Thermometer Puzzle: fill the mercury, solve the grid
Fill each thermometer with mercury, starting at the bulb, so every row and column matches its number. Every puzzle has exactly one solution and can be solved by logic alone.
What is a thermometer puzzle?
A thermometer puzzle is a logic puzzle played on a square grid filled with thermometers. You fill some thermometers with mercury, always starting from the bulb and with no gaps, so that the number of filled cells in each row and column matches the clue printed beside it.
The puzzle is also known as Thermometers, Thermo puzzle or Thermometer logic puzzle. Like Sudoku and nonograms, it uses no arithmetic beyond counting and needs no guessing. Every deduction follows from the rules.
Three rules, endless logic
Thermometer puzzles take a minute to learn and much longer to master. These are all the rules.
Fill from the bulb
Mercury starts at the round bulb and rises toward the flat end of the tube, just like a real thermometer.
No gaps, ever
A thermometer can be empty, partly filled or full, but the mercury must be one continuous column from the bulb.
Match every clue
The number outside each row and column is how many cells in that line hold mercury. Satisfy them all to win.
Built for puzzle lovers
Every feature is there to make solving smoother, fairer and more satisfying.
✓ One solution, guaranteed
A built-in solver checks every puzzle before you see it. If a puzzle has more than one answer, it's thrown out, so you never need to guess.
💡 Hints that teach
A hint fills in a cell you could have worked out from the clues and tells you which row and column decide it. If you've made a mistake, the hint fixes that first.
🌡️ Animated mercury
Tap any cell and mercury fills smoothly from the bulb up to it. Drag to fill several cells; long-press or right-click to mark air.
📏 Six sizes, two styles
From a 4×4 warm-up to a 15×15 marathon, each with straight or curved thermometers. That's 12 kinds of puzzle in total.
📅 Daily puzzle & streaks
A new daily thermometer puzzle every day, the same for everyone, with a streak counter to keep you coming back.
🖨️ Printable sheets
Make printable thermometer puzzles with answer keys, for classrooms, travel or pen-and-paper fans.
Choose your challenge
Bigger grids mean more thermometers and longer chains of logic. Here's a rough guide.
| Size | Level | Typical solve time | Best for |
|---|---|---|---|
| 4×4 | Mini | Under 1 minute | Learning the rules |
| 6×6 | Easy | 1–3 minutes | Beginners and quick breaks |
| 8×8 | Medium | 3–8 minutes | Practising the core techniques |
| 10×10 | Hard | 8–20 minutes | Experienced solvers |
| 12×12 | Expert | 15–35 minutes | Long logic chains |
| 15×15 | Master | 25–60+ minutes | A serious workout |
Want more? Switch any size to curved thermometers: bent tubes cross several lines and make each deduction harder.
Five techniques every solver should know
The full strategy guide explains each one with diagrams.
Zeros and full lines
A clue of 0 empties the whole line; a clue equal to the grid size fills it. Both are free starting points.
Learn more →Mercury flows down to the bulb
If any cell is filled, every cell between it and the bulb must be filled too, even in other rows.
Learn more →Air flows up to the tip
If a cell is empty, every cell beyond it toward the tip is empty as well.
Learn more →The squeeze
When a line needs more mercury than the other thermometers can supply, the shortfall must come from the bulb end of the long one.
Learn more →Capacity limits
A thermometer crossing a line can add at most one cell to it. Count the maximum each line can hold.
Learn more →Ready for more?
Read the step-by-step worked example, then try a 10×10 puzzle.
Thermometer puzzles vs other logic puzzles
If you like these puzzles, you'll find thermometers familiar but different in useful ways.
| Puzzle | What you do | Clues | Key idea |
|---|---|---|---|
| Thermometer puzzle | Fill thermometers with mercury | Row and column counts | Fill order: always from the bulb, never with gaps |
| Nonogram (Picross) | Shade cells to reveal a picture | Run lengths per row and column | Block positions and overlaps |
| Sudoku | Place digits 1–9 | Given digits | Each digit once per row, column and box |
| Battleships | Place a fleet of ships | Row and column counts | Ship shapes and no touching |
| Star Battle | Place stars in regions | Region shapes | N stars per row, column and region |
Thermometers combine the row and column counting of Battleships with the shape logic of region puzzles. The one rule that sets them apart is that the fill order is fixed.
Frequently asked questions
What is a thermometer puzzle?
A thermometer puzzle (also called Thermometers or Thermo) is a grid logic puzzle. The grid is covered with thermometers, and you fill some of them with mercury. Mercury always starts at the round bulb and rises toward the flat end with no gaps. The numbers outside the grid say how many filled cells each row and column must contain.
How do you solve a thermometer puzzle?
Start with lines whose clues are 0 (all empty) or equal to the grid size (all full). After that, think about what each row and column can hold: mercury must fill from the bulb, so if a cell is filled, every cell between it and the bulb is filled too, and if a cell is empty, every cell beyond it toward the tip is empty. Our strategy guide covers 10 techniques with diagrams.
Does every puzzle have only one solution?
Yes. Our solver checks every puzzle before you see it and rejects any that have more than one solution. On larger boards a few cells may come pre-filled (shown on a tinted background) to guarantee this, so you never need to guess.
Is Thermometer Puzzle free?
Yes. There are no accounts, paywalls or limits. You get unlimited puzzles in six sizes, straight and curved thermometers, a daily puzzle and printable sheets.
What is the difference between straight and curved thermometers?
Straight thermometers run in a single horizontal or vertical line. Curved thermometers can bend around corners, so one thermometer can affect several rows and columns at once. That makes them harder to read. Mercury still fills from the bulb along the tube.
Can I play on my phone?
Yes. The board resizes to fit any screen. Tap a cell to fill mercury, long-press (or switch to Air mode) to mark empty cells, and drag to fill several cells in one motion. Your progress is saved automatically on your device.
Are thermometer puzzles good for your brain?
They exercise deductive reasoning, working memory and pattern recognition, just like Sudoku or nonograms. Each puzzle needs pure logic, no arithmetic beyond counting, so they make a relaxing but focused mental workout.
Today's puzzle is waiting
Everyone gets the same daily puzzle. Solve it and build your streak.