Thermometer puzzles explained: the complete guide

1. What a thermometer puzzle is

A thermometer puzzle is a logic puzzle played on a square grid that is completely covered by thermometers, each with a round bulb at one end and a flat tip at the other. You decide how far each thermometer is filled with mercury. Mercury starts at the bulb and has no gaps, and the number of filled cells in every row and column must match the clue beside it. The puzzle uses no arithmetic beyond counting, and every puzzle on this site has exactly one solution.

Read more: the full rules with diagrams, or play a puzzle now.

2. Other names, and a common mix-up

The puzzle is also called Thermometers, Thermo or thermometer logic puzzle. The name is easily confused with Thermo-Sudoku, a Sudoku variant in which digits must increase along thermometers drawn over the grid. That is a digit-placement puzzle with full Sudoku rules. A thermometer puzzle has no digits in the grid at all.

Full guide: Thermometer puzzle vs Thermo-Sudoku.

3. Its family: line-clue puzzles

Thermometers belong to a family of puzzles with clues beside every row and column. In Battleships the clues are counts, as in thermometers, but you place a fleet of ships that may not touch. In nonograms the clues list the lengths of filled runs in order. All three share the "overlap" style of reasoning, so skills transfer well between them.

Full guide: Thermometers vs nonograms vs Battleships.

4. Sizes and variants

Puzzles come in square grids from 4×4 to 15×15, with either straight thermometers (each in a single row or column) or curved ones that bend around corners. Larger grids have more thermometers and longer chains of logic. Some large puzzles also include a few pre-filled cells that guarantee a single solution.

Full guide: thermometer puzzle sizes and variants. To play a particular kind: 6×6, 10×10, 15×15, curved.

5. The math behind it

Because mercury fills from the bulb, a thermometer of length L has only L + 1 possible states. On a typical 10×10 puzzle that leaves about 10¹⁹ possible arrangements instead of the 2¹⁰⁰ (about 10³⁰) you would get if every cell were independent. Two more facts do a lot of work: the row clues and column clues always add up to the same total, and a thermometer's share of any line can only grow as it fills. That last property is why the squeeze and ceiling deductions are always valid.

Full guide: the math of thermometer puzzles.

6. How our puzzles are made

Each puzzle is generated in your browser: a random layout of thermometers, a hidden fill level for each, and clues counted from that solution. A logic-only solver then checks that the clues alone decide every cell. If they don't, the generator reveals the single most helpful cell and checks again, then removes any revealed cells that turned out to be unnecessary. A puzzle ID is the seed for this process, which is why a shared link always opens the same grid.

Full guide: how our thermometer puzzles are generated.

7. Thermometer puzzles for learning

Short rules, no arithmetic beyond counting and a guaranteed path that needs no guessing make the puzzle a good fit for teaching step-by-step reasoning. Small grids suit beginners and children, while larger and curved grids stretch confident solvers.

Full guide: thermometer puzzles for kids and classrooms, with a 45-minute lesson plan and printable sheets.

8. Ways to play

• Endless: unlimited puzzles online in six sizes and two styles, each with a shareable ID.
• Daily: one daily puzzle per size, the same for every player, with streaks.
• On paper: printable puzzles with answer keys.

9. Quick answers

Short answers about the rules, the game's controls, hints, saving and sharing are collected in the thermometer puzzle FAQ. For solving help, go to the complete guide to solving thermometer puzzles.

Every guide in this collection

1. Thermometers vs Thermo-SudokuTwo different puzzles that share a name and a shape. Here is how to tell them apart.Explainer
2. Thermometers vs Nonograms & BattleshipsWhat the three line-clue puzzles share, and where their logic differs.Comparison
3. Sizes & variantsStraight vs curved, grid sizes, pre-filled cells and difficulty.Reference
4. The math of thermometer puzzlesFill levels, counting arrangements, checksums and why logic is enough.Deep dive
5. How our puzzles are generatedFrom random thermometers to a puzzle with one solution, step by step.Behind the scenes
6. For kids & classroomsTeaching logic with thermometer puzzles, with a ready-to-use lesson plan.Teachers & parents
7. FAQQuick answers about the puzzle, the game and solving.Reference

Questions