How hard is a Hard puzzle here?

It starts with 28 givens, against 33 for Medium and 40 for Easy. With fewer clues, expect to need pencil marks and to look for pairs of candidates as well as single squares. Clue count is the setting, not a measured solving time, so some Hard grids fall quicker than others.

Is there anything harder?

Yes. Expert starts with 24 givens, and now and then 25, or very rarely 26, when no sparser grid with one solution turns up. Choose it from the Difficulty menu, or open the Very Hard Sudoku page on this site.

How do pencil marks work?

Press Notes off to switch it to Notes on, then press digits to add or remove small candidates in the selected square. Entering an answer clears that square's notes, and Undo restores a note change as well as a digit.

Can I print hard Sudoku puzzles?

Yes. Print puzzle prints only the 28 starting digits in a large black and white grid, and Print answers prints the solution on its own sheet. Choose Save as PDF in the print dialog to keep a copy.

Hard Sudoku

Hard Sudoku opens here with 28 of the 81 digits given, so 53 squares are empty and most of the grid has to be reasoned out rather than spotted. Every puzzle has exactly one solution, proved by a solver before each clue was removed, so there is always a logical answer to find. Turn Notes on to keep pencil marks for the candidates in each square, Undo up to 80 steps, and use Hint only if you want to: it marks the session assisted. The timer keeps running while you work, and Print puzzle and Print answers give the grid and its answer key on paper. For a sparser grid still, switch the level to Expert.

SUDOKU · HARD · 28 CLUES

Fill every row, column and box

0:00

Difficulty

Choose a square, then press a number.

Keyboard: focus the board, use arrows to choose a square, then press 1–9. Notes are pencil marks kept in this browser. Every generated puzzle has exactly one solution; the level sets the clue count (Easy 40, Medium 33, Hard 28, Expert 24, now and then 25 or, rarely, 26). Print puzzle prints the starting grid without your entries; Print answers prints the solution.

Common questions

How hard is a Hard puzzle here?: It starts with 28 givens, against 33 for Medium and 40 for Easy. With fewer clues, expect to need pencil marks and to look for pairs of candidates as well as single squares. Clue count is the setting, not a measured solving time, so some Hard grids fall quicker than others.
Is there anything harder?: Yes. Expert starts with 24 givens, and now and then 25, or very rarely 26, when no sparser grid with one solution turns up. Choose it from the Difficulty menu, or open the Very Hard Sudoku page on this site.
How do pencil marks work?: Press Notes off to switch it to Notes on, then press digits to add or remove small candidates in the selected square. Entering an answer clears that square's notes, and Undo restores a note change as well as a digit.
Can I print hard Sudoku puzzles?: Yes. Print puzzle prints only the 28 starting digits in a large black and white grid, and Print answers prints the solution on its own sheet. Choose Save as PDF in the print dialog to keep a copy.

Every generated puzzle has exactly one solution, checked by a solver before each clue is removed, and every given agrees with that solution. Conflicts are checked by row, column and 3×3 box; the level sets the clue count, which is not a measured human solving time.