How to Solve Thermometers Puzzles

Updated September 2026

Thermometers is a logic puzzle where the grid is divided into tubes, each with a bulb at one end. Mercury always fills from the bulb outward, so a thermometer can be filled partly, fully, or not at all, but never with a gap. The numbers beside each row and under each column say exactly how many cells in that line are filled. This guide explains the deductions that solve these puzzles without guessing.

The bulb rule is everything

Because mercury fills from the bulb, a filled cell means every cell between it and the bulb is also filled. An empty cell means every cell beyond it, toward the tip, is empty too. Every deduction in Thermometers comes back to this rule.

In ujem you apply it directly: tapping a cell fills its thermometer up to that cell, and tapping a filled cell drains it from that point. You are really choosing how high each thermometer rises.

Start with full and empty lines

A row or column with a clue of zero has no filled cells at all. Every thermometer that crosses that line must stop before reaching it. That often means some thermometers are completely empty, since their bulb sits on the zero line.

A line whose clue equals its length is completely full. Every thermometer that crosses it must be filled at least up to that line, which also fills every cell between that line and each bulb.

Thermometers that lie along a line

When a thermometer runs along a row, the row's clue limits how far it can fill. If a row has a clue of 2 and a thermometer of length 4 lies along it with its bulb in the row, at most two of its cells can be filled, and only the two nearest the bulb.

Conversely, if a line's clue is large and the only way to reach it is to fill most of one long thermometer, that thermometer must be filled up to the needed length.

The overlap trick

When a line's clue is large compared with the space available, some cells must be filled no matter how you satisfy it. Count the maximum cells the other thermometers in the line could provide. If they cannot reach the clue on their own, the remaining thermometer must contribute the difference, starting from its bulb.

The same logic works in reverse for small clues. If a line already has as many filled cells as its clue, every other cell in it must be empty, and the bulb rule then empties the cells beyond them.

Use the colour feedback

Clues turn green when a line is exactly right and red when it has too many filled cells. Use this to check your work as you go, but do not rely on trial and error. Each change you make affects both a row and a column, so a guess in one place can quietly break a line elsewhere.

A practical plan

1. Mark zero lines and full lines first and apply the bulb rule to them.
2. Look at thermometers lying along a single row or column and limit their fill.
3. Use the overlap trick on lines with large clues.
4. Once a line is satisfied, empty everything else in it.
5. Recheck crossing lines after each change, then repeat.

Thermometers puzzles look intimidating at larger sizes, but they always come down to the same few deductions. Master the bulb rule and the overlap trick, and each new grid becomes a satisfying chain of forced moves.