What is a Thermometers puzzle?
Thermometers is a visual logic puzzle played on a square grid filled with thermometer shapes. Each thermometer has a bulb, a path and a tip. Your job is to decide how much mercury belongs in every thermometer.
The numbers outside the grid tell you exactly how many cells are filled in each row and column. A thermometer can be empty, partly filled or completely filled, but the mercury always starts at the bulb and moves continuously toward the tip.
- Fill thermometer cells with mercury.
- Mercury must begin at the round bulb.
- Filled cells cannot skip over empty cells in the same thermometer.
- Every row clue and column clue must be matched exactly.
- The puzzles on this page are checked for a unique solution before they appear.
How to play Thermometers online
Click or tap a cell to cycle through filled, empty and unknown. Use filled cells for mercury and empty marks when a square cannot contain mercury. The grey thermometer outlines show the direction from bulb to tip.
Use Check when you want feedback without revealing the answer. Hint fixes one useful square, Undo steps back through your moves, and Solution shows the complete answer if you want to study the logic.
- Start by scanning rows or columns with a clue of zero.
- Mark a full row or column when its clue equals the grid size.
- When you fill a cell, every earlier cell in that thermometer must also be filled.
- When you mark a cell empty, every later cell toward the tip must also be empty.
- Keep comparing thermometer pressure with the remaining row and column totals.
Thermometers rules
The central rule is continuous fill. If the third cell of a thermometer is filled, the bulb, first cell and second cell must be filled too. If the second cell is empty, every cell farther toward the tip must also be empty.
The counting clues are exact. A row clue of 4 means exactly four filled cells in that row, not at least four. The same rule applies to every column clue.
- Thermometers may be empty, partly filled or full.
- Mercury never starts in the middle of a thermometer.
- Mercury never leaves a gap.
- Row clues count filled cells across the row.
- Column clues count filled cells down the column.
Thermometers strategy tips
Good Thermometers strategy comes from turning one clue into a chain of consequences. A zero row empties every cell in that row, and any empty mark can push farther toward the tip of its thermometer. A full row does the opposite: every cell in the row fills, and that fill can flow back to the bulb.
On harder puzzles, look for overlap. If a row needs three more filled cells and the only way to get them would also force extra cells in another row, that path is impossible. This kind of cross-checking is the heart of Thermometers logic.
- Use zero clues first because they remove whole lines immediately.
- Use maximum clues to force every cell in a line.
- Apply bulb-to-tip pressure after every fill or empty mark.
- Watch rows and columns that are almost complete.
- Compare the earliest possible filled cells with the latest possible empty cells.
The slider trick: every thermometer is a dial
The fastest way to understand Thermometers is to stop thinking about single cells and start thinking about each thermometer as one dial. Because mercury always fills continuously from the bulb, a thermometer with five cells can only sit at one of six settings: empty, or filled up to the first, second, third, fourth or fifth cell. It never skips and never starts in the middle, so the whole puzzle is really about choosing one setting per thermometer that makes every row and column count come out right.
That view turns vague guessing into simple bookkeeping. For each thermometer, keep the smallest and largest setting still possible. Every empty mark lowers the largest setting, because the mercury cannot reach that far; every filled mark raises the smallest setting, because the mercury must reach at least there. When a thermometer's smallest and largest settings meet, it is solved, and that often forces neighbouring lines to move too.
- A thermometer of length N has only N+1 possible fill levels.
- Mercury never skips a cell and never starts past the bulb.
- Track each thermometer's minimum and maximum possible fill.
- An empty cell lowers the maximum; a filled cell raises the minimum.
- When the minimum meets the maximum, that thermometer is locked in.
Counting tricks with the row and column clues
Every row and column clue is an exact count, so simple arithmetic does a lot of the solving. Subtract the cells you have already filled in a line from its clue to get the line's remaining capacity. If that capacity equals the number of cells still open in the line, fill them all; if it is zero, mark them all empty. Each forced mark then flows along its thermometer toward the bulb or the tip.
Counting also shows where mercury is most likely. When a line still needs fills, the cells next to a bulb are the safest to fill first, because filling any cell drags every cell behind it back to the bulb. In a line with a very low clue, the cells near a tip are the safest to empty. Balancing a row's leftover capacity against the columns it crosses is what cracks the hardest boards.
- Remaining capacity = the clue minus the cells already filled in that line.
- Capacity equal to the open cells means fill them all.
- Capacity of zero means empty every remaining cell.
- In a line that still needs fills, start from the cells nearest a bulb.
- In a low-count line, empty the cells nearest a tip first.
Thermometers vs Nonograms
Thermometers and Nonograms look like cousins: both are grid puzzles solved with number clues around the edges, and both reward patient row-and-column logic. If you already enjoy picture-logic puzzles, Thermometers will feel familiar from the first move.
The clues work differently, though. A Nonogram clue describes the lengths of separate runs in a line, while a Thermometers clue simply counts how many cells are filled in total. The twist that makes Thermometers its own puzzle is the thermometer shape: mercury must fill continuously from each bulb, so a filled cell forces its neighbours back toward the bulb in a way Nonograms never do. That single rule gives the puzzle both its name and its flow.
Grid sizes and difficulty
The 6x6 Thermometers board is the friendliest place to learn because each clue has fewer cells to control. The 8x8 board feels like a compact daily puzzle, while 10x10 gives longer thermometer chains and more row-column interaction.
Easy puzzles use shorter chains and more direct clues. Medium puzzles add more curved thermometers and tighter counts. Hard puzzles use larger grids, longer pressure chains and clue patterns that require several deductions to work together.
- Choose 6x6 easy if you are learning the rules.
- Choose 8x8 for a balanced Thermometers puzzle online.
- Choose 10x10 for a bigger logic puzzle challenge.
- Easy, medium and hard change the thermometer layout and solving pressure.
- New puzzle creates another checked puzzle for the chosen setting.