Printable Killer Sudoku

What comes out of the printer

The stylesheet carries a print section, so what you get is not a screenshot of the screen. The number pad, the Erase, Notes, Hint, Check and New puzzle buttons, the timer and the message line under the board are all removed. The grid is set to fifteen centimetres across so it lands at a sensible size on A4 or Letter instead of filling the sheet edge to edge, and the shading on the selected cell is dropped, because a grey square on paper reads as a smudge. The difficulty label above the grid stays, which is useful if you print several.

The dashed cage outlines get darker. On screen they sit at reduced opacity so they do not compete with the solid box rules; on paper, and particularly on a black-and-white printer, that would leave them faint or missing altogether, so they go to full contrast.

Print it before you fill anything in

This one catches people out, and it costs a sheet of paper each time. Digits you have already typed print along with the board: the sheet shows the state of the puzzle at the moment you pressed print, not a blank one. If you want an empty grid, print first and solve afterwards. If what you want is to carry a half-finished board away from the screen, that works too, and it is the one case where printing mid-solve is exactly the right thing to do.

Getting several different puzzles

Press New puzzle and print again. Every press builds a fresh board with a fresh cage layout, so a stack of sheets off this page will be a stack of different puzzles at the same difficulty. There is no set of pre-made sheets behind any of this and no numbered puzzle to request; the board is constructed when you load the page. A killer sudoku printable in that sense is not a document that already exists somewhere, it is one your browser makes on request. Nothing about this page is special in that respect either, since every board on the site prints the same way, but this is the one set up to be used like that.

A wrong digit costs more on a caged grid

Losing Check and Hint is the ordinary price of paper, and it lands harder here than on a printed ordinary sudoku. On a plain grid a mistake is eventually contradicted by a given, and the givens are printed, cannot be wrong and are not going anywhere, so there is always something true to work back from. A killer board has no givens at all. Every digit on the sheet is one you put there yourself, so an error made in the fourth minute has nothing to collide with except your own later reasoning, and by the time a cage refuses to add up you have no way of telling how far back the fault began. The difficulty label accounts for none of this, which is worth remembering before deciding the printed ones are graded too generously.

What you get back is room to write. The standard method on paper is to put a cage’s possible combinations in tiny figures inside its outline along the top edge, rather than trying to maintain candidates cell by cell across the whole grid. A three-cell cage totalling 8 gets 134 and 125 written into it once, and that single annotation does more work than nine cells of pencil marks.

Why the cages are drawn the way they are

Two structures overlap on a killer grid and both have to stay readable: the three-by-three boxes, and the cages, which follow no alignment at all and can be any shape. Draw both with solid lines and you get a grid nobody can parse. So the boxes keep the solid rules and the cages take a dashed outline set slightly inside the cell, which is the convention printed puzzles of this kind have always used and one most solvers arrive already knowing. The inset has a second job: it keeps the dashes clear of the digit sitting in the middle of the square, and clear of the cage total tucked into the corner.