Hard sudoku
Hard is graded by a move that places nothing. Every grid here forces at least one step whose entire output is candidates struck off other cells, and the placements you were hunting for only become available once you have made it.
Hi! I am Ollie. Welcome to Sudoku Springs!
Every grid here is built to hard and checked against it before you see it. Needs a pair or a locked digit. Have fun!
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chalk arch Fill every row, column and box with 1-9. The Chalk Arch board of Sudoku Springs is graded hard, and a level of the trail takes four to six cleared boards.
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sun terrace Fill every row, column and box with 1-9. The Sun Terrace board of Sudoku Springs is graded hard, and a level of the trail takes four to six cleared boards.
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pebble mosaic Fill every row, column and box with 1-9. The Pebble Mosaic board of Sudoku Springs is graded hard, and a level of the trail takes four to six cleared boards.
What hard is allowed to ask for
Hard is fenced at both ends. The floor is a naked pair, so this board cannot be finished by scanning and counting alone; the ceiling is a naked triple, so nothing in it reaches past the standard set.
Grids are built as you play, so this page never runs out, and each one is proved to have exactly one solution before it reaches you.
A hard grid, opened three moves
This is not a drawing. The engine dug it at hard from the fixed
seed opening:hard:0, so it is here tomorrow too, and it reached you with
28 starting digits. These techniques finish it in 54 moves, and the hardest one it forces is a pointing pair.
Its first 3 moves, in the order the solver makes them. Recompute the candidates from the digits above and you will get the same answer, because nothing below leans on an elimination you cannot see.
- hidden single — In row 3, 5 has only one cell left it can go in. The cell is r3c1, where only 1, 2 and 5 were still legal, so it is 5.
- hidden single — In row 2, 5 has only one cell left it can go in. The cell is r2c9, where only 2, 3, 5 and 8 were still legal, so it is 5.
- hidden single — In row 4, 5 has only one cell left it can go in. The cell is r4c2, where only 2, 3 and 5 were still legal, so it is 5.
The techniques you will meet
Every one of those has a page with a real worked example, generated by the same solver that grades these grids.
This board turns on a pointing pair, and the free rung names the line the eliminations land on, not the box where the confinement is visible. So you are told the technique and the row or column it acts on; the box, the digit, and however many cells are doing the confining, are yours to work out or to buy one rung at a time.
Printable sudoku: hard
Each sheet is 12 grids at one level, two to a page, answers at the back. Free, no sign-up, and yours to photocopy for a class.
Other levels: easy, medium, expert.
Under every grid is the first move, in the same words the hint in the game uses: it names the row, column or box to look in and the digit to look for, and never the cell, so it unsticks you at the start without solving anything for you. The answer page names the hardest technique each grid actually needed, measured by the solver that built it. See all four sheets, with a preview of each.
Other levels
- Easy Scan the boxes. No pencil marks needed.
- Medium Pencil marks start to pay off.
- Expert Goes past the standard techniques. Take your time.
A hard grid can call for any of four eliminating techniques, and they are worth learning as a set rather than one at a time, so start from sudoku techniques. If you are spotting them without asking for help, expert sudoku is where the set stops being enough.