Expert Thermo Sudoku

Digits must increase from the bulb end to the tip along each thermometer. Temperature direction constrains every placement.

Easy Medium Hard Expert

Expert Thermo Sudoku. The hardest thermometer puzzles on the site — every placement follows from precise bound calculations and logical reasoning.

8
7
5
9
1
2
5
8
3
9
4
1
8
4
3
2
4
1
9
3
6
3
2
8
4
Mistakes
0/3
Score
-
Time
00:00
All Thermo Sudoku Puzzles →
Progress0%

What is Thermo Sudoku?

Difficulty
★★★★☆
4/5
Constraint Type
Line Constraints
Typical Givens
20–28
Avg. Solve (Expert)
75 min

Solving Techniques for Expert Level

Technique Description Level
Range Restriction from Length A thermometer with N cells must use N strictly increasing distinct values. The bulb is at least 1 and the tip is at most 9, giving a calculable valid range for each cell. Beginner
Minimum/Maximum Bounding The bulb value is ≤ 9 − (N − 1). The tip value is ≥ N. Apply these bounds immediately to prune all thermometer candidates. Beginner
Overlapping Thermometer Deduction Two thermometers sharing a cell give tighter combined bounds than either alone. Intermediate
Forcing Chains via Thermometers When a value is forced into a thermometer cell, all cells closer to the bulb must be smaller and all closer to the tip must be larger. Advanced
Short vs. Long Thermometers A 2-cell thermometer is simply a greater-than relationship. A 9-cell thermometer forces exactly 1–9 in order. Intermediate

Techniques to Master at Expert

  • X-Wing technique — Expert thermo endgames run on the extremes: thermometers force 8s and 9s away from tips of short lines, leaving textbook two-per-row X-Wing geometry.
  • Swordfish technique — When several long thermometers parallel each other, mid-range digits get confined to the same three columns across three rows — exactly the Swordfish setup expert grids exploit.

Average Solve Time by Difficulty

Easy
10 min
Medium
22 min
Hard
45 min
Expert
75 min
Want a full walkthrough of rules, strategies, and solving steps? How to Play Thermo Sudoku →

Related Variants

Arrow Sudoku

Thermo and Arrow both constrain digits along drawn lines — increase swaps for summation, but the cell-by-cell bounding feels the same.

German Whispers

Another drawn-line rule: instead of strictly increasing, neighbouring line digits must differ by at least 5.

Renban Sudoku

Line cells must form a consecutive set rather than an increasing one — the same range-tracking instinct, loosened by one degree.

Frequently Asked Questions — Expert Thermo Sudoku

Is expert Thermo Sudoku solvable without guessing?
Yes. Every expert puzzle on Sudoku.by has a unique solution reachable by logic alone. Expert Thermo Sudoku uses complex chains of thermometer bound deductions combined with advanced Sudoku techniques. If you feel stuck, there is always a bound update or a multi-unit interaction you have not yet applied.
What advanced strategies appear in expert Thermo Sudoku?
Expert puzzles require bent naked sets (groups of cells spanning multiple units that together restrict candidates), thermometer chains where the bound of one thermo depends on the resolved value of a cell it shares a row with from another thermo, and occasionally X-Wing patterns applied to the narrow candidate ranges that thermometers create.
How do thermometers create unusually tight constraints at expert level?
At expert level the thermometers are longer and more numerous. A long thermometer spanning 6 cells can reduce the valid range for its middle positions to just 2 or 3 digits. Combined with row and column constraints that may already exclude 5 or 6 digits, these tight ranges interact to force placements that would be invisible without complete bound tracking.
How many clues does an expert Thermo Sudoku have?
Expert Thermo Sudoku puzzles have very few pre-filled digits — sometimes none at all. All deductions flow from thermometer bounds and standard row/column/box uniqueness. Every solve step must be earned through calculation and elimination rather than reading off an obvious given.
How long does an expert Thermo Sudoku take?
Most players take 60 to 90 minutes on expert puzzles. Experienced solvers comfortable with bound tracking and multi-thermo interactions may finish in 40 to 60 minutes. If you have spent 20 minutes without a placement, recalculate bounds from scratch on the two or three longest remaining thermometers — the constraint you need is almost always hiding there.

More questions? See the full Thermo Sudoku guide.