How to Play Thermo Sudoku
Fill the 9x9 grid so that every row, every column, and every 3x3 box contains the digits 1 through 9 exactly once — just like classic Sudoku. The twist: each thermometer drawn on the grid adds one rule.
- Thermometers increase: Digits along a thermometer must strictly increase from the round bulb to the tip. Equal digits are never allowed anywhere on a thermometer.
- The bulb is smallest: The large circle at one end always holds the smallest digit on its thermometer.
- Position bounds: A cell's spot on a thermometer limits its digits — the second cell of a four-cell thermometer must be at least 2 and leave room for two larger digits after it.
- Conflict checking: A cell that breaks the increasing rule is ringed in amber immediately, so you can spot mistakes before they spread.
How this Thermo Sudoku works
- Daily puzzle: One shared puzzle per difficulty per day — the same grid for every player, so you can compare times. The Daily Puzzle button brings you back to it.
- New Puzzle: Pulls a fresh grid from a bank of 150 thermometer puzzles (50 each of Easy, Medium, and Hard), every one pre-verified to have a single logical solution.
- Notes: Toggle Notes (or press N) to pencil in candidates. Correct placements automatically clean up notes in the affected row, column, and box.
- Keyboard play: Arrow keys move the selection, 1–9 place digits, Backspace erases.
Thermo strategy: start with the longest thermometers
Long thermometers are the most constrained cells on the board, so they are the best place to begin. A five-cell thermometer on a 9x9 grid can only span a narrow range of digit runs, which quickly pins down its bulb and tip. Our Thermo Sudoku rules & strategy guide walks through position bounds and the “squeeze” technique in detail. New to variants? Start with the 6x6 mini Thermo Sudoku for a faster, friendlier grid.
Frequently Asked Questions
What is Thermo Sudoku?
Thermometer Sudoku is a Sudoku variant with thermometer shapes on the grid. Along each thermometer the digits must strictly increase from the bulb to the tip, on top of the usual row, column, and box rules.
How do thermometers work?
The bulb holds the smallest digit, and each step toward the tip must be strictly larger than the last — equal digits are never allowed on a thermometer.
Is Thermo Sudoku harder than regular Sudoku?
Not necessarily — the thermometers give you extra clues. Each cell's position limits its digits, so long thermometers are the easiest place to start. Many players find it a gentle, visual introduction to variant Sudoku.
Can I play on my phone?
Absolutely. The board and number pad are designed mobile-first. And if you enjoy this, the free Sudoku - Brain Puzzles iOS app adds smart hints that teach thermometer logic, daily streaks, and iCloud sync — plus Arrow, Killer, Kropki, Mega 16x16, and more.