Play Thermometers Online

Free thermometer puzzles — fill mercury from the bulb using row and column clues. Choose your grid size and difficulty. No account needed.

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Thermometers logic puzzle game — fill mercury in thermometers using row and column number clues

What Is a Thermometers Puzzle?

Thermometers (also known as Thermometer puzzles or Mercury puzzles) is a logic puzzle played on a rectangular grid filled with thermometer shapes. Each thermometer has a bulb (rounded end) and extends in one direction — up, down, left, or right. Number clues along the edges of the grid tell you how many cells in each row and column must be filled with mercury.

The key rule is simple but powerful: mercury must be filled from the bulb end. You can never have a gap in the mercury — if a cell is filled, every cell between it and the bulb must also be filled. A thermometer can be completely empty, partially filled, or completely full, but the filled portion always starts at the bulb.

Thermometers puzzles first appeared in puzzle magazines in the early 2000s and have become a staple of online logic puzzle collections. They combine spatial reasoning with number-based deduction, making them approachable for beginners yet deeply satisfying for experienced solvers.

How to Play Thermometers

If you're new to thermometer puzzles, here's a step-by-step guide:

  1. Read the clues. Each row has a number on the left, and each column has a number on top. These numbers tell you exactly how many cells in that line must be filled with mercury.
  2. Identify the thermometers. Each thermometer is shown with a round bulb at one end. The mercury always fills starting from the bulb.
  3. Fill cells. Tap or click a cell to fill it with mercury (shown in blue). Tap again to clear it.
  4. Mark empties. Switch to Mark mode (✕) to flag cells you've determined must be empty. This is optional but very helpful for harder puzzles.
  5. Respect thermometer order. Remember: if you fill a cell, every cell between it and the bulb must also be filled. You cannot skip cells within a thermometer.
  6. Win! The puzzle is solved when all row and column clues are satisfied and all thermometer rules are followed.

Thermometers Strategy Tips

1. Start With Zero Clues

If a row or column clue is 0, every cell in that line must be empty. Mark all of them immediately. This is the easiest and most impactful first step — it eliminates cells across multiple thermometers and often triggers a chain of deductions.

2. Look for Full Lines

If a clue equals the total number of thermometer cells in that row or column, fill them all. For example, if a row has a clue of 4 and contains exactly 4 thermometer cells, every one of them must be filled.

3. Use Forced Fills

If filling a thermometer cell forces other cells to be filled (because mercury must be continuous from the bulb), count whether the clue can accommodate those forced fills. If a row clue is 2 but a thermometer's bulb is 3 cells away from a potential fill, that fill is impossible.

4. Cross-Reference Rows and Columns

Information from one direction constrains the other. If you determine that a thermometer must be filled to at least level 3, that affects every row and column the thermometer passes through. Work back and forth between rows and columns to narrow down possibilities.

5. Count Remaining Cells

Track how many filled cells a row or column still needs. If the remaining unfilled thermometer cells in a line exactly equal the remaining clue count, fill them all. If the remaining count is 0, mark all remaining cells as empty.

6. Consider Thermometer Length

Short thermometers (2–3 cells) are more constrained than long ones. A 2-cell thermometer can only be empty, half-filled, or full — just 3 possibilities. Focus on short thermometers first to build a foundation of known cells.

Thermometers vs Other Grid Puzzles

If you enjoy Thermometers, you'll likely enjoy similar grid-based logic puzzles. Nonograms (Picross) also use row and column clues to determine which cells to fill, but without the thermometer constraint. Battleship puzzles use row and column counts to place ships on a grid. Nurikabe and Tapa involve shading cells based on number clues with connectivity rules.

What makes Thermometers unique is the directional fill constraint — the mercury-from-the-bulb rule adds a layer of spatial logic that other counting puzzles lack. This makes Thermometers particularly satisfying when you realize that filling one cell cascades into multiple forced deductions.

Difficulty Levels Explained

This game offers three difficulty levels for each grid size:

  • Easy: Fewer thermometers with generous clues. Most deductions are straightforward — look for zero clues, full lines, and short thermometers.
  • Medium: More thermometers and tighter clues. You'll need to cross-reference rows and columns and use forced-fill logic to progress.
  • Hard: Dense grids with subtle clues. Requires careful counting, elimination, and multi-step deduction chains. Every puzzle is still solvable without guessing.

Frequently Asked Questions

Thermometers is a logic puzzle played on a grid containing thermometer shapes. Each row and column has a number clue indicating how many cells must be filled. Mercury in each thermometer must be filled starting from the bulb — you can never have a gap.
Start with rows or columns where the clue is 0 (mark everything empty) or where the clue equals the total cells (fill everything). Then use the thermometer fill rule — mercury must be continuous from the bulb — to make deductions. Cross-reference between rows and columns to narrow down possibilities.
Yes! This Thermometers game is fully responsive and works on any modern smartphone browser. Tap cells to fill or empty them. The board automatically adjusts to fit your screen.
No. Every puzzle on this page can be solved using pure logic — no guessing or trial-and-error required. If you're stuck, try the Check button to find errors, and look for rows or columns where the remaining count is very low or very high.
The bulb is the rounded end of the thermometer — it's where mercury starts filling from. If any cell in a thermometer is filled, every cell between that cell and the bulb must also be filled. Think of it like a real thermometer: mercury rises from the bottom.

More Puzzle Games

If you enjoy Thermometers, you might also like these puzzle and logic games:

Nonograms

Use row and column clues to fill cells and reveal a hidden picture.

Nurikabe

Shade cells to form a connected wall around numbered islands.