Fill the grid so that every row, every column, and every 2×3 box contains the digits 1 through 6 exactly once.
The boxes are rectangles — two rows tall and three columns wide — marked by the thicker lines after row 2, row 4, and column 3.
Start with the rows, columns, and boxes that already hold the most digits: where only one digit is missing, it places itself.
That technique, the obvious single, carries you through most 6×6 puzzles.
If 6×6 still feels like a stretch, warm up on 4×4 Mini Sudoku first —
it teaches the same rule on a smaller board and is the format we recommend on our
Sudoku for Kids page. And when hard 6×6 stops slowing you down,
take the leap to a full easy 9×9 Sudoku — the logic is identical, just bigger.
Frequently Asked Questions — Hard 6x6 Sudoku
How hard is hard 6x6 Sudoku?
Hard 6×6 puzzles begin with only about 14 of 36 cells filled. Early on, almost no cell can be placed by looking at a single row or box — every placement comes from combining constraints across the grid. It is the closest a 6×6 gets to the feel of a real 9×9 challenge.
What techniques do I need for hard 6x6 Sudoku?
Systematic candidate elimination is the core skill. Work box by box, list what each empty cell can still hold, and hunt for cells with a single remaining candidate. Naked pairs inside a 2×3 box or a row appear regularly and are usually the key that unlocks the middle of the solve.
Can a hard 6x6 Sudoku have more than one solution?
No. Every puzzle on Sudoku.by is verified by a solver during generation to have exactly one solution, regardless of difficulty. If you reach a contradiction, one of your earlier placements is wrong — use undo and re-check.
I finished hard 6x6 — what next?
You are ready for the full grid. Try an easy 9×9 Sudoku: the logic is identical, just with nine digits and 3×3 boxes. The habits you built here — scanning, elimination, candidate tracking — transfer directly.