How to Play Thermo Sudoku

Digits must strictly increase along thermometer shapes

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The Rules

Standard Sudoku rules apply: fill every row, column, and 3×3 box with the digits 1–9, each appearing exactly once.

Thermo Sudoku overlays thermometer shapes on the grid. Each thermometer has a bulb (circle) at one end and a tip at the other. The digits along any thermometer must strictly increase from the bulb to the tip. Combined with standard Sudoku constraints, this creates powerful chains of deductions — knowing the bulb is at least 1 and the tip is at most 9, you can calculate the range of valid values for each cell on the thermometer based on its position.

At a Glance

20–28
Typical givens
Line Constraints
Constraint type
~10m
Easy solve time
~22m
Medium solve time

How to Solve Thermo Sudoku

Beginner
Range Restriction from Length
A thermometer with N cells must use N strictly increasing distinct values. The bulb is at least 1 and the tip is at most 9, giving a calculable valid range for each cell.
Beginner
Minimum/Maximum Bounding
The bulb value is ≤ 9 − (N − 1). The tip value is ≥ N. Apply these bounds immediately to prune all thermometer candidates.
Intermediate
Overlapping Thermometer Deduction
Two thermometers sharing a cell give tighter combined bounds than either alone.
Advanced
Forcing Chains via Thermometers
When a value is forced into a thermometer cell, all cells closer to the bulb must be smaller and all closer to the tip must be larger.
Intermediate
Short vs. Long Thermometers
A 2-cell thermometer is simply a greater-than relationship. A 9-cell thermometer forces exactly 1–9 in order.
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Common Questions

Which end is the bulb? ▾

The circle (round end) is the bulb — digits increase from the bulb towards the tip.

Can a thermometer cross box boundaries? ▾

Yes — thermometers can cross any boundary.

Must digits be strictly increasing? ▾

Yes — strictly increasing means no two adjacent thermometer cells may share the same digit.

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