Hard Sukaku

A digit-free variant — only pencil marks are given, not final answers. Solve from candidate sets rather than placed digits.

Easy Medium Hard Expert

Hard Sukaku. Locked candidates and multi-step elimination chains are required — a dense board with no shortcuts.

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Mistakes
0/3
Score
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Time
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Tap a number to eliminate it from selected cell
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What is Sukaku?

Difficulty
★★★☆☆
3/5
Constraint Type
Pencilmark Elimination
Typical Givens
All candidates shown
Avg. Solve (Hard)
35 min

Solving Techniques for Hard Level

Technique Description Level
Candidate Scanning Start by scanning rows, columns, and boxes — any candidate appearing in only one cell in a group must be placed there. Beginner
Cross-Hatch Elimination When a digit is placed (or reduced to one candidate), eliminate it from all cells in the same row, column, and box. Beginner
Naked Singles A cell with only one remaining candidate is a naked single — the digit is forced. In Sukaku, this happens automatically when you eliminate down to one candidate. Intermediate
Hidden Singles A candidate that appears exactly once in a row, column, or box must be placed in that cell regardless of how many other candidates that cell shows. Intermediate
Pair Elimination Two cells in a group that share only the same two candidates form a naked pair — both candidates can be eliminated from all other cells in that group. Advanced

Ready for the ultimate test? Try Expert Sukaku — the hardest puzzles on Sudoku.by.

Techniques to Master at Hard

  • Hidden Pairs technique — Hard sukaku starts with deliberately bloated candidate lists — hidden pairs are the tool that cuts through, stripping the excess from exactly two cells per unit.
  • X-Wing technique — Because every candidate is visible, X-Wing rectangles can be spotted by pure pattern-reading — scan each digit's printed positions row by row for the two-and-two shape.

Average Solve Time by Difficulty

Easy
8 min
Medium
18 min
Hard
35 min
Expert
62 min
Want a full walkthrough of rules, strategies, and solving steps? How to Play Sukaku →

Related Variants

Killer Sudoku

Another puzzle that opens with zero placed digits — cage sums replace pencilmarks as your only raw material.

Jigsaw Sudoku

Sharpens the same candidate-tracking discipline sukaku trains, with region shapes that twist your eliminations.

Diagonal Sudoku

Adds two more houses to feed your elimination engine — ideal for solvers who enjoy sukaku's systematic pruning.

Frequently Asked Questions — Hard Sukaku

What advanced techniques do hard Sukaku puzzles require?
Hard Sukaku puzzles require locked candidates (a digit confined to one row or column within a box, letting you eliminate it from the rest of that row or column), naked and hidden triples, and sometimes X-Wings. You must track candidates across multiple units simultaneously rather than working one cell at a time.
What is a locked candidate in Sukaku?
A locked candidate occurs when a particular digit can only appear in cells that are simultaneously in one box and one row (or one box and one column). Because that digit must go somewhere in the box, and all valid positions share the same row or column, you can safely eliminate that digit from every other cell in that row or column outside the box.
How important are naked pairs and triples in hard Sukaku?
Extremely important. A naked pair is two cells in the same unit that together contain exactly two distinct candidates. Those two digits must occupy those two cells, so you can remove them from all other cells in the unit. Naked triples extend this to three cells and three digits. Missing these patterns is the main reason solvers get stuck on hard puzzles.
How long does a hard Sukaku take to solve?
Most players spend 25 to 45 minutes on a hard Sukaku. If you are comfortable with naked pairs and locked candidates, you may finish faster. The elimination chains are longer than in medium, and a single deduction often requires checking four or five units before it becomes visible.
What is the best order to scan cells in hard Sukaku?
Start with the units that have the fewest remaining candidates in total. A box or row where most cells are already solved is the richest source of locked-candidate and hidden-single deductions. Work outward from those constrained areas rather than scanning the whole board randomly.

More questions? See the full Sukaku guide.