Hard printable sudoku
30 sudoku dug down to the bone. Each one still has exactly one solution, which is the part most hard sudoku collections quietly get wrong.
What makes these hard
Each grid here is stripped down to around 26 starting clues, which leaves about two thirds of the 81 squares empty. That is the whole mechanism. The rules are the same, the grid is the same 9 by 9, and there is no gimmick: you simply have much less to work from.
At this clue count the scanning technique that carries an easy puzzle runs out within a few moves. You will need to write candidate notes into the empty squares, and you will need at least one technique beyond "only one digit fits here".
Still exactly one solution, and that is the point
This is where most free hard sudoku collections quietly fail. It is easy to make a grid difficult by removing more numbers. It is harder to check that what remains still has a single answer, and if nobody checks, you end up with a puzzle that has two valid solutions and an answer sheet that is simply wrong about one of them.
Every board on this page is dug out of a complete solved grid one square at a time, and a square is only left blank if the puzzle still resolves to exactly one answer without it. Any removal that would create ambiguity is put back. The count is then verified again before the puzzle is published. So these are hard, but they are never a coin flip: there is always a logical next move.
Techniques you will actually need
- Candidate notes. Not optional here. Pencil the possible digits into each empty square before you look for anything clever, and print at full size so there is room to do it.
- Naked pairs. Two squares in the same row, column or box that can only take the same two digits. Neither digit can go anywhere else in that unit, which usually cascades.
- Hidden singles. A digit that can only go in one square of a unit, even though that square could also take other digits. Easy to miss precisely because the square looks busy.
- Pointing pairs. If a digit within a box is confined to one row, it can be eliminated from that row everywhere outside the box.
Reading the card before you print
Each description carries the clue count plus how many blank squares already accept only one digit, and how few clues the barest row and the barest box were left with. Two boards with identical clue counts can feel very different, and those numbers are where that difference shows. A board with very few immediate openings will need candidate notes from the first move.
Printing
Set scaling to 100 percent or "actual size", never "fit to page". These need every millimetre of square, because you will be writing several small digits into most of them. Avoid draft mode: it thins the grid lines, and on a board you will stare at for half an hour that matters. The 15mm margin is built in, so A4 and US Letter both work as is.
Answer sheets
The solution downloads separately and shows the digits you had to find in a different colour from the ones you started with, so you can see at a glance where you went wrong rather than comparing all 81 squares. Each puzzle number identifies its board permanently.
How to use these sheets
- Pick a sheet and press Print. The file is a PDF sized for A4 and Letter paper.
- Print it in black and white at normal quality. None of these sheets need colour ink except the colour by number pages.
- Keep the answer page for yourself, or print it too if you want to check the work later.