What Is Thermo Sudoku?
Thermo Sudoku is a standard 9×9 Sudoku with thermometer-shaped paths drawn across the grid. You still place digits 1 through 9 so every row, column, and 3×3 box contains each digit once. The extra rule is directional: values rise strictly as you move from a thermometer's round bulb toward its tip.
The format is also called Thermometer Sudoku. It feels familiar to regular Sudoku players because the underlying grid never changes, yet the thermometers create useful upper and lower bounds long before a row or box is nearly complete. Every puzzle on this page is checked for one solution before it is published.
How to Play Thermo Sudoku
Start with normal Sudoku scanning, then treat each thermometer as an increasing sequence. A path can cross several rows or boxes, so one small deduction can influence distant parts of the grid.
- Use digits 1–9 exactly once in every row, column, and 3×3 box.
- Find the round bulb of each thermometer and follow its path toward the tip.
- Make every digit larger than the digit before it on that same thermometer.
- Use pencil marks to combine Sudoku candidates with the range allowed by a cell's thermometer position.
- Finish only when all standard Sudoku units and every thermometer are valid.
Thermo Sudoku Rules
A thermometer must increase at every step, not merely from its first cell to its last. If a four-cell path contains 2, 5, 4, 8, it is invalid because 4 is lower than the preceding 5. Different thermometers do not share a separate digit set; their cells still obey the row, column, and box rules of the underlying Sudoku.
Digits on a thermometer do not have to be consecutive. Sequences such as 1–3–7 and 2–5–6–9 are legal. What matters is strict order. The bulb is always the lowest value on its path, while the tip is always the highest.
How Thermometers Change Sudoku Logic
A path's length immediately limits its digits. On a five-cell thermometer, the bulb cannot exceed 5 because four larger values must still fit after it. The tip cannot be lower than 5 for the opposite reason. More generally, the first cell of a length-L path is at most 10 minus L, and each later position inherits a tighter minimum.
These bounds become stronger when a digit is known in the middle. If a four-cell path reads blank–4–blank–blank, the bulb must be 1, 2, or 3, while the remaining cells must be greater than 4 in increasing order. Combining those ranges with row and box exclusions often produces a single candidate.
Thermo Sudoku Solving Tips
- Scan long thermometers first because their early and late cells have the narrowest ranges.
- When a bulb is high, reserve enough larger digits for every cell after it.
- When a tip is low, work backward and reserve enough smaller digits before it.
- Check where a thermometer crosses a row, column, or box that already contains several digits.
- Keep notes based on both Sudoku exclusions and the thermometer's position, not on either rule alone.
- Revisit a whole path after placing one digit; the new lower and upper bounds can cascade through every remaining cell.
- Avoid assuming consecutive values. Skipped digits are often necessary and can prevent false deductions.
Common Thermo Sudoku Mistakes
- Reading a thermometer from the tip toward the bulb instead of from the round bulb outward.
- Assuming the digits must be consecutive rather than simply increasing at every step.
- Checking only adjacent filled cells and forgetting to leave enough values for the rest of the path.
- Writing notes from standard Sudoku alone without applying the thermometer's lower and upper bounds.
Thermo Sudoku Difficulty Levels
Easy puzzles use shorter paths and generous givens, making positional ranges visible early. Medium puzzles ask you to combine a few thermometers with standard candidate elimination. Hard and Expert boards use longer interactions, fewer direct singles, and more deductions that travel through a thermometer into a different row, box, or column. Difficulty is measured from the solving work, not only from the number of starting digits.
Thermo Sudoku vs Classic Sudoku
Classic Sudoku relies only on row, column, and box uniqueness. Thermo Sudoku keeps all three rules and adds ordered paths, so a digit can be impossible even when it causes no immediate duplicate. If you are new to Sudoku, Classic Sudoku is the clearest starting point. If ordinary candidate scanning feels familiar, thermometers provide a natural next challenge without changing the 9×9 grid.
Uniquely Solvable Puzzles
Each published board is validated by a solver that applies Sudoku constraints and thermometer ordering together. The solution counter stops as soon as it finds a second completion; only boards with exactly one completion are accepted. The answer is not printed into the page source sent to players, and checks reveal conflicts rather than displaying the full grid.
Frequently Asked Questions
What is Thermo Sudoku?
Thermo Sudoku is a 9×9 Sudoku with thermometer-shaped paths. Standard row, column, and 3×3 box rules apply, and digits must also increase from each round bulb toward its tip.
How do thermometers work in Thermo Sudoku?
Read a thermometer from its bulb to its tip. Every next cell must contain a larger digit than the cell before it, and the rule applies at each step along the path.
Do thermometer digits need to be consecutive?
No. A path such as 2–5–8 is valid because the digits rise strictly. The thermometer does not require 2–3–4 or any other consecutive sequence.
Does Thermo Sudoku use normal Sudoku rules?
Yes. Every row, column, and 3×3 box must still contain digits 1 through 9 without repeats. Thermometer ordering is an additional constraint, not a replacement.
Can thermometers cross boxes?
Yes. A thermometer may travel through more than one 3×3 box or row. Every cell still belongs to its normal Sudoku units while also following the path order.
What number can go in a thermometer bulb?
The bulb contains the lowest digit on its thermometer. Its maximum depends on the path length because enough larger digits must remain for every cell toward the tip.
What is the best Thermo Sudoku strategy for beginners?
Start with long thermometers and mark their positional ranges. Then combine those limits with ordinary row, column, and box exclusions instead of trying to solve either rule in isolation.
Are these Thermo Sudoku puzzles uniquely solvable?
Yes. Every published puzzle is checked with a solver that counts solutions using both Sudoku and thermometer constraints, and only puzzles with exactly one solution are accepted.
Can I use pencil marks?
Yes. Notes mode lets you record candidates in unsolved cells. Notes, values, elapsed time, and move history are saved locally so an unfinished puzzle can be resumed in the same browser.
Is Thermo Sudoku free to play?
Yes. You can play every difficulty online for free with no download or signup required.