Number & arithmetic
Thermometer Sudoku — rules and where to play
THERMOMETER SUDOKU · Thermo Sudoku / Thermo
Thermometers sit on the grid and their digits must strictly increase from bulb to tip. It adds a directional constraint, and the reasoning feels nothing like plain Sudoku.
- Origin
- 2008 · Belgium
- Difficulty
- ★★★★★ 3–5 / 5
- Time
- 10–50 min
- Grid
- 9×9
- Sites
- 5
Rules
- All standard Sudoku rules apply.
- Along each thermometer, digits strictly increase from the round bulb to the tip.
- A thermometer's length sets the minimum spread of digits it needs.
How to start
Solve the long thermometers first
A nine-cell thermometer can only be 1–9 in order, so the whole thing locks at once.
Use length in reverse on short ones
On an n-cell thermometer, the bulb can be at most 10−n.
It compounds well with other variants
Thermometers often combine with killer cages or arrows, and the overlap is valuable.
Where players get stuck
- Reading "increasing" as "non-decreasing" — equal digits are not allowed.
- Missing that a thermometer can bend, and misjudging which end is the bulb.
Play Thermometer Sudoku
5 sitesThe default web editor for Sudoku variants. Build a grid, send the link, or just solve.
Open → SudokuPadFreeThe variant player a lot of people use now, and it edits too. There's an app and a Steam build.
Open → Penpa+FreeA fat editor for Japanese types: lots of shapes and marks, and it will ingest a puzz.link.
Open → Grandmaster PuzzlesFreeThomas Snyder's own handmade Sudoku and variants — world-champion difficulty, not generated filler.
Open → GridPuzzleFreeDozens of grid logic types in the browser, with progress tracking and dailies.
Open →Also on multi-type portals
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About this page
- Last updated 2026-09-22
- Source Curated compilation of established puzzle rules and listed third-party play sites
- Audience Players learning Thermometer Sudoku rules or looking for online sites to play
- Covers Rules summary, how to start, common pitfalls, and the dedicated play sites listed on this page
- Does not cover Hosted puzzles, worked solutions, official contest rankings, third-party uptime or pricing guarantees
- Not advice This page is puzzle reference content only. It does not constitute financial, legal, medical or other professional advice.
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