Play Thermo Sudoku Online

Thermo Sudoku: The Thermometer Logic Puzzle

Thermo Sudoku (also called Thermometer Sudoku) is a fascinating Sudoku variant that adds a visual and logical twist to the classic number puzzle. Grey thermometer-shaped lines are drawn across the grid, and the digits along each thermometer must strictly increase from the round bulb end to the narrow tip. Combined with standard Sudoku rules, these thermometer constraints create deeply satisfying puzzles that demand careful deduction.

🤔 What Is Thermo Sudoku?

A Thermo Sudoku puzzle uses a standard 9×9 grid divided into nine 3×3 boxes, exactly like regular Sudoku. The key difference is the addition of thermometer lines — visual paths that snake through the grid. Each thermometer starts with a round bulb and ends at a tip. The digit in the bulb cell must be the smallest on that thermometer, and each subsequent cell must contain a strictly larger digit than the one before it.

For example, if a thermometer covers four cells, the digits along it might be 2, 5, 7, 9 — but never 3, 3, 6, 8 (no repeats on ascending path) or 5, 4, 7, 9 (not strictly increasing).

📋 Rules of Thermo Sudoku

Thermo Sudoku combines standard Sudoku rules with one elegant additional constraint:

1. Standard Sudoku rules — Every row, column, and 3×3 box must contain the digits 1–9 exactly once.
2. Thermometer rule — Along each thermometer line, digits must strictly increase from the bulb (round end) to the tip. No two cells on the same thermometer may contain the same digit.

The thermometer constraint is purely about ordering — the actual values don't matter as long as each one is larger than the previous. Every puzzle has exactly one solution reachable through pure logic.

⭐ Difficulty Levels Explained

Our Thermo Sudoku offers four difficulty levels that affect both the number of given digits and the thermometer configuration:

• Easy — More given digits and shorter thermometers (3–4 cells). The thermometer constraints quickly reveal placements. Ideal for learning the variant.
• Medium — Fewer givens with thermometers of 3–5 cells. Requires combining thermometer logic with standard Sudoku elimination. A solid daily challenge.
• Hard — Significantly fewer clues and longer thermometers (4–6 cells). Demands advanced techniques like thermometer range analysis and cross-constraint deduction.
• Expert — Minimal givens with many long thermometers (4–7 cells). Requires deep logical chains and expert-level Sudoku skills.

🧠 Essential Thermo Sudoku Strategies

Mastering Thermo Sudoku requires a blend of classic Sudoku techniques and thermometer-specific logic:

1. Thermometer Range Analysis

For each thermometer, calculate the minimum and maximum possible values at each position. On a thermometer of length L, position k (starting from 0 at the bulb) must be at least k + 1 and at most 9 − (L − 1 − k). This immediately eliminates many candidates.

2. Bulb and Tip Constraints

The bulb of a thermometer is always the smallest digit on that line. On a 5-cell thermometer, the bulb can only be 1–5. The tip can only be 5–9. These constraints are powerful entry points for solving.

3. Overlap with Box Boundaries

When a thermometer crosses from one 3×3 box to another, the increasing constraint interacts with the no-repeat rule in both boxes. This often creates powerful eliminations — a digit forced on the thermometer in one box can be removed as a candidate in the same box's other cells.

4. Parallel Thermometer Logic

When two thermometers share a row, column, or box, their increasing constraints interact. If both thermometers have bulbs in the same box, those bulbs compete for the lowest available digits. This kind of interaction is key to cracking harder puzzles.

5. Standard Sudoku Techniques

Don't forget naked singles, hidden singles, pointing pairs, and other classic strategies. Thermo Sudoku is still Sudoku at its core — the thermometer constraints supplement rather than replace traditional logic.

📊 Quick Reference: Thermometer Position Ranges

Use this table to instantly know the minimum and maximum values at each position on a thermometer:

• 3-cell thermometer: Bulb 1–7, Middle 2–8, Tip 3–9
• 4-cell thermometer: Bulb 1–6, Pos 2: 2–7, Pos 3: 3–8, Tip 4–9
• 5-cell thermometer: Bulb 1–5, Pos 2: 2–6, Pos 3: 3–7, Pos 4: 4–8, Tip 5–9
• 6-cell thermometer: Bulb 1–4, Pos 2: 2–5, Pos 3: 3–6, Pos 4: 4–7, Pos 5: 5–8, Tip 6–9
• 7-cell thermometer: Bulb 1–3, through to Tip 7–9

🆚 Thermo Sudoku vs. Other Sudoku Variants

How does Thermo Sudoku compare?

• vs. Classic Sudoku: Classic uses only given digits as clues. Thermo adds visual thermometer lines with ordering constraints, providing a different type of logical information.
• vs. Killer Sudoku: Killer uses cage sums (arithmetic). Thermo uses increasing sequences (ordering). Both add constraints beyond standard Sudoku, but the logic feels very different.
• vs. Arrow Sudoku: Arrow Sudoku requires digits along an arrow to sum to the digit in the circle. Thermo requires strictly increasing order instead — no arithmetic needed.
• vs. X Sudoku: X Sudoku adds diagonal constraints. Thermo adds path-based ordering constraints. They can even be combined for an extra challenge!

📜 History of Thermo Sudoku

Thermo Sudoku emerged from the broader world of Sudoku variants that exploded in the 2000s and 2010s. While the exact origin is difficult to pinpoint, thermometer-based constraints appeared in puzzle magazines and competition circuits in the early 2010s.

The variant gained massive mainstream attention through Cracking the Cryptic, the popular YouTube puzzle channel run by Simon Anthony and Mark Goodliffe. Their enthusiastic solving videos introduced Thermo Sudoku to millions of viewers and inspired puzzle setters worldwide to create increasingly creative thermometer puzzles.

Today, Thermo Sudoku is one of the most popular Sudoku variants, regularly featured in the World Puzzle Federation competitions and beloved by the global puzzle community.

💪 Benefits of Playing Thermo Sudoku

• Strengthens sequential reasoning — tracking increasing sequences builds a different type of logic than standard Sudoku.
• Improves spatial awareness — following thermometer paths across the grid exercises visual-spatial skills.
• Develops constraint management — juggling thermometer ordering with row/column/box rules deepens your ability to handle multiple constraints simultaneously.
• Visually engaging — the thermometer graphics make the puzzle visually distinctive and satisfying to solve.
• Highly addictive — the "aha" moments when a thermometer cascade resolves multiple cells are deeply rewarding.

🎮 More Sudoku Variants to Explore

• Classic 9×9 Sudoku — The original puzzle. Start here if you're new.
• Killer Sudoku — Cage sums replace given digits. Logic meets arithmetic.
• X Sudoku — Both main diagonals must also contain 1–9.
• Hyper Sudoku — Four extra window regions add 4 more constraint groups.
• Jigsaw Sudoku — Irregular regions replace the standard 3×3 boxes.