sudoku

• Fill notes after the easy moves. Do not start with notes; first exhaust singles so the candidate lists are as short as possible.
• Hunt for cells with two candidates. They are the seeds of naked pairs and later chains.
• Check pointing pairs. If a digit's candidates in a box all lie in one row or column, remove it from the rest of that line.
• Look for hidden pairs. Two digits that fit in only the same two cells of a unit can clear all other candidates from those cells.
• Update notes immediately. After each number, erase it from its row, column and box. Stale notes are the top cause of mistakes on hard grids.
• Take breaks. A fresh look often reveals a pattern you walked past a minute ago.

The pairs and pointing pairs mentioned here are explained with examples in Sudoku techniques explained, and Sudoku strategies covers how to organize a long solve.

Common mistakes to avoid

• Skipping notes. Trying to solve a hard puzzle in your head leads to missed eliminations.
• Forgetting to erase notes. A candidate that should have been removed will hide the pair or single you need.
• Misreading pairs. A naked pair needs two cells in the same row, column or box that share exactly the same two candidates.
• Guessing to save time. A wrong guess on a hard grid can cost far more time than finding the real deduction.
• Overusing hints. With three hints per game, save them for when you have checked your notes thoroughly.

Hints, notes, timer and other tools

Everything in the game above is free and available at every level:

• Hints: three per game. A hint fills in one correct cell. Press H as a shortcut.
• Notes: pencil notes are essential on hard puzzles. Turn them on, fill in candidates after the easy moves, and keep them updated. Press N to toggle.
• Undo and erase: take back any move as many times as you like with Undo or Ctrl+Z.
• Timer: your solving time is shown above the board. Select it or press Pause to stop the clock while you take a break.
• Mistake counting: switch it on to end the game after three wrong numbers, a good challenge once you are comfortable.
• New puzzle: press New Puzzle for a fresh hard grid whenever you like.
• Autosave: close the tab mid-game and your puzzle is waiting when you come back.
• Keyboard: arrow keys to move, 1–9 to enter a number, Backspace to erase.

Prefer to check a puzzle from a newspaper or book? The Sudoku solver solves any grid you type in.

Print hard Sudoku

Like solving with a pencil? Generate a free sheet of hard Sudoku puzzles on our printable Sudoku page, choose the layout and paper size, and print or save it as a PDF.

Ready for another level?

Once you can finish hard puzzles cleanly and keep your notes accurate, explore the neighboring levels:

Want a fresh puzzle everyone plays? Try the Daily Sudoku, or switch to a different grid size with Sudoku variants such as 4×4 and 6×6.