Thermometer Puzzle

How to use the Thermometer puzzle

1. Choose a Size and a Difficulty above the board. The default is 6×6. Press New puzzle to make a new board. If a size cannot produce the level you asked for, the closest level is used and the puzzle title shows the real level.
2. Tap a cell on a thermometer to fill it from the bulb up to that cell.
3. Tap the top filled cell again to lower the mercury by one cell.
4. Drag along a thermometer to set its level in one move.
5. To mark a cell as empty, right-click it or use the Empty (X) chip. On a filled cell, this also empties every cell above it.
6. Watch the numbers at the edges. A row or column number turns grey when you have met it and red when you have exceeded it.
7. Press Hint when you are stuck. It highlights the cells involved and explains in one sentence which rule proves the next step. Press Check to see cells that break a rule or differ from the solution, shown in red, or a “No mistakes so far” message with the percentage done.
8. When the puzzle is solved, the page shows “Solved in m:ss” and offers Next puzzle.

With a keyboard, the arrow keys move between cells, Space fills up to the cell, X marks it empty, and Backspace clears it. Undo and Redo use Ctrl+Z and Shift+Ctrl+Z (Cmd on a Mac). Restart clears the board but keeps the same puzzle.

Thermometer puzzle rules

A thermometer puzzle is a grid covered by thermometers, and you fill some of their cells so that every row and column matches its number. Each thermometer is a path of 2 to 6 cells, either straight or bent, with a round bulb at one end. Every cell belongs to exactly one thermometer, and about half the cells end up filled.

To fill a thermometer, start at the bulb and fill upward with no gaps. A thermometer may be empty, partly filled or completely full. The number at the right of each row is how many cells in that row must be filled. The number below each column is how many cells in that column must be filled.

How to solve thermometers

The best way to solve a thermometer puzzle is to use logic one step at a time, starting with the basic rules. Easy puzzles can be solved with four rules alone. Each one is named the same way the site’s solver names it.

• Line full: if a row or column already has its number of filled cells, its other open cells are empty.
• Line all: if a row or column has exactly as many open cells as it still needs, all of them are filled.
• Fill below: if a cell is filled, every cell nearer the bulb is also filled.
• Empty above: if a cell is empty, every cell further from the bulb is also empty.

Medium puzzles also need two more rules. Overflow applies when filling a cell would fill every cell down to the bulb and push a row or column over its number, so the cell must be empty. Underflow applies when leaving a cell empty would empty every cell above it and a row or column could no longer reach its number, so the cell must be filled.

Hard puzzles need trial and error, where you test one possibility and follow it through to see whether it breaks a rule. For example, in a 6×6 Medium puzzle (seed 3), the first hint shows that if the cell in row 3, column 4 stayed empty, every open cell above it in its thermometer would be empty too, and column 5 could no longer reach its number of 5. So that cell must be filled.

Frequently asked questions

Is the thermometer puzzle free, and do I need an account?

Yes, it is free, and you do not need an account or sign-up. Your progress and settings are saved automatically in your browser, and nothing is uploaded.

Can I play a thermometer puzzle on a phone?

Yes, it works on phones, tablets and computers. On a phone or tablet, the chips above the board choose what a tap does: Fill or Empty (X). A third chip, Move, lets you scroll a zoomed board instead of playing. Zoom out, Fit and Zoom in change the board size, and Fit is the default.

Can I print a thermometer puzzle?

Yes. Press Print to print the puzzle on its own page, with its size, difficulty and rules. Tick “Print the answer page too” to add the solution on a second page. In the print dialog you can choose “Save as PDF,” and both A4 and Letter paper work.

Does every thermometer puzzle have one solution?

Yes. Each puzzle is generated in your browser and kept only when the site’s own solver proves it has exactly one solution. This means every puzzle can be finished by logic alone.

Can I share a puzzle or continue it on another device?

Yes. Copy link copies a link with the size, seed and difficulty in it. It opens exactly the same puzzle, so you can send it to a friend or finish it on another device.

Can I sell thermometer puzzles printed from this page?

Yes. Thermometer puzzles from this page are royalty-free for personal, classroom and commercial use, including puzzle books on Amazon KDP, printables on Etsy, Teachers Pay Teachers and Gumroad. No credit is required. The only thing you may not do is copy the generator or the site itself to run a competing service; see the terms.

Related tools

• Aquarium puzzle: use it if you enjoy filling regions to match row and column totals.
• Kakurasu: try it when you want another grid puzzle built around sums of numbers.
• Tents puzzle: play it for a logic grid where you place tents next to trees.
• Binary puzzle: choose it for a grid of 0s and 1s with strict row and column rules.
• Star Battle: pick it for a region-based logic puzzle where you place stars.
• Hitori: use it for a grid puzzle where you cross out numbers to keep the board clear.
• Nonogram: play it online when you want to reveal a picture from row and column clues.
• All puzzles on the site: browse every puzzle in one place to find your next challenge.