Hard Cube/3D Sudoku

A classic 9×9 grid folded onto the faces of a cube. Rows and columns wrap across face boundaries in three dimensions.

Easy Medium Hard Expert

Hard Cube Sudoku. Cross-grid locked candidates and multi-unit tracking across 153 cells — a serious challenge for experienced solvers.

Score
0
Mistakes
0/3
Time
00:00
Grid A
8
2
5
6
3
6
5
7
1
7
4
2
9
5
6
8
4
2
7
9
1
7
8
9
2
6
4
6
7
5
9
3
8
9
4
4
9
6
8
1
4
5
6
4
8
1
4
9
2
Grid A Shared Grid B
Grid B
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Hard · Cube/3D · 2 grids (153 cells)0%
How to play Cube/3D Sudoku
Two 9×9 Sudoku grids overlap at a shared 3×3 corner box. Grid A (blue tint) covers rows 1–9, columns 1–9. Grid B (green tint) covers rows 7–15, columns 7–15. The shared box (rows 7–9, cols 7–9) satisfies both grids. Each grid must independently satisfy standard Sudoku rules.
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What is Cube / 3D Sudoku?

Difficulty
★★★★☆
4/5
Constraint Type
Twin Grids
Typical Givens
~30 per grid
Avg. Solve (Hard)
60 min

Solving Techniques for Hard Level

Technique Description Level
Shared Box Exploitation The overlapping 3×3 box satisfies both grids. Digits placed here eliminate from rows and columns in both Grid A and Grid B simultaneously. Beginner
Grid A/B Isolation Outside the shared box, each grid operates independently. Apply standard 9×9 Sudoku logic to each grid in turn. Intermediate
Cross-Grid Cascade Completing the shared box often cascades through the adjacent rows and columns of both grids, rapidly resolving nearby cells. Intermediate
Overlap Constraint The shared box must satisfy: Grid A's bottom-right box rules, AND Grid B's top-left box rules, AND both grids' overlapping row/column constraints. Advanced
Twin Digit Counting Each digit appears 9 times in Grid A and 9 times in Grid B, but only once in the shared box. This counting helps verify near-complete grids. Advanced

Ready for the ultimate test? Try Expert Cube / 3D Sudoku — the hardest puzzles on Sudoku.by.

Average Solve Time by Difficulty

Easy
15 min
Medium
32 min
Hard
60 min
Expert
95 min
Want a full walkthrough of rules, strategies, and solving steps? How to Play Cube / 3D Sudoku →

Frequently Asked Questions — Hard Cube Sudoku

What makes hard Cube Sudoku difficult?
Hard Cube Sudoku puzzles have fewer given digits, forcing you to use advanced Sudoku techniques within each grid and exploit the cross-grid interaction heavily. The shared region's dual membership creates complex constraint webs that require tracking candidates across 18 rows and 18 columns simultaneously.
How do locked candidates work across grids in hard Cube Sudoku?
A locked candidate in Grid A's shared region can eliminate candidates in Grid B's corresponding rows and columns even though those cells are in a different grid. For example, if a digit can only appear in two cells of the shared box that both lie in row 8 of Grid A, you eliminate it from the rest of Grid A's row 8. But since those cells also belong to row 8 of Grid B, you eliminate it from the rest of that row in Grid B as well — a cross-grid locked candidate.
Should I use pencil marks for hard Cube Sudoku?
Yes, and you need a system that tracks both grid memberships. Mark candidates for overlap cells with special notation indicating which constraints are Grid A constraints, which are Grid B constraints, and which apply to both. Without disciplined marking, it is easy to miss a cross-grid elimination and waste significant solving time.
What is the most powerful deduction source in hard Cube Sudoku?
The shared 3x3 box is consistently the richest source of deductions at hard level. Because it is constrained by three boxes (its own in Grid A, its own in Grid B, and indirectly by the surrounding boxes of both grids), it tends to resolve before other parts of the board. Prioritize every deduction that touches the shared region.
How long does a hard Cube Sudoku take to solve?
Expect 40 to 70 minutes on hard Cube Sudoku puzzles. The larger board and dual-grid constraints mean each step requires verifying against more rules than in standard Sudoku. If stuck, return to the shared region and check whether any cross-grid locked candidate has been missed.

More questions? See the full Cube Sudoku guide.