A LITTLE FOCUS. A FRESH START.
Play Thermo Sudoku
Play free Thermo Sudoku online. Digits increase from each thermometer bulb, adding ordered clues to five levels of Sudoku puzzles.
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LEARN THE TWIST
How to play Thermo Sudoku
Thermo lines have a direction: numbers strictly increase away from the bulb. Renban lines use a consecutive set in any order instead.
The rules
Fill each row, column, and 3×3 box with 1–9, without repeats. Digits strictly increase along each thermometer, starting at its round bulb. They do not need to be consecutive.
A useful first move
Use the thermometer’s length to bound each cell. Once a middle digit is known, every earlier cell must be smaller and every later cell must be larger.
A four-cell thermometer can read 1, 3, 6, 9 from bulb to tip; consecutive digits are not required. Its bulb cannot exceed 6, because three larger digits must fit after it.
SOLVE IT STEP BY STEP
Leave enough room on both sides of a known digit
Thermometers are ordered paths. Their digits increase strictly from the round bulb, so you can reason about upper and lower limits before you know an exact value. A long run of cells needs enough distinct numbers to fit before the tip.
- Bulb
- 1, 2, 3
- Second cell
- 2, 3, 4
- Tip
- 6, 7, 8, 9
Start with the length.
On any four-cell thermometer the bulb is at most 6: even a bulb of 6 needs 7, 8, and 9 after it. The cells’ minimum values from the bulb are 1, 2, 3, and 4.
Work backward from 5.
The second cell is below 5, so it is at most 4. The bulb must be smaller again, so it is at most 3. Combining these limits gives {1, 2, 3} at the bulb and {2, 3, 4} in the second cell.
Work toward the tip.
The last cell must exceed 5, leaving 6, 7, 8, or 9. Do not assume it is 6: thermometers require an increase, not a step of exactly one.
Look for a squeezed segment
If two known values leave exactly as many intermediate digits as there are cells between them, the segment is forced. For example, a thermometer segment reading 3, empty, empty, 6 must be 3, 4, 5, 6. A wider gap leaves several possibilities, so return to the crossing rows and boxes.
Common mistakes to avoid
- Read from the bulb, even if the thermometer bends or points downward.
- Strict increase forbids repeats and decreases, but it permits gaps such as 1, 3, 5, 8.
Try the reasoning yourself
On a five-cell thermometer, the third cell is 7. What are the last two digits?
Show the answer and why
They must be 8 and 9 in that order. Two strictly larger digits must fit after 7, and those are the only two available in 1–9.
Controls, difficulty, and saved games
Choose Beginner, Easy, Medium, Hard, or Expert above. Select an empty cell, then tap a number or use your keyboard. Notes let you pencil in candidates; three hints are available. Three mistakes end the puzzle.
Every mode and difficulty keeps a separate save in this browser. You can switch puzzles and return to your place. Clearing site data removes these saves; they do not sync with another device or the iPhone app. Settings include night mode and optional automatic cell advancement.
Correct entries earn points. High scores are stored by mode and difficulty and update only when beaten; completed-puzzle count and day streak are shared.
FIFTEEN WAYS TO PLAY
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A familiar grid, a different kind of logic. Every mode is free to play in your browser.