🌡️ How to play Thermometer Sudoku (Thermo)

By Miguel Arechaga

Thermometer Sudoku, or Thermo Sudoku, is one of the most visual and popular variants, a regular feature in Cracking the Cryptic puzzles. Thermometer shapes are drawn on the board, and the rule is simple: numbers increase along each one, from the bulb to the tip. Here are the rules and a few tips.

What is Thermometer Sudoku?

It's a standard 9x9 sudoku with several thermometers drawn on it: shapes with a round bulb at one end and a path running through adjacent cells. The extra rule is that the numbers must strictly increase along the thermometer, starting at the bulb (lowest value) and ending at the tip (highest value).

Rules and how to play step by step

1. The sudoku rules apply: 1 to 9 without repeats in each row, column and 3x3 box.
2. On each thermometer, every cell must be greater than the previous one as you move from bulb to tip.
3. You start with some clue cells and fill in the rest with logic. Every puzzle has a unique solution.

Tips to solve Thermometer Sudoku

• Length matters: a thermometer with N cells forces the bulb to be at most 10-N and the tip at least N. For example, on a 5-cell thermometer the bulb ranges from 1 to 5 and the tip from 5 to 9.
• The bulb tends toward low numbers and the tip toward high numbers; start there when in doubt.
• When you know a value halfway along the thermometer, it immediately bounds the minimum of the following cells and the maximum of the earlier ones.
• Combine thermometers with the classic elimination techniques of sudoku.

How it differs from regular sudoku

Compared to classic sudoku, Thermometer adds order relationships between cells (greater/less than), not just the no-repeat rule. That lets you deduce many values by range. If you enjoy variants with marks and extra constraints, you'll also enjoy Consecutive Sudoku and Killer Sudoku.