Hard Sudoku
Minimal givens. Expect several passes and some pencil marks in the margins.
Puzzle 1
28 clues · 9×9
Puzzle 2
28 clues · 9×9
Puzzle 3
28 clues · 9×9
Puzzle 4
28 clues · 9×9
Puzzle 5
28 clues · 9×9
Puzzle 6
28 clues · 9×9
Puzzle 7
28 clues · 9×9
Puzzle 8
28 clues · 9×9
Puzzle 9
28 clues · 9×9
Puzzle 10
28 clues · 9×9
Puzzle 11
28 clues · 9×9
Puzzle 12
28 clues · 9×9
About hard sudoku
Hard grids give you 28 numbers and ask for the other 53. With this few givens the opening is genuinely slow, and it is normal to place only three or four digits in the first few minutes before the grid begins to open up.
Pencil marks stop being optional here. Fill in candidates across a box or a row, then look for a digit that appears as a candidate in exactly one cell of that unit; it has to go there regardless of what else that cell could have been. That single technique solves most of what scanning cannot.
Frequently asked questions
How many clues does a hard sudoku have?
28 givens out of 81 cells, so 53 are left to you. Twelve fewer than an easy grid, which is enough that the opening moves are genuinely slow.
Why can I not get started on a hard sudoku?
Because scanning alone rarely finds the first few cells at this density. Start by pencilling candidates into one box that already has several numbers, and look for a digit that can only sit in one cell of it.
Are pencil marks necessary for hard sudoku?
In practice, yes. Hard grids are close to unsolvable by scanning alone, and the standard route through them is noting candidates and eliminating them as placements accumulate.
How long does a hard sudoku take?
Twenty to forty minutes is typical. The shape is usually slow for the first third, then it collapses quickly once eight or ten cells are placed.