Hard killer sudoku — play free online

100,000 hard puzzles, every one verified to have exactly one solution: bigger cages, fewer certainties. Innies, outies and combination cross-referencing become essential.

Cages stretch to 5 cells and the free information dries up. You'll want the full strategy guide: the rule of 45 stops being a trick and becomes the method.

Puzzle numbers are permanent — #4,821 is the same puzzle for everyone, forever — so you can send a friend a link and race them. Progress saves automatically in your browser, pencil marks tidy themselves, and there's a dark mode for late-night solving.

Not sure this is your level?

Try the daily puzzle — difficulty rotates through the week, easy on Mondays through expert on Sundays, so you can find your ceiling one day at a time.