Thermometer Puzzle Generator
A thermometer puzzle covers the grid with thermometers, each a straight run of squares with a bulb at one end. Mercury rises from the bulb and never leaves a gap behind it, so once you know a square is empty every square further along the same thermometer is empty too. A number beside each row and above each column says how many of its squares are filled. There is nothing else to go on — and every grid here is checked to have one answer and no other.
What this generator does
Tiles the grid with thermometers, fills them at random, counts the totals that result, and keeps the layout only if those totals allow one answer and no other.
How to use this tool
- Choose a grid size and how long the thermometers should be.
- Print or copy the grid; the round end of each thermometer is its bulb.
- Fill squares from the bulb along each thermometer, never skipping one.
- Match every row total beside the grid and every column total above it.
- Work from the extremes first — a total of zero empties a whole row, and a full row fills every bulb it touches.
- Show the answer to check the finished grid.
Understanding the controls
- Grid
- Five to nine squares a side, with nine reachable only when the thermometers are long.
- Thermometers
- How long a thermometer may be. Longer ones make a grid far easier to pin down with the totals alone, which is why the short setting stops at six squares a side, the medium at seven and the long at nine.
- Seed (optional)
- Type anything to rebuild the same puzzle later, which is how you reprint a page you have lost. A seed here is for repeating a puzzle, not for keeping one secret — it carries no cryptographic strength.
Common use cases
- A shading puzzle whose rule is a run rather than a count of neighbours
- A printable page for a puzzle club, a classroom or a newsletter
- A quick logic puzzle that needs no arithmetic beyond counting
- A change of pace for anyone who solves picture-shading grids
- Reprinting the identical grid later from its seed
How this generator works
This puzzle has no clue-removal pass, and that turns out to be what limits it. Every row and column total is always printed, so a layout cannot be made harder by taking a clue away: it either has one answer or it has several, and the only remedy is to draw another. What runs short on a large grid is therefore not time but supply, because there are only two totals per line however many thermometers there are. Measured over three hundred random layouts each, a five-square grid of short thermometers had one answer about a third of the time, a seven-square grid of short thermometers about one time in eighty, and a ten-square grid almost never. The offered combinations are the ones where a pinned-down layout still turns up quickly. Solving is cheap by comparison: every thermometer is straight, so raising its mercury touches one row and one column only, and the search takes the longest thermometers first while holding every line bounded from both sides.
Randomness and fairness
The layout of the thermometers, and how far each one is filled, come from your browser's cryptographic random source. Giving a seed replaces that source with a repeatable one, so the same puzzle can be printed again; the seed is a reference, deterministic, not a cryptographic key.
For how randomness is produced across the whole site, see how Generate Random works.
Limitations and good to know
- The grid stops at nine squares a side, and the shorter thermometer settings stop sooner — measured limits on how often a puzzle can be pinned down at all.
- Difficulty is not graded. Every total is printed, so the puzzle is always solvable by reasoning — but how much of it falls out at once varies and is not measured.
- Most random layouts have more than one answer and are thrown away, so the time a puzzle takes to appear varies.
- Every total is printed, so the puzzle cannot be made harder by hiding some of them.
Privacy and your data
The layout is drawn, filled and checked entirely in your browser. Nothing you type as a seed, and no puzzle made from it, leaves the device.
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