Printable Hard Sudoku for Pencil-Mark Solvers

A hard sudoku on SudokuIt starts with 21 to 23 givens, the same as a medium one. What makes it hard is the logic. None of the forty hard puzzles we checked could be finished with singles, and most of your time goes into pencil marks rather than placing digits.

What you need to know to solve it

• Pointing pairs and claiming groups come up in almost every hard puzzle. Treat them as routine.
• Naked pair and hidden pair. Seven hard puzzles in ten use at least one.
• Y-Wing. Half of our hard puzzles need a wing somewhere. The most common one is actually the W-wing. The Y-Wing guide teaches the shape all wings share.
• X-Wing and Swordfish turn up in about one hard puzzle in ten.

Then there is the part no list covers. In more than half of our hard puzzles, every technique up to the wings runs dry with around 40 cells still empty. What finishes them is a longer chain of reasoning: if this cell is a 4, then that one is not, so the next one must be. If you want very hard sudoku to print, this is it.

How long it takes

Expect an hour or more, and do not be surprised if one puzzle takes two sittings. Treat both numbers as a rough guide.

How to print it

Here the layout matters. Before the real work starts you will write every candidate into about 58 empty cells, and each cell is a little under 1 cm. Give every digit a fixed spot, like a phone keypad: 1 in the top left corner, 5 in the centre, 9 in the bottom right. Each mark is then about 3 mm high, and a gap where a digit used to be is easy to see. Cells with only two candidates, which pairs and wings are built from, show as two small marks in an otherwise empty square.

Use a fine pencil, 0.5 mm if you have one, and a good eraser. Print two copies. Dense candidate grids smudge, and when a wrong elimination sneaks in, a clean copy beats trying to trace it back. If your printer can scale up or print on A3, use it.

Tick Solutions and keep the second page folded. When you are sure a cell is right and the grid still breaks, check that one cell against the answer, not the whole page. If you pencilled candidates online first, tick Notes and they print in the cells.

Who this is for

Experienced solvers who already use pencil marks, and anyone working through pairs and wings who wants real grids to practise on. If medium puzzles still take you a long time, stay there a while longer and pick up the pairs first.

See every level on the printable sudoku page, or play hard sudoku online with hints.