Thermometer puzzle FAQ
Quick answers about the puzzle, the game and solving. Can't find yours? Start with how to play.
The puzzle
What is a thermometer puzzle?
A thermometer puzzle is a grid logic puzzle. You fill thermometers with mercury from the bulb upward, with no gaps, so that each row and column contains the number of filled cells given by its clue.
What are thermometer puzzles also called?
They are also known as Thermometers, Thermo puzzles, Thermometer logic puzzles and, in puzzle-competition circles, simply Thermo. "Thermo-Sudoku" is a different puzzle: a Sudoku with thermometer-shaped digit constraints.
Is a thermometer puzzle the same as Thermo-Sudoku?
No. In Thermo-Sudoku you write digits that must increase along each thermometer, inside a normal Sudoku. In a thermometer puzzle you only shade (fill) cells to match row and column counts. No digits are placed.
Do thermometer puzzles need math?
Only counting. Each clue is the number of filled cells in a line, and every deduction compares counts. There is no arithmetic beyond simple addition.
Do I need to guess?
Never. Each puzzle on this site has exactly one solution, and you can reach it by logic. Our smart hints always show a cell that follows from the current board when one exists.
Playing on this site
How do I mark a cell as empty?
Right-click or Shift-click with a mouse, long-press on a touch screen, press X on the keyboard, or switch to Air mode in the side panel.
Is my progress saved?
Yes. Your board, time and hint count are saved automatically in your browser. Return to the same puzzle (or reload the page) and you'll pick up where you left off.
How do I share a puzzle with a friend?
Press Share. You'll get a link containing the puzzle's size, style and ID, so your friend gets exactly the same grid.
What do the hints do?
If you've made a mistake, a hint fixes it first. Otherwise it fills in a cell that the current row and column clues already force, and tells you which row and column decide it.
Why are some cells already filled?
On larger boards a few cells may be revealed at the start. They guarantee the puzzle has only one solution and are shown on a tinted background.
Can I play offline or print puzzles?
Once the page has loaded, puzzles are generated in your browser without contacting a server, so you can keep playing even if your connection drops. For pen and paper, use the printable puzzles page.
Is there a dark mode?
Yes. The site follows your device's light or dark setting, and you can switch it with the moon icon in the header.
Solving
Where should I start on a new puzzle?
Look for clues of 0 (the whole line is air) or clues equal to the grid size (the whole line is mercury). Then spread each certain cell along its thermometer.
What is the squeeze technique?
If a line needs more mercury than its other thermometers can supply, the remainder must come from one particular thermometer. Since mercury fills from the bulb, its first few cells are certain. See the squeeze.
How long does a puzzle take?
Roughly under a minute for 4×4, 1–3 minutes for 6×6, 8–20 minutes for 10×10, and up to an hour or more for 15×15, depending on experience.