Best SudokuThermo Sudoku
Thermo Sudoku adds thermometers to the grid. Each thermometer starts at a round bulb, and the digits must get strictly bigger as you follow it away from the bulb. There’s no other cage or sum: the order alone limits what can go where, which makes it a great logic variant.
The rules
- Fill the grid so every row, column and 3×3 box holds 1 to 9.
- Along each thermometer, numbers must get bigger, starting at the bulb.
- Numbers on a thermometer can’t repeat.
An example puzzle
An Easy Thermo Sudoku puzzle with 12 starting digits. It has one solution.
What each cell can be
Because digits rise along the thermometer, a cell near the bulb can’t be large and a cell near the tip can’t be small. For a thermometer of length n, the cell in position k (counting the bulb as 1) is at least k and at most 9 − n + k. Here are the ranges from the bulb to the tip.
| Length | Allowed digits, bulb to tip |
|---|---|
| 3 cells | 1 to 7 · 2 to 8 · 3 to 9 |
| 4 cells | 1 to 6 · 2 to 7 · 3 to 8 · 4 to 9 |
| 5 cells | 1 to 5 · 2 to 6 · 3 to 7 · 4 to 8 · 5 to 9 |
| 6 cells | 1 to 4 · 2 to 5 · 3 to 6 · 4 to 7 · 5 to 8 · 6 to 9 |
Solving tips
- Start with the longest thermometers. Their cells have the narrowest ranges.
- A placed digit changes its neighbours’ bounds. A 3 in the second cell means the bulb can only be 1 or 2, and the cell after it is at least 4.
- Thermometers can bend and even cross a box edge. Only the order along the thermometer matters, not whether cells touch a row or column.
- Mix in normal Sudoku logic: a thermometer often leaves a cell with two candidates that a row or box then settles.
More ways to play
Print Thermo Sudoku puzzles · All variants · Sudoku techniques