The numbers only have to go up. They don't have to be consecutive, so 1 → 3 → 6 → 9 is perfectly fine, but they can never stay the same or go down.
Thermo Sudoku rules in 30 seconds
- 1Every row, column and 3×3 box contains 1–9 exactly once.
- 2Along each thermometer, digits strictly increase from the bulb to the tip.
- 3Equal digits on the same thermometer are never allowed.
- 4Thermometers can bend; follow the line cell by cell.
See it on a board
Watch: how to play Thermo Sudoku
Still not sure? Some puzzles click faster when you see them solved. This 13-minute video from Steven Scott shows Thermo Sudoku in action.
Reading a thermometer
The bulb is the round end, and it holds the smallest digit on that thermometer. Each step along the line toward the tip must be larger than the one before. So if a cell in the middle of a thermometer holds 3, every cell before it is smaller than 3 and every cell after it is larger.
2 → 4 → 7 is valid. 2 → 2 → 7 isn't, because the digits must strictly increase. 5 → 3 → 8 isn't either, because it goes down.
Length is a clue
Thermometers limit digits before you place anything. On a five-cell thermometer the bulb can't be more than 5, because four bigger digits still have to fit after it. The tip can't be less than 5, because four smaller digits must come before it.
In general, a cell with n cells before it on the thermometer is at least n + 1, and a cell with m cells after it is at most 9 − m. On a nine-cell thermometer, every digit is fixed: 1 through 9 in order.
Worked example
- 1A four-cell thermometer has a 6 at its third cell.
- 2The two cells before it must be smaller than 6 and increasing, so the bulb is at most 4.
- 3The tip comes after the 6, so it must be 7, 8 or 9.
- 4If the tip's row already has 7 and 8, the tip is 9, found without any other clue.
Thermo Sudoku tips and strategies
Start with the longest thermometers
Long thermometers pin each position to a narrow range of digits.
Work out min and max
For each cell, count how many cells come before and after it. That gives its lowest and highest possible digit.
Use known digits in both directions
A placed digit in the middle of a thermometer caps everything before it and floors everything after it.
Combine with normal elimination
A range like 3–5 becomes a single digit fast once the row and column rule out the others.
Common mistakes to avoid
- Reading the thermometer from the tip instead of the bulb.
- Assuming digits on a thermometer must be consecutive.
- Following the physical shape instead of the order of cells along a bent line.
Play Thermo Sudoku in 10+ Number Puzzles
- Thermometers stay visible under the numbers while row, column and box highlights show related cells.
- Auto Notes fills candidates using both the Sudoku rules and the thermometer order.
- Notes are the natural way to track each cell's minimum and maximum.
- You have 3 lives. A wrong digit costs one.
- Five difficulty levels, each a ladder of stages from Easy to Genius.
- Go online for free: ranked 1v1 duels, rooms where you race, co-op or play in teams, and tournaments.
- 27 more puzzle types in the same app, and online play never costs coins.
Thermo Sudoku FAQ
Do Thermo Sudoku digits have to be consecutive?
No. They only need to increase. 1, 4, 5, 9 is a valid thermometer.
Can a digit repeat on a thermometer?
Never. Strictly increasing means each digit is bigger than the one before, so repeats are impossible.
Where do I start a Thermo Sudoku?
At the longest thermometer. Work out the minimum and maximum digit for each of its cells, then cross-check with rows and columns.
