X Sudoku 6×6 Hard — Set 99
Solver Notes
Hard 6x6 X-Sudoku introduces naked pairs on the diagonals. You will need to track which numbers remain available across three constraint types. 8-12 minutes.
X-Sudoku — Free Online & Printable
X-Sudoku adds a diagonal constraint to the classic sudoku rules. Both main diagonals — running from top-left to bottom-right and from top-right to bottom-left — must each contain the numbers 1 through 9 without repetition. This is the most accessible sudoku variant: the two extra constraints actually make many cells easier to deduce, since numbers on the diagonals are additionally restricted. If you''ve mastered standard sudoku and want a fresh twist without learning complex new rules, X-Sudoku (also called Diagonal Sudoku) is the ideal starting point.
About 6x6 Grid
The 6x6 grid strikes a balance between accessibility and depth. With 36 cells in 2x3 boxes, puzzles take 3 to 10 minutes depending on difficulty. The 6x6 is large enough to showcase variant rules meaningfully — Killer cages have room to create genuine arithmetic puzzles, Windoku windows overlap in interesting ways — while remaining completable in a single sitting. Many players find the 6x6 their "daily driver" format: challenging enough to be satisfying, short enough to fit into a break.
Hard Difficulty
Hard difficulty introduces multi-step deductions where you must track constraint chains across multiple regions simultaneously. Required techniques include naked pairs/triples (two or three cells sharing the same possible values), pointing pairs (where a candidate is confined to one row or column within a region), and intersection removal. For variant-specific techniques: Killer Hard requires cage combination analysis where several possibilities exist and must be eliminated systematically. Average time: 20-40 minutes for 9x9.
How to Get Started
- Choose "New Puzzle" to generate a fresh puzzle with a unique solution
- Click or tap cells to enter numbers (1-6 for 6x6)
- Use the constraint highlights to track which numbers are still available
- Print the puzzle if you prefer solving on paper