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Thermo Sudoku
Play Thermometer Sudoku: classic rules plus thermometers whose digits must increase from the bulb to the tip. Free, with difficulty levels.
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Thermo Sudoku draws thermometers across the grid. Each starts at a rounded bulb and runs to a tip, and the digits along it must strictly increase as they move away from the bulb.
That single ordering rule is surprisingly powerful. A long thermometer sharply limits which digits can sit at each end — a five-cell thermo, for instance, can only begin with 1, 2, 3, 4 or 5 — so the bulbs and tips quickly reveal placements the grid alone would hide.
It is a favourite of the online-solving community for good reason: clean to read, rich to solve. Choose a difficulty and Playoku generates a fresh, uniquely-solvable thermo board.
Rules
- All classic Sudoku rules apply.
- Along each thermometer, digits strictly increase from the bulb to the tip.
- Digits never repeat on a thermometer (they are strictly increasing).
Frequently asked questions
How do thermometers work in Sudoku?
Each thermometer has a bulb end and a tip. The digits must increase strictly as you move from the bulb toward the tip.
Is Thermo Sudoku hard?
The ordering constraint does a lot of the work, so many thermo puzzles have few givens — but long thermometers create elegant, satisfying chains of logic.
Explore more Sudoku
Try another variant, play the classic game, or sharpen up with the strategy guides.
Play classic Sudoku free →