Expert Sandwich Sudoku

Clues outside the grid give the sum of digits sandwiched between the 1 and the 9 in each row and column.

Easy Medium Hard Expert

Expert Sandwich Sudoku. The hardest puzzles on the site — sum-range intersection and parity reasoning across rows and columns. No guessing.

7
21
25
0
22
17
0
15
2
0
9
15
0
0
0
22
28
0
9
1
4
4
6
8
3
3
6
8
7
4
6
3
7
9
5
8
1
3
9
2
6
Mistakes
0/3
Score
-
Time
00:00
All Sandwich Sudoku Puzzles →
Progress0%

What is Sandwich Sudoku?

Difficulty
★★★★☆
4/5
Constraint Type
Outside Clues
Typical Givens
4–10 clue values
Avg. Solve (Expert)
85 min

Solving Techniques for Expert Level

Technique Description Level
Clue 0 Means Adjacent 1 and 9 A sandwich clue of 0 means 1 and 9 are side-by-side with nothing between them. Place this pair immediately. Beginner
Clue 35 Means 1 and 9 at Ends A clue of 35 = 2+3+4+5+6+7+8. All middle digits are sandwiched, placing 1 and 9 at the two ends of the row/column. Beginner
Position Enumeration For each clue, enumerate all valid positions of 1 and 9 such that the enclosed digits sum to the clue. Cross-reference with column clues. Intermediate
Sandwich Intersection Row and column clues intersect at one cell. Combining row positions of 1 and 9 with column positions pinpoints key placements. Advanced
Digit Forcing from Both Sides Use clues from both the row and column sides to triangulate the positions of 1 and 9 simultaneously. Intermediate

Average Solve Time by Difficulty

Easy
12 min
Medium
26 min
Hard
52 min
Expert
85 min
Want a full walkthrough of rules, strategies, and solving steps? How to Play Sandwich Sudoku →

Frequently Asked Questions — Expert Sandwich Sudoku

Is expert Sandwich Sudoku solvable with pure logic?
Yes. Every expert Sandwich puzzle on Sudoku.by has a unique solution reachable through logical deduction. No guessing is needed, though the chain of reasoning to determine 1 and 9 positions may span many rows and columns before any digit can be placed definitively.
What strategies are unique to expert Sandwich Sudoku?
Sum-range intersection: for each row, compute the set of cells that must be inside the sandwich for at least one valid 1/9 arrangement. These cells share a common digit constraint regardless of exact arrangement. Combining this across intersecting rows and columns creates a powerful elimination network.
How does parity help in expert Sandwich Sudoku?
Sandwich sums have a parity determined by the number of sandwiched cells and the specific digits. If a sandwich sum is odd and the number of sandwiched cells is even, the average sandwich digit is non-integer, which forces specific high/low digit distributions. This parity reasoning can eliminate candidate arrangements without explicit enumeration.
How many clues does an expert Sandwich puzzle have?
Expert puzzles have the fewest pre-filled digits inside the grid and rely almost entirely on sandwich sum clues. Some rows or columns may have clues that appear indistinguishable from multiple arrangements until constraints from neighboring rows and columns eliminate the alternatives.
How long does expert Sandwich Sudoku take?
Expert Sandwich Sudoku typically takes 60–120 minutes. If you are stuck, identify every row and column where only two valid 1/9 position arrangements remain, then look for a cross-constraint that eliminates one — that is almost always where the decisive progress lies.

More questions? See the full Sandwich Sudoku guide.