9×9 · Medium to expert
Thermo Sudoku
Digits must strictly increase along each thermometer, starting from the rounded bulb end.
About thermo
What thermo sudoku feels like.
Thermometer sudoku draws 'thermometers' across the grid — a rounded bulb with a line trailing from it. The digits along each thermometer must strictly increase as you move away from the bulb. Combined with normal sudoku rules, these ordered lines sharply limit which digits can go where.
Rules
- Standard sudoku rules apply: each digit 1–9 appears once per row, column, and 3×3 box.
- Each thermometer has a bulb (the rounded end) and a line of cells.
- Digits strictly increase along the thermometer, starting from the bulb.
- So the bulb holds the smallest digit on its thermometer, and each cell is larger than the last.
History
Thermo sudoku is one of the most popular modern variants, widely featured by online solving communities and variant competitions.
- Key rule
- Digits increase along each drawn thermometer, from the bulb
- Difficulty range
- Medium to expert
- Tips
- A thermometer of length n forces its bulb to be at most 10−n (a length-4 thermo starts at 6 or less).
- The tip (far end) holds a large digit — often near 9 on longer thermometers.
- Overlapping thermometers and boxes chain their constraints; follow them like a ladder.